Synthesizer Build part-1: SYMMETRICAL POWER SUPPLY (1,5A) with independent voltage adjustment.

The essential building block for our DIY synth. with outputs for +/- 15V, 12V and 5V at 1.2 Ampères.

Please scroll half way down the article to find the second version (which is modular in set-up) with stripboard layouts

WARNING, THIS PROJECT REQUIRES YOU TO WIRE UP A MAINS TRANSFORMER. BE VERY CAREFUL WHEN HANDLING MAINS POWER. IT CAN BE DEADLY AS YOU PROBABLY KNOW. IF IN DOUBT ASK AN EXPERT OR POST QUESTIONS IN THE FACEBOOK GROUP. DON'T TAKE CHANCES WITH MAINS ELECTRICITY!!

In Oktober 2019 I set myself the task of building my own synthesizer. I started by slowly buying in the components I was going to need, as my budget allowed (and that wasn't much ^__^).
The first thing I needed for this project was a symmetrical power supply to give me positive and negative voltages, because practically everything in a synthesizer runs on a dual powersupply.
I used the LM317 and LM337 for this project because they are easy to work with and fully protected inside against short circuits and over heating. The LM3XX series regulators can deliver up to 1,5 Ampères.
This is the schematic I came up with and it works very well: (click picture for full screen view)

(STRIPBOARD LAYOUT FURTHER DOWN THE ARTICLE!)

I based this schematic on the datasheet schematic for the LM317 and LM337. It calls for a 5K trimpot but I used 10K's because that's what I had and it works fine. Afterall you can trim it down to below 5K just as easy, so no problem there. The values of the electrolytic caps I'm using is way over the top compared to the datasheet but it does help stabilize the voltage especially because some modules, like Sample and Holds or Sequencers can pull a pulsed current from the powersupply. Here's a screenshot of the original schematic from the Texas Instruments datasheet:

The transformer you use for this power supply must be about 2 to 3 Volt higher in output voltage than the needed output voltage of the powersupply. There is going to be some voltage drop over the voltage regulators and the diode rectifiers that must be accounted for. However the voltage will also be higher than the transformer delivers when it comes out the diodes so we must also account for that. I find 2 x 17Vac secondary outputs to be about the sweet spot. You can go a bit higher if you like.

In the schematic above you can see that the output capacitors are 1000µF. They don't have to be this big. 100µF will do nicely too. The caps discharge through the 220 Ohm and 10K potmeter when you switch off.
If you need a power supply that can handle more current, say 10 Ampères for instance, then you can easily adjust this circuit by adding a 2N3055 on the positive side and an MJ2955 on the negative side. You can even put more power transistors in parallel over eachother to get even bigger current specs. Move the capacitors that come after the LM3XX's past the transistors but put an extra 100nF capacitor between the base and the null to suppress transients etc. Use Google to find more specific schematics for that, if you need it.

But for a modular synthesizer those alterations are not needed because the individual modules I'm going to build won't draw much current at all. It's mostly in de 20 to 80 milli amps at most. The only transformer that I had that was big enough for this project didn't have a center tap but it had two independent secundary outputs. One at 21V and one at 17V. This was close enough. I connected one wire from each output together to make a center tap and made a circuit board to build the powersupply on. (Make sure when you connect two secundary windings together like this, that you measure the AC output before proceeding with the next steps. If you connect the wrong wires the 2 voltages will try to cancel eachother out. It won't damage the transformer but you won't get any voltage out.) I didn't have etching fluid anymore so I cut different islands on the copperside of the copperclad circuit board I was using, with a dremmel tool with a milling bit. I had an old 25 Amp. Graetz diode bridge in my collection of components and after I drilled all the holes the build was pretty straight forward.
I used little trimmer potentiometers to set the voltage with. Once you set it, you don't have to touch it again but if you want to make it continuously adjustable you can of course use 10K panel mounted potentiometers with a knob. I put some LED's on the output side to indicate that all is working well. The difference in brightness will indicate if the voltages are set differently from eachother. You could also go the luxurious route and use two panel voltmeters to indicate the voltage but as I intend this to power my synthesizer that will not be necessary.
But if you build this as a stand-alone power supply, it would be a good idea to use two meters on the outputs. Beware with cheap Chinese digital voltmeters. They look great but they put a ton of noise on the voltage rails!! Use analog meters instead if you can. They look even better and are noise free.
The power supply as described above outputs between 1,5 and 25 Volts (dependent on the transformer you use) at a maximum current of 1,5 Ampere. If you want a more powerful version you could use the LM3XX regulators to drive a 2N3055 and a MJ2955 NPN and PNP power transistors as mentioned above and then you can draw up to 10 to 15 Amps. Be aware that the pinouts for the LM regulators differ from eachother. The correct pin numbers are noted on the schematic above.
The ripple is very low on the output. It's actually better than some you buy ready made. Make sure the electrolytic capacitors you use are rated 50 volts or higher. (I used 35V ones and that seems to work fine too but don't go any lower!) and make sure they are oriented the right way. Plus on + on the positive side and plus to ground and minus to negative voltage on the negative side. The voltage at the secundary directly after rectification can go up to 10 volt higher than the AC voltage from the transformer. Don't present more than 35V to the input pin of the regulators and be sure to use big heatsinks on both of them.
I've measured the ripple and noise of the power supply using the method outlined by Dave Jones from the EEVblog on YouTube in his video EEVblog #594 - How To Measure Power Supply Ripple & Noise
I used my simple home build 'brute force power load' described in the article from januari 2017 and under a load of 1 Ampere the Ripple Voltage Vrms was 6mV and Vpeak-to-peak was 10mV. Those are very good results.

Here's the stripboard layout for the powersupply. There are minor differences in values and components because this layout is based on an eBay kit, instead of the above datasheet schematic but it works perfectly, I assure you. You can mount the voltage regulators on a single big heatsink but they must be electrically insulated from the heatsink and eachother.

[NB: 11-Sept-2021 I just built one of these again today using this layout and it worked rightaway.]

You can leave the 10µF electrolytic capacitors over the trimpotmeters out. You don't need to include them. I put them in as an added protection for the LM3** voltage regulators but they are not needed. They are also not included in the schematic above.   

I included an L-Bracket symbol to show which part of the stripboard can be used for mounting behind a panel or in a case. The components are quite spread out so you can put them closer together to make the board smaller, I leave that up to you.

Here are some pictures of the first powersupply . As you can see there's some room left on the circuitboard for extra voltage regulators to get other voltages from the same supply:

I added the inductor coil in series with the Ground or Zero Volt pole to suppress any high frequency noise. It's just something I added as a test but you can leave that out.

It's a week later and I finished the power supply as I need it for my synthesizer project. It now has -15/0/+15V,  -12/0/+12V and -5/0/+5Volt. I looks a bit of a mess as my projects usually do but it works just fine. Here's a picture of the finished psu:

[EDIT: This is future Eddy speaking 5 years later. Having built over 60 synthesizer projects which have all been connected to this powersupply I can say it's a solid design. Many a time I have tested a module and had a short circuit somewhere but the powersupply always survived it.]

Obviously you can't plug in all modules in the same holes so I've build a power bus system to which I can connect every module I build. It's a bit crude and I use a lot of hot-glue to stick it all in place but it works just fine and it will all be invisible once the case is ready.

Below you see the connector I build. The pins carry the following voltages: From top to bottom in the pic below, the top 2 pins are ground or 0V. Then I took out two pins and stuffed the holes in the female connector with hot glue. This is to get an a-symmetrical distribution so you can't put the connector in the wrong way around. Then there's -15, -12 and -5 Volt and then we get +5, +12 and +15 Volt. I kept the plus and minus pins as far away from eachother as possible for safety reasons.

PART TWO. SECOND POWERSUPPLY for stage two of my synthesizer.
So, as I'm writing this we're 6 months on in the synthesizer build and I'm about to add a second stage to go on top of the synthesizer I have already built.
So I need a second power supply. The first design as seen above works so well that I'm repeating it for the second stage with a few minor changes. I'm using multiturn potmeters for the LM317 and 337 voltage regulators so I can set them very accurately. In fact, the one I just built has plus and minus 15.00V that's accurate to 1/100th volt. I'm using all LM3xx regulators for this because I have a lot of them and because their voltage doesn't drop if you pull more current from them which is important because otherwise the VCO's would become out of tune. I'm using the same schematic as above and I made separate stripboards for every stage of the supply. Here's the stripboard layout:

Bridge rectifier board. (Don't forget to cut the copper strip underneath the fuses and to tin all the copper strips that carry current.). You can hang more than one voltage regulator print off of the bridge rectifying board as long as the transformer and rectifying diodes can handle the maximum current of the combined regulator boards.

Voltage regulator board. (Again, make sure to tin all the copper strips that carry current). 

(Last revised: 30-Jan.-2021: Corrected connection of negative voltage indicator LED.)

SOME NOTES ON THE DIFFERENT COMPONENTS TO USE:
Don't get confused by Capacitors being of a different value on the stripboard than on the schematic. The electrolytic caps on the Rectifier board are the big ones. They can be 1000µF to 2200µF or even higher and they do the main ripple suppresion. On the regulator board the electrolytic caps can be smaller, like 100µF because the main ripple suppresion has already been done and these are there to suppress noise and such. 100µF is enough for that.
Diodes also. The diodes around the voltage regulators are simply safety valves. Their purpose is to prevent the output having a higher voltage than the input, which could damage the regulator. The circuit will work fine even if you leave the diodes out. You can use any type of diode you wish 1N4148, 1N4001 upto 1N4007. It doesn't really matter. The diodes on the bridge rectifier however must be types that can handle at least 100V and 1,5 Ampères. You can not compromise on those but there are many different types you can choose from so I didn't specify which type number of diodes to use. You can also use a Graetz Bridge Rectifier, that's 4 big diodes in one case. I saw a 1000V/4Amp one for 50 cents on eBay. Those will work fine and they won't even get warm.
Like I mentioned before, the voltage regulators must be mounted to a heatsink. Either use two separate heatsinks (make sure they can't touch eachother) or use one big one for both regulators but in that case you MUST insulate the regulators electrically from the heatsink otherwise you'll get a very short lived microwave oven with integrated laser lights. ^____^
Naturally the voltage regulators do not have to be mounted on the stripboard itself. You can mount them on the backside of a front panel, using that as a heatsink, or on the side of a metal case you're using and then you can wire them up to the stripboard with normal electrical wire. Use your imagination but do make sure the regulators are not electrically in contact with eachother, otherwise, boom!
The LED's are simply there as a visual indicator that the circuit is under power and they are not critical to the working of the circuit, so you can do without them if you wish. Use 15K current limiting resistors with the LEDs though, because lower values can get hot.

Should you have problems like not getting the right voltages out of the powersupply then check your resistor values. Are you sure the 220 Ohm is not a 220K? This has happened on at least two occasions I know of that's why I'm writing it here as a reminder.

Here are some pictures of the finished power supply. Tinning all the copper strips that carry current is important because they get very thin around the holes in the stripboard. 
I've mounted the whole powersupply on a long piece of MDF ready to accommodate the power-bus system I need to build.

I found some very old vintage diodes with a metal case which I think look very cool and they work fine. They are sturdy too because I had some short circuits in testing and the fuse went 2 times but the diodes didn't mind and I use slow fuses too so they did get some current through them.
Use plenty of heat sink compound on the LM's. The electrolithic caps are 1000µF each; all four of them and that's all the capacitance I put in. 1000µF in the rectifier and 1000µF on the output side of the voltage regulators. The rectifier caps have 10K 1Watt resistors over them to make sure the are drained of voltage when the powersupply is switched off. (It's 2K2 in the picture but they were getting slightly warm so I changed them for 10K's)

SIMPLE FIXED VOLTAGE POWERSUPPLY  using the 7812 and 7912 voltage regulators.

Finally I want to close off this article with a very simple powersupply that uses fixed voltage regulators. The 7812 for positive voltage and the 7912 for negative voltage. These can do up to 1,5 Ampères but I wouldn't use it for more than 1 Amp. otherwise they get very hot even with heatsink.

Btw, you must use these on a heatsink too, just like the previous designs, and you must make sure they don't electrically touch the heatsink if you have both regulators on one heatsink. Otherwise you get a short circuit and a lot of magic smoke. The circuit schematics for this one can be found by clicking here

Here is the layout I made for this PSU. It's very small and can fit anywhere. The LEDs are there to indicate if power is present on the outputs. You can mount those in a panel if you make a panel for your powersupply. I always mount leds like this near the ON/OFF switch for the PSU (Power Supply Unit).

(Last revised: 24-Aug-2022: Corrected a mistake where the LEDs were not connected to ground properly, thanks to a observant reader. )

Okay that's it for this one. If you have any questions you can leave them in the comments or post them on our special Facebook Group for this website, where we have a cool little community who will love to help you out.

ELECTRO-MAGNETIC FIELD DETECTOR.

Here is an easy to build EM Field detector with 4 stage LED strength indication and which has a wide range of applications. This circuit will detect electromagnetic fields and also static electricity. It detects the mains hum on a 240 Volt (or 110V) wall socket or cable without having to touch the object. It is enormously sensitive to any changes of the EM field surrounding it so it could be used to detect lightning (proof is in the video below) or maybe even ghosts. (No video proof of that alas! At least not yet  ^__^ ). Please note: this circuit can not be used as a metal detector.

Here is the circuit (click on image to display full screen):

(Last revised: 02-june-2020: Changed 1M potmeter for 20 to 50K potmeter.)

Parts list:

Transistors:
8 x BC547 

Resistors:
1 x 680 Ohm 
4 x 470 Ohm
1 x 220 Ohm 
1 x 4K7 
1 x 3K3 
2 x 2K2 
1 x 100K
1 x 1M 

Potmeter:
1 x 20K or 50K potmeter (use either a trimpotmeter or a panel potmeter if you're building this into a case.) 

LEDs (3mm):
3 x green, 1 x yellow, 1 x red

Diode:
1 x 1N4148

Miscellaneous:
9V battery clip, 1 switch (SPDT Toggle Switch ON-ON), 1 Bullet conncector for antenna. (optional)

Before I go on with the rest of the explanation, here's a video showing this EMF Detector in action in a lightning storm. In the background audio you can hear the crackle of the lightning on an AM radio I had switched on, and you can see that the meter lights up as the radio crackles and lightning occurs. Sometimes it even detects the build up of the electric field in the air before lightning happens. I'm not influencing the meter in any way. I'm just holding it by the 9 volt battery underneath. Here it is:

I designed this because I always found it a shame that these "everything detectors" or 8 Million times amplifiers never had a strength indicator so you could actually see if and how it's working. So I tried combining two pre-existing circuits and see if I could make them work together and it turned out to work very well. The first of these circuits is this 4 LED signal strength indicator

and the second is this circuit which is the actual detector stage, consisting of the 8 million times amplifier.

You can easily build this on a piece of stripboard.
The circuit needs only 8 transistors (BC547 or 2N3904), 5 LEDs and 11 resistors. The extra (5th) LED is there simply to function as a on/off indicator and could be left out if you so wish. I used 3mm LEDs on this project but 5mm will work too. Don't use LEDs that draw a lot of current though like bright white LEDs or blue LEDs. The circuit is fed from a normal 9 Volt battery.

The sensitivity of the circuit can be changed with the 20K or 50K potmeter. If you're using it like me, without a case, you can use a trim-potmeter. If you're building this into a little case then use a panel potmeter for sensitivity. Make sure there's a grounding point when you build it into a case. Some connector from where you can ground it.

The circuit is very sensitive and it reacts to all sorts of things. If you hold this EMF Detector  near any mains cables it will instantly detect the voltage, I noticed that if you hold it near metal it will detect that too and even in an open space it will sometimes indicate a field even if there's nothing visible there but it's not a malfunction because it will keep indicating on the same spot in the room. 

This meter works best if it is grounded properly, either by connecting minus to a metal case in which you build the meter and then holding it in your hand  or by  grounding it to some metal item (do NOT connect to ground of mains power supply!!!)
Here are some pictures of the detector I build:

Enjoy building this awesome little "everything detector" ^____^ oh and hey, while you're here, please leave a comment! That'll be cool! :-)

BAOFENG BF-F9V2+ Impressions.

This is just a little overview of my experience with the Baofeng BF-F9V2+ hand transmitter.

[Please keep in mind this review was written in 2018. I don't think this particular model is still available.]

I ordered mine from eBay and even though they're Chinese made transmitters, they all come from the United States. The are sold by Foscam or Baofengradios.us
Beware that since they come from the States, they come with US chargers that don't fit in European or UK wall sockets! (As I am situated in The Netherlands I just soldered on a European wall plug and covered the connections in hot glue and then with electrical tape. Works like a charm and it's safe too.)
I did a check on May the 12th 2019 and couldn't find a listing of these Hand Transmitters anywhere. It seems the BF-F9V2+ models are out of stock everywhere. It seems the new kid on the block is the Baofeng BF-UVS9+. This is the new model for 2019 but it doesn't seem to offer anything more than the older model except a higher capacity battery (3800 mAh instead of the 1800 mAh of the F9V2+), and a newly designed case which does look very cool and comes in 3 colours, black, silver and red.
It's also an 8 Watt model. They call it Tri-Band but they include in that the FM broadcast band on which you of course can not transmit. There are however also models that include the 200MHz range and are true tri-band hand transmitters.

Here's a picture of my BF-F9V2+:

Build Quality and Range:
This is a 3rd generation version of the well known UV-5R series of HT's and the exterior looks exactly like the UV-5RV2+ but it's most distinguishing feature is that it has the 8 Watt RF power option (although it doesn't really do 8 Watt, more on that later). I am familiar with FM transmitters and I know that in order to double your range with any transmitter, the general rule is that you need a 10 fold increase in output power. So I didn't think the extra power would have much influence on the range of this HT but it did have an effect. Not so much a big increase in range but an increase in penetrating power. The BF-F9V2+ has very good penetrating force. I must remark here that the first thing I did when I got this set is remove the original antenna and replace it with an original Nagoya NA-701. That is important because all the consequent testing is done with that antenna! Well, the signal goes through buildings and what have you like a hot knife through butter. For instance, I was underneath an overpass, a kilometer away from my house where the receiver stood (I used a UV 5RE+ with a Nagoya NA-701 antenna as receiver because at that time I didn't have a second BF-F9V2+ and I used a digital voice recorder to record the received signals.) I was inside my car with all windows closed and underneath that particular concrete overpass you're literally below ground level, and still the signal came in crystal clear. I was impressed I can tell you. That would not have worked had I used a normal UV 5R. An other thing I noticed is that the BF-F9V2+ has a better sound quality out of the build-in speaker than the UV-5RE plus. The BF has better low frequency sound. The UV sounds very tinny, with much more high frequency elements in the sound. The BF uses a new chip set with noise reduction features that block out noises caused by signal intensity changes and it also has a tail tone elimination feature.
The unit feels very solid. It's made from industrial plastic and has an aluminium frame inside. The loudspeaker is protected by a metal gauze over which they mounted a black anodized aluminium protection plate. This has a bevel in it which gives a shiny effect when the light hits it. It looks very cool. Also the lettering on the front around the LCD screen has a holographic effect which gives it all the colours of the rainbow if you hold it at different angles. On the back there is a spring loaded belt-clip which is made from plastic but feels very sturdy. It's the same one as on the UV-5R. I think you'd have to misuse it a lot in order to break it. The LCD display is protected by a hard plastic window so you can't touch the LCD screen itself whereas the UV-5R just has the display with the metal case around it. The UV-5R LCD-display is soft plastic and scratches easily in itself, but because of the raised metal edges it hardly ever scratches with normal use. The BF-F9V2+ has a hard plastic window over the display which protects the display but this plastic window is not protected by any metal so it will scratch in normal use. So that's a bit of a paradox. The protective hard plastic window should protect the display underneath from scratching (which it does) but because it is right at the surface of the unit it will scratch over time because things will rub against it. None of this is really important though in my opinion. They are meant to be used and hold up in daily use and they do. All the Baofeng units are very sturdy and excellent value for money.
One tiny thing on the front panel is different to the UV-5R; it's missing the 'Band' button. Leaving that out was a wise decision by the designers, because I don't think anyone ever uses that button. It's not needed because the band is selected automatically when you enter the frequency.
The unit comes in a nice brown box with a 74 page manual for which you'll need glasses with enormous magnifying power to be able to read it. The letters are 0.9 mm high. (Yes I actually measured them ^__^) It is however a very useful manual. It tells you everything you need to know, including how to program it. If you're sensible though, you should get a programming cable from eBay so you can use "Chirp" to program it. For those of you who don't know Chirp. It's free software you can download from the net and which makes programming this unit a breeze.
You can download CHIRP directly by clicking here.

The frequency range is as follows:

65 to 108 MHz (FM Broadcast band, receive only)

136-174 MHz (VHF) I found that it still works as low as 131 MHz and as high as 177 MHz.  You can transmit and receive from 131 to 177 MHz without problems. You can type in 130 MHz on the BF-F9V2+ and it will transmit but it only transmits weird beeping sounds. You can see it's not designed to transmit on frequencies as low as that, but the software does allow you to type it in. If you type in frequencies between 178 and 179 MHz and press the PTT button it just goes on transmitting even if you let go of the PTT button. You need to switch the unit off in order to stop transmission. It works fine up to 177 MHz though.

400-519.990 MHz (UHF) The UHF range is listed as 400 to 480 but goes up to 519.990 without problems. However, I tested the signal on my oscilloscope and it looks more like a modulated AM signal than an FM signal so I wouldn't use these frequencies if I were you. You can type in 520.000 MHz but again the transmission will go on even if you let go of the PTT button and you need to switch off to stop it. It's not designed to go that high. Best keep within the advertised ranges at all the bands.

The 5 bar signal indicator does actually work. It doesn't just go on or off if you receive a signal. It actually shows more bars as the signal gets stronger. This does not work if you press the Monitor Button. Then you just get all 5 bars at once. So there's a working signal strength indicator on all Baofeng HT's  (not just this model) but it's so small you can hardly read it. Oh well. =)

FUN FACT:

Did you know that the ON/OFF and Volume knob can actually be used as a tool to tighten or remove the fastening ring on the antenna connector? If you pull the knob off the radio you'll see 4 little stubs in there that fit inside 4 little notches in the screw ring that fastens the antenna connector. So you can use it to remove or fasten that ring should it come loose.

RF Power:
I measured the output power on a Diamond SX-600 SWR/Power meter with a 50 Ohm Dummy-Load attached as antenna and I measured an output power of 6 Watts on the 2 meter band (VHF) and 4 Watts on the 70cm band (UHF). That is less than advertised but it is still 2 Watts more compared to the UV-5R on both bands which I measured at 4 Watt and 2 Watt respectively.

The BF-F9V2+ with 2 UV-5RE+'s behind it. Note the lettering which changes colour according to the angle at which you hold it. It's the same in outward appearance as the UV-5RV2+.

BF-F9V2+ with the Nagoya NA-701 antenna. This is the antenna that I find the most useful for these HT's. Although the Nagoya NA-771 is a bit better it is also 38 Centimeters long (15 Inches). The NA-701 is 20 cm (8 Inches) long, which is much more practical. The original Ducky antenna is just 14,5 cm (5.7 Inches) and is pretty much useless. Get rid of it as fast as possible.

The inside is Aluminium and the battery is in a sturdy industrial-plastic casing.

Here is the complete manual in PDF form: click here

EDIT: In the recently started conflict between Russia and the Ukraine, I saw a Ukrainian soldier on a news item with a Baofeng UV5-R. It seems they are ideal for communication in close quarter fighting because of their long range, clear sound and long battery life.

That's is it for now, thank you for reading this. I hope you found it useful and if you did, I would love for you to leave a comment. Let me know your thoughts on the Baofeng products!
See you later!

LED Oscilloscope with 100 LEDs.

Hello everyone,

One word before we start: Don't build this project if you're in need of an oscilloscope for measurements or checking waveforms. This scope has no trigger and therefor no stable waveform unless you exactly match the frequency of the waveform with that of the timebase. Furthermore, 10 by 10 LEDs is way too low a resolution to check waveforms with. If you need an oscilloscope for audio waves you can start with a cheap one from eBay for $20. (Buy one with acrylic case!)
This project is just a fun thing to build with LEDs, Something that actually visualizes audio in a small way. And that's all it is; just a bit of fun.

Okay, with that said, here we go:

Following on from my 81 LED chaser with 2 NE555s I now set out to build a LED oscilloscope using the same type of LED matrix I used in the last project, only this one has 100 LEDs instead of 81.  First of all I'll show you the circuit schematics I used for this project. You can easily find this on Google and it's a very simple design. Actually easier to build than the 81 LED chaser.

I made some changes to the way the NE555 was configured. To test the schematics I build this pulse generator on a breadboard and took some measurements with my oscilloscope and the pulses that came off only had a duty cycle of 6%. Maybe that was meant to be and actually works better, but I changed it to a design that gave a 50% duty cycle. I wanted to be able to extend the range of frequencies by adding the possibility of switching between capacitors on the 555 and I wasn't sure how this short duty cycle worked on the higher frequencies I intended to put in, and I also didn't have a 500K potmeter. I only had a 100K so I needed a design that gave me a good frequency range with a 100K potmeter. Here's what I came up with:

When set to the highest capacitor value (330 nF) this gives a range of 17,5 Hz to 6,2 kHz. Then, by choosing the lowest value capacitor, it goes up to about 650 kHz. That's a nice range for a timeline I thought. The ranges overlap a lot and you only really need the first and last setting but I liked having some choise and it adds yet an other switch to the front panel which always looks cool :)
After I had build this circuit I came to the conclusion that the higher frequencies for the timeline don't look good at all because this scope doesn't have a trigger-mode. So with high speed signals it just looks asif the LEDs are on all the time. So you don't have to bother with the alteration to the NE555 and just keep to the original schematic. I just thought I'd include it in this article in case you had the same idea ;)
Do not forget to put in the 470µF electrolytic capacitor (even if the circuit is fed from a 9 volt battery). This prevents oscillations on the positive voltage rail caused by the NE555. (I had the same problem with the 81 LED chaser. This is a well known issue with the normal NE555 chips but if you use a LM7555 cmos version of the 555, this problem won't occur). The capacitor makes sure you get a nice ripple free supply voltage, which needs to be 9 Volts btw. I also put a Schottky diode in the positive voltage rail to prevent damage from accidental polarity reversals. This circuit draws between 24 and 34 milli-amperes (depending on the frequency of the timeline) so it can easily be fed from a 9 volt battery.

I proceeded to build the LED matrix first and I wanted to make a better job of it than I had done with the LED chaser. So I again had to grind down 100 LEDs on 4 sides to make them fit tight together on the perforated circuitboard. I had to glue on an extra bit of circuitboard because I could only fit 9 rows of LEDs on there and I needed room for ten rows. After I had soldered them all in place I took my Dremel tool and shortened all the negative leads so I could fit the positive rails over the negative rails without them touching and creating a short circuit. Here's a picture of the backside of the LED matrix which came out very well. (You can see the extra strip of circuitboard I glued on at the left side):

After that I proceeded to solder together the rest of the electronics which was quite straight forward really. I did end up with a mess of wires and knobs etc. but that was unavoidable. But it was going to be build into a nice case anyway. Here's the finished product mounted in its case but still very accessible because only the display is glued into place so changes and repairs can easily be made.

Here's a closer look at the switch with the different capacitors on it, to change the frequency range:

And here it is in full working order:

Problem solving:
I did encounter a little problem after I had assembled the scope in its case. I had made a BNC connector on the front panel to attach a probe to but it turned out that the ground wire caused the 3rd column of LEDs to turn off so I cut the ground wire for now. I need to mount the BNC connector in such a way that it is insulated from the case completely.
I also build in the microphone with the little amplifier which you can see in the schematic on the lower right. This works very well and I put in a switch to choose between the microphone or the probe. But I wanted an amplifier that is a bit more powerful and has a volume control button that I can put on the front panel aswel. In the next paragraph I explain how I did that.

The Amplifier:
Like I mentioned above, I wanted to build a more powerful microphone amp with a volume control to put in this scope. Well, recently I did just that. It took me just over an hour to build it and put it in and it works very well. The mike is much more sensitive now and reacts even to random noise it picks up. I build it with a 2N3904 transistor as a pre-amp for the electret microphone and then a LM 386 to amplify the signal. Here's a little sketch of the circuit:

(Last revised: 08-Feb-2020 Changed 10µF cap on pin7 for 100nF.)

Here's a picture of the scope with the new volume control added to the front panel:

Here's a little video of the scope working (with a bit low battery) with the synthesizer I build. There are no knobs on the scope this time because I needed them all for the synthesizer, LOL :)  :

I can really recommend using this design over the microphone amp in the main circuit schematic. This one works much better.

Okay, that's it for this project. I hope you enjoyed this read and if you did please consider supporting me by subscribing to my YouTube channel EdEditz or by following this blog or clicking on the adds.

If anyone has any idea how to incorporate a trigger section into this scope I would love to hear from you!!
If you have any questions or remarks, feel free to post them beneath in the comments or on my YouTube channel. I always love to hear from you!!!

81 LED Chaser circuit using 2 NE555's!

Hi everyone!

This last week I've been busy making a little LED chaser circuit. I found a schematic online that used one NE555 to drive two CD4017 decade counter chips that drive the LED's. I thought I could improve on that by adding a second NE555 and it worked beautifully.

BTW, AliExpress now sell a kit of this design, only it has 9 x 9 LEDS, for about $5,-

I started out by building the LED display on a separate perforated circuitboard. I wanted to have the LEDs very close together to get a nice dense row of lights and the board I used was exactly the right size to fit a 9 by 9 LED matrix. I used 5mm LEDs because I have about a thousand of those in my junk box, salvaged from an old display unit. They had short leads but long enough to still use. I used a green perforated circuit board but to fit them on next to eachother I had to trim each LED on 4 sides with a Dremel tool because these LEDs have a broad rim at the bottom. Anyway, I managed to fit them all on the board in a 9x9 matrix. Then I soldered all the Cathodes together, row by row. Then the same for the Anodes, to give me an X and Y axis to work with.

You can use other types of LEDs if you want and choose a resistor value for the collectors of the transistors that works best with your preferred LEDs. Test the LED/Resistor combination on a breadboard and choose a value that makes the LED shine at normal brightness.

After that I started soldering the actual circuit.
Now, the original schematic, that you can find on the internet, only uses one NE555 to drive the movement of the LED lights and so you can really only change the speed of the X-axis of the display, or the Y-axis according to the schematic below, but I soldered the display in such a way that the initial movement was horizontal. So I thought why not add an extra NE555 and make the Y-axis adjustable aswell so you can get much more variation in the patterns displayed. So that's what I did. I made a new schematic and here it is: (Btw, the collectors of the vertical row of transisitors are all connected to the + of the power rail, just like the top transistor. It's not shown in the schematic because that would make the drawing very messy.)
(Click on the images to see them in full scale and right click to download them.)

Btw, instead of using the BC547 transistors you could also use the 2N3904 but in that case you need to change the 220 Ohm resistors for 100 Ohm resistors, but my advise would be to test the LED/resistor combination on a breadboard and see which resistor value works best and makes the LED shine at a normal brightness. You could also try 2N2222 transistors). The 10K's at the Base of the transistors should always be the same to protect the Base input.

OSCILLATION PROBLEMS (solved ^__^):

After having soldered on the first NE555 squarewave generator, I tested the output signal with my oscilloscope and I found that there were bursts of pulses with a frequency of about 60kHz on the output squarewave. This is a common problem of the NE555 which does not occur with the CMOS version ICM7555.
Here's a screenshot of the Squarewave from the NE555 with the pulses on top:

I added a big electrolytic capacitor of 470 µF to the output of the voltage regulator and that solved the problem. I proceeded to solder in the rest of the components. The 10K resistors for the base of the transistors I stuck into the same hole as the base, to save space. I had a bunch of cheap resistors from China which had very thin leads so they just fitted into the hole together with the base of the BC547s. That way I only needed 4 holes per transistor resistor combination.
I tested the circuit a few times during assembly to make sure everything worked because once it was all put together it would be very difficult to trouble shoot this thing with all the wires going everywhere. Luckily it all worked as I had imagined, especially the second NE555. It worked just as I thought it would do. I had a problem though with the two 100K potmeters. The ones I used were old ones from a valve radio and they turned out not to be up to the job. I didn't have more 100K pots but luckily I did have two 50K stereo potmeters, so I soldered the wires on, in such a way that the double 50K was in series and formed one 100K potentiometer and that worked very well. It's important that the potmeter goes all the way down to zero Ohm to get the fast movement of the LEDs and the ones I used did that very well.
Btw, if you decide to build this and want the display to appear as I have it in my video, with the lights going from left to right working their way down, you'll need to experiment with how you solder the wires from the transistors to the display. The way it's drawn in the schematic the light would go from top to bottom instead of left to right down the rows. Beware of that.

I made a video about this circuit which shows how it works with a little animation sequence, which you can watch here:

TESTING:

I did some measurements of the pulses and they are pretty messy to look at but they work just fine to trigger the CD4017s. I was surprised at the fast rise-time of the output pulses from the 4017s. They rise in about 12 nano seconds! Here are some screenshots from the scope:

This is the X-axis pulse going to the LEDs:

This is the Y-axis pulse going to the LEDs:

Here's a closeup of the rising edge of the output pulses showing how fast they rise. You could build a Time Domain Reflectometer with pulses this fast:

So now that I had it all working, I decided to round off this project by building the whole thing into a nice case. I found an old sewing tin which had just the right size. I spray painted it black and with a Dremel tool I made holes for the display and the knobs. Then using hot glue I glued in the display board. I left the board with the actual electronics on it floating. I didn't glue it down. All the wires connecting it to the display were enough to keep it in place and I needed the lid to be removable to make it possible to exchange the battery. I used a 9 volt battery which I kept in place with a strip of copper, bent to fit around the battery and glued to the bottom of the case. I lined the inside of the case with gaffer tape to prevent accidental short circuits should the print touch the case. I had just received a batch of knobs from China that looked a lot like the knobs on a Mini-Moog synthesizer and I put those on the potmeters.
After it was all assembled it looked like this:

This is the inside of the case:

(The hot glue underneath was meant to protect the wiring when I was testing the circuit.)

Doesn't it look cool?? Of course it doesn't do anything useful, but it's so much fun to play around with and also to build. Actually, you could use it as a game: try and make a diagonal line appear that doesn't move across the screen. It's possible but requires a very delicate touch on the controls. Perfect to while away those busy office hours, lol! And you could use it as a prop for a movie. Say like an artificial scanner of some sort, for tracking down ghosts  ^___^

Okay that concludes this blog post. I hope you enjoyed it and if you did please leave a comment, either here on on the YouTube video. If you want to support my channel you can do so by subscribing. That would really help me out and it costs you nothing :) Win/win situation!  :) But please leave a comment. I always love to hear from you!!
I've also recently opened a Patreon Page through which you can support my work. I don't have any rewards set up as of yet but I will in the very near future. Here's the link:
https://www.patreon.com/EdEditz

Simple but effective Transistor Curve Tracer circuit.

This curve-tracer uses only 6 transistors and produces a beautiful curve display on an oscilloscope in X-Y mode. And it doesn't even matter which transistors you use to build it with!! I build it up on a bread-board first and it works very well! The component that is being tested does not get hot, unlike some Chinese Curve Tracers. 

I was impressed by the simplicity and effectiveness of this little circuit and since I didn't see it available on the internet I thought I'd share it with the world.
Here is the circuit:

(At the bottom of this article there's an improved version of this design.)

This circuit can handle all sorts of components, NPN transistors (bi-polar junction transistors), diodes etc. However it does not work with FET transistors, like Jfet's, P or N channel mosfets etc. You can build a PNP version of this circuit. Instructions for that are at the bottom of this article. As you can see, very simple design.  

The traces are displayed from right to left which is the wrong way around for a normal graph display but you can change that by inverting the X channel on your scope. However most analog scopes won't allow you to invert the X-channel, usually only the Y-channel, so you'll just have to get used to that. That's what you get with such a simple design. (The graph for the PNP version requires you to invert the Y-channel (channel-2) on your scope, so that will look normal on almost any scope.)
TUT is the Transistor Under Test, TUN stands for Transistor Universal NPN, TUP is Transistor Universal PNP and DUG stands for Diode Universal Germanium. I used a AA119 for the diode, but I would advise you to use a Schottky Diode instead. I tried it with a 1N5819 and that works just as well as a Germanium diode. 

For the NPN's I used the 2N3904's (I also tried the BC547 and it works just as well) and for the TUP I used a BC559. I set it all up on a bread-board and it worked like a charm.
The circuit diagram advises a power supply voltage of 6 Volt but I use it with 10 Volt and I feel that works better. You can turn the voltage up to 15 Volt without any problems. I haven't tried higher voltages then 15V but I think the voltage is only limited by the transistors you use in the circuit and a higher supply voltage gives you more data about the transistor under test. As to power consumption: it hardly uses any power at all. At 6 Volt it draws a current of 4.8 mAmps and at 15 Volt 11.8 mAmps. That's equivalent to 0.177 Watts.

A reader of mine kindly sent in a link to a Falstad animation that shows how the step generator actually works which is very interesting to see. Here's the link so you can have a look for yourself:
- Click here for circuit animation -

The number of traces that appear on the screen is determined by the ratio between C5 and C4 and can be varied by changing the value of C4. A higher value will take longer to discharge creating more steps inbetween fully charged and fully discharged. Don't change C5 because that will change the length of the traces. Using a 2N3904 as test transistor, with a 68nF capacitor for C4 I got 5 traces on the screen and with an 82nF I got 6 traces. It also changes if you vary the power supply voltage. Depending on the type of transistor under test you can loose a trace if you turn the voltage up above a certain value. For instance with the AC187 under test, the display goes from 5 to 4 traces if the voltage goes above 11.4 Volt. Btw, should you ever short circuit this thing and it doesn't work any more, your best bet is to replace T4 and T5, They are the first components to go. Believe me, I speak from experience ^____^
One more thing: make sure the ground of the oscilloscope is NOT connected to the ground of the powersupply. Otherwise this circuit won't work, you'll short out the transistor under test.

Here are some tips on how to set your digital scope (in my case the the Rigol DS1054Z) for best possible display of the curves:

1. Set your scope into X-Y Mode.
2. Put your probes on DC coupling so you can measure voltages with the cursors of your scope.
3. Set the bandwidth limit for both probes to 20MHz
4. Invert Channel 1 probe.
5. Set the horizontal timeline to 2.00mSec/Div
6. Set both probes to 1x multiplication (and don't forget to set it in the scope to 1x aswell)
    1x will give you sharper lines but if you want to do measurements it's better to put the probes at 10x to reduce the influence of the scopes impedance on the resistance in the circuit.
7. Set channel 1 (X) to 1 Volt/Division
8. Set channel 2 (Y) to 500 mVolt/Division
9. Go into the 'Acquire' menu and set the Memory Depth to 60K

This gives a good starting point from which you can fine-tune your scope depending on the type of transistor you are testing.

With these settings you should get a picture like this one:

This is what the graph from an NE555 based curve tracer looks like. You can see the lines are a tiny bit thicker although the difference is almost negligible.

You can also measure diodes with this circuit. Just put them between the emitter and collector of the TUT (Transistor Under Test) points and you'll get the characteristic diode curve. The anode must be connected to the collector and cathode to the emitter clip.

Here is a picture of the PCB layout including the parts list and a picture of how the curve appears on an analog scope:

I've posted a video about this circuit on my YouTube channel which you can watch below. The video description contains a link to the original article, written in Dutch, of which I made a PDF file which you can download for free. That link is also posted at the bottom of this article. I recently found an English language version of this same article on Archive.org and that link is also at the bottom of this article. If you plan on making this circuitboard then beware of the transistor polarity! The layout used in this article was designed for transistors like the BC547 with Collector on the left and Emitter on the right so if you plan on using the 2N3904 the Collector and Emitter will be reversed so you must put those in the other way around.

In the video the traces are displayed pointing the wrong way, namely to the left. This is because at the time of filming I hadn't figured out yet that you could invert channel 1 to get it pointing in the right direction.

This circuit makes your oscilloscope an even more useful instrument than it already is because it allows you to easily match transistors together, which is sometimes necessary if you're building a high quality audio amplifier or a precision oscillator or for high accuracy current mirrors or if you (like me) are building your own synthesizer and need matched pairs of transistors for the filters (like the famous Moog Ladder Filter).
In the video I made the remark 'it's not that accurate of course because it's a very simple design' but I was thinking about that and that is actually a bit nonsensical because the oscilloscope doesn't lie. In fact this circuit is as accurate as your scope is and using the cursors you can make some very accurate measurements and calculate all sorts of parameters from these traces.

I've done some more experimenting and I've designed my own PCB with my own design layout. The new circuitboard has a lot of surface area for mounting clips to put the transistor under test in. I simply made some small coils out of copper wire and soldered them to the circuitboard. It works very well. I made the PCB so that it has a big ground plane, to reduce noise and it seems to have done the trick because I get nice thin/sharp lines when the curves are displayed on the scope. The breadboard versions always had thicker lines, at least on a digital scope. Recently I've also soldered on some short wires with alligator clips so I can easily measure big power transistors like the 2N3055 in TO-3 housing.
Here are some pictures of the new PCB. It doesn't look very professional because I simply draw the circuit layout straight onto the board with a permanent marker and then etch it. This is a double sided board:

Some screenshots showing the curves I get from this new circuitboard:
This is the curve from a 2N3904:

This is the curve from an AC187 Germanium Transistor. You can see that the back-traces are very prominent. That's also a drawback of the simple design. However it doesn't matter because all the information you can get from a graph like this is easily visible, plus you can dial it down a bit if you have a scope with an intensity graded display. The improved version at the bottom of this article eliminates those back traces

This is an AC176 Germanium transistor and at the top you can see the different wave-forms that make up the curves in the X-Y display. The yellow signal is from the X-axis (horizontal) and the blue from the Y-axis (vertical):

Below is the curve of a PNP transistor. The OC79 Germanium PNP transistor to be precise. So if you see a curve like this, you know your transistor is a PNP type and should be measured with the PNP version of this circuit. Notice how the wave-forms have changed. The blue Y-axis signal has changed from a square-wave to a Shark Fin wave.

This next curve is from a BC547.
Now we can do some calculations on this with the help of our cursors and determine the Collector current of the middle trace for instance. We select the 'Cursors' on our scope and set the first horizontal cursor on the middle trace and the second on the 0 Volt line. The readout of cursor AY says we are at 1.45 Volts. We know the collector resistor (R7 in the circuit) has a value of 330 Ohms so the current through that resistor and therefore also through the Collector of the transistor is 1.45/330=4.39 mA.

CALCULATING HFE:

If we now want to calculate the Hfe or Beta or amplification factor of this transistor, at this value, we need to know the Base current. The Base current is biased through resistor R8 which is 270K. I soldered some copper-wire to each end of R8 so I could connect a probe to it. That's the purple waveform in the picture below. The probe we use to measure the voltage drop over R8 has an impedance of 10MOhm so the total resistance of R8 will drop to 262.9K.

The image below shows the voltage drop over R8 which is 3.64 Volt. If we divide that by 262900 Ohm we get 13.8 µAmpere. Hfe is then Ice/Ibe = 0.00439/0.0000138=318.1 for the middle trace.

From these curves you can also get a rough indication of the collector/emitter impedance; the flatter the horizontal bit of the trace is, the higher the impedance.

PNP version:
I recently build a PNP version of this design and it's very easy to do. Here's a video I made about that:

Like I mention in the video you need to switch the NPN transistors with PNP's, and switch the PNP transistor C5 for an NPN, reverse the polarity of the diode (DUG) and of capacitor C6 and don't forget to switch the power supply connections! Then, on the oscilloscope, you need to invert channel 2 (the Y-channel) and that's all. If you design your own circuitboard for this you could make both versions on one circuitboard for ease of use. Just let your imagination run wild and I'm sure you could build a tester that is better than many of the Chinese products advertised on eBay. I later did the same. More on that further down the article.

Here is the revised circuit schematics for the PNP version:

Here's a JPEG image of the original article in English from the Elektor magazine. I made a Photoshop compilation of the article and turned it into a single Jpeg image with high enough resolution to zoom in and easily read the text. Simply click on the image to enlarge it and then right click on it and choose 'Save As...' Then you can zoom in on it.

AN IMPROVED VERSION OF THE TRANSISTOR CURVETRACER.

WATCH THESE TWO VIDEOS TO SEE HOW THE IMPROVEMENTS CAME TO BE.
Youtuber @Indepth electronics made two videos about this circuit that you absolutely have to see if you're interested in the working of this circuit. The first one is an in depth explanation of how the circuit works and the individual parts that it is built up from.  

The second video is even more interesting because here he explains how the circuit can be alterred to make it work even better and to how to get rid of the back-traces. This is a really brilliant analyses and if you're an electronics student you should really watch this video. 

Here's the schematic, described in the two videos above, of the NPN version of the improved curve tracer design by YouTuber @indepth_electronics. The probe connections are the same as with the simple original version.

With the PNP version it's simply a matter of changing everything to the opposite part. Plus becomes Minus. NPN becomes PNP. Diode polarity is switched.

Here are the settings I used on my digital scope, the Rigol DS1054Z in XY mode:

Chan 1 (X) to 2V

Chan 2 (Y) to 65mV

For the NPN version Channel 1 is inverted and for the PNP version channel 2 is inverted and you can switch the bandwidth filter on if the traces are too wide and noisy.

Set the sample rate to 60ks/sec

Make sure the ground connection of the oscilloscope is NOT connected to the ground of the circuit or powersupply because that will short out the transistor under test making measuring a bit difficult :)

Here's the bill of materials for the improved version:

Disregard the points beyond 20, that's just a summation of connections but you must not forget the eurorack powerconnector 10 pin male.

MAKE SURE THAT INSTEAD OF 4,7 OHM RESISTORS FOR R25 AND R26 YOU PUT IN 15 OR 18 OHM RESISTORS.

ALSO MAKE SURE TO CHANGE C5 AND C11 FROM 100nF TO 120nF (FOR 6 TRACES) OR 150nF (FOR 8 OR 9 TRACES IN THE DISPLAY.)

I have designed PCB's for this version in both NPN and PNP versions. I received them back on August 14th 2025 and I built one up and it works fine on a digital scope. However at first the signal was very noisy. I had an idea it was because of the low collector resistor so I changed the 4,7 Ohm resistors for 15 Ohm ones and that did the trick. Now it displays a beautiful and sharp Transistor Curve with 6 traces. With C5 and C11 at 100nF I got 4 traces but then I put in 120nF caps and now I get 6 traces. The PCBs I had made also have the Ian Fritz method transistor matching circuits on them for both NPN and PNP transistors. Sorry, but the boards are no longer for sale. But you can design your own. Just follow the KiCad tutorial on this website if you don't know how.

Here's the KiCad schematics for the PCB including transistor matching circuits:

Here's a new demo video I made showing the improved curve tracer on a digital oscilloscope:

OTHER MEDIA:

https://www.youtube.com/watch?v=44lk_BGHFXY

Youtuber 'The Tube Roaster' made a cool little video about the original circuit too. 

Click the link below to watch it:

https://www.youtube.com/watch?v=b2Qb6y-Ttkk

There's an other video made by Youtuber "W4DXZ Signal Shack". He demonstrates the original circuit on both an analog scope and a digital Siglent 1102CML+ oscilloscope. A very nice demonstration which you can watch here: 

https://www.youtube.com/watch?v=DEtsD_TqJiU

Feel free to make your own video if you wish and you can use any picture/video from this website. No problem. And if you do, please send me the link and I will share it in this article.

DOWNLOAD THE ORIGINAL ARTICLE (PDF IN DUTCH OR ENGLISH) HERE:

Transistor curvetracer article (Dutch)

Here's a new link from Archive.org to the original article in English from Elektor Magazine from September 1980:

https://archive.org/details/ElektorMagazine/Elektor%5Bnonlinear.ir%5D%201980-09/page/n37/mode/2up

That concludes this article. Hope you enjoyed it.

If you have questions about this circuit or see any mistakes in the text, please leave them in the comments. Leave a comment anyway please.

I see from my statistics that this article is a very popular one on my website and gets, on average, more than a 100 visitors every day so this curve tracer must have been build by many people. So if you want to do me a favour in return, make a little video about it and show it here in the comments. It would be so cool to see your home made curve tracers.:) You can put a link to a youtube video in your comment.

Beware comments are moderated and don't appear right after posting. I have to approve them first.