Synthesizer Extra's No. 01: SIMPLE AD/AR using the 7555

A fantastic little AD/AR envelope generator that is super easy to build and works very well.  Perfect little adition to the DIY synthesizer.

I was looking for a better AD/AR design than the one I had built and used upto now and I came across the Thomas Henry design. Here is the schematic.
This design uses the CMOS version of the NE555, the 7555 and it can be built on a piece of stripboard that is the same size as the LMNC version that I first used.

If you're new to all this; AD/AR stands for Attack Decay/Attack Release. It's a little Envelope Generator creating a control voltage that can open and close a Voltage Controlled Amplifier (or you can drive a filter with it. There are lots of options.)
In the picture below is the stripboard layout I made for it. I added all the features that Sam Battle has in his design, like the Arcade push-button with internal light and I added a little thing of my own, an option to double the output voltage of the Envelope from 0 to +5V to 0 to +10Vpp. I always find it handy to have a bit of extra charge on the envelope if I want to use it to control a filter for instance. You can of course connect a potmeter to the +10V output and so turn the output amplitude up or down from 0 to 10V. That way you can do away with the +5V output altogether. Since I already made and wired up the panel for this, I couldn't use the potmeter option. I simply exchanged the old stripboard for this new one and soldered all the wires back in place. You can also wire up the opamp in such a way that it inverts the envelope. That would be easy enough to do. As a final extra I put in buffer stages for the envelope output, for both the +5V and the +10V output. You could also wire one of those up to be an inverter. Lots of options here. One thing that is different from the LMNC version is that the Arcade Push Button won't work as long as a key is pressed down. The Gate signal has priority in this design.

The layout below is an early version and although it works fine, it is a bit messy. So I made a new layout which you can find a bit further down the article. I'm leaving the old one up in case someone who built it needs to reference it for troubleshooting or something.

SKIP THESE 2 LAYOUTS AND GO FURTHER DOWN TO THE UPDATED LAYOUTS TO BUILD THIS PROJECT.

Beware if you are using standard 24 x 56 holes stripboard, that the layout only goes from A to U not to X. So only 21 strips!

(Last revised: 1-March-2020: Changed attack and release pots from linear to logarithmic. 4-Oct.-2021: Cosmetic changes to layout.)

Here's a close-up of just the stripboard:

Bill of Materials:

Here's a look at how fast this little AD/AR is and it is super fast! It reacts to the Gate signal with practically no delay what so ever as you can see from these scope images. The risetime is about 12 µSec. The same as the risetime of the Gate signal (Gate = yellow, AD Out = blue). The gate signal has a bit of a skew in it half way up. That's due to some circuit specific stuff elsewhere in the synth but not really relevant because we are zoomed in so much it's practically instant. I mean, it's 12 millionth of a second in total:

UPDATED LAYOUT:

I made an updated version of the layouts above. I built it and changed the old one for this new one and everything works as it should so it's verified.  In the previous layout the output stages and 0 to 10V is a bit clumsy, although I guarantee that it works fine! The layout below is just a bit neater because I gained some knowledge over the past year and applied it here:

Beware if you are using standard 24 x 56 holes stripboard, that the layout only goes from A to U not to X. So only 21 strips instead of the usual 24!

SCHEMATIC:

Here is the schematic for this version. As you can see I added two buffers (which is a bit overkill but I wanted to use all the opamps available) and one opamp with a gain of x2 to get a 0 to +10V output. The buffers help to prevent this AD/AR from 'hanging' if you use it with inputs that have a bit of a low impedance (see text below). Both outputs are connected to a switch so you can choose between them. You could of course connect sockets to both outputs, instead of the switch and have two outputs permanently available, a 0 to +5V and a 0 to +10V. That's up to you.

The AD/AR works as follows: In AR or Gate mode, the Attack remains high for as long as you keep the key on the keyboard pressed down. After you let go the Release kicks in and the signal will fade out in the time you have set with the Release potmeter.
In AD or Trigger mode the Attack/Decay cycle still needs to have been completed before you can trigger it again but as soon as the Attack cycle has been completed the Decay kicks in, regardless of whether the key is still pressed down or not. For fast trigger sequences the Attack and Decay need to be set to short times because it won't trigger again until the cycle is completed, and that's perfectly normal.
So with the Attack a tiny bit open and Decay/Release fully closed you get a powerful envelope pulse of either 5V or 10V depending on how the switch is set.
In Gate mode you can have both Attack and Release fully closed to get fast short envelope pulses as the video below will demonstrate.
If you build this circuit with separate inputs for Trigger and Gate, and you feed it both at once, the Gate signal will take priority.

Here are some pictures of the stripboard using the new layout. I had made a mistake at first because I forgot this layout only had 21 strips instead of 24 so I made some cuts in the wrong place. That's why I placed the warnings with the layouts. And that's why there are some horizontal wirebridges in the lower ground strip (bottom black line).

12V vs 15V:

A little word on operating this from a dual 12V power supply. It will work but you'll need to change one resistor at the output. (R7 on the schematic). The 2K2 (R7) becomes a 3K3. This is necessary to give pin 6 on the 7555 the correct threshold voltage. I myself put in a 5K trimpot for R7 so I could experiment with the threshold voltage. It turned out that changing the resistance value of R7 mainly influenced the amplitude of the Envelope. In other words, you can set the initial envelope voltage with it. So after I learned this I took the trimmer potmeter back out and put in a 3K3 resistor.

The 'hanging' issue:
Because the resistor voltage divider at the original output influences how this AD/AR works I decided to add some extra buffer stages at the end, to stabilize the working of the circuit. I noticed that impedance differences, when connecting it to certain filters in my synth, can make the AD/AR hang sometimes. The release won't activate like it should, probably because the threshold voltage on pin 6 is disturbed somehow. I didn't want to rebuild the whole stripboard so I used a little piece of stripboard with just a single TL072 on it and buffered the +5V aswell as the +10V outputs. I stuck it onto the main board with hot-glue. It now works perfectly. No hanging or anything. I incorporated these buffers on the stripboard layout so they are now part of this design.

This design works a lot better for me than the LMNC one. This AD/AR reacts to trigger signals with an amplitude of +4 V and upwards and gate signals from +1.8 V and upwards with a maximum frequency of at least 60Hz. For triggering to work well, you need to open up the Attack a tiny little bit. The circuit is so fast that the envelope pulse shuts off before it has time to reach full potential. I tried different things to fix this little issue but I wasn't successful upto now. Anyway, it's nothing serious having to turn up the Attack a tiny little bit when using Trigger pulses. When you use Gate signals there's no problem.

I do strongly advise you use a logarithmic potmeter for the 1 M Attack potmeter. I used a normal linear one first but had trouble setting short attack times accurately. I've now put in a logarithmic one and it makes a world of difference. Works so much better. I really need to change the Release potmeter into a Logarithmic one too. That would make it much easier to dial in the Resonance or Cut-Off frequency when I use this to activate a filter. For the 4,7µF capacitor you can use a normal electrolythic capacitor. You don't need to use a Bi-polar capacitor in this circuit, unlike the LMNC one. You can put in extra electrolythic capacitors in parallel with the 4,7µF cap. to stretch the Attack time to the maximum length you want. I put in a 3,3µF and two 1µF caps for a total of 5,3µF which gives me almost 10 seconds maximum attack time. If you need longer Attack times just put in a 10­­µ­F cap.

Here are some technical specifications:
Minimum Attack time: 692 µSec
Minimum Decay time: 248 µSec
Maximum Attack time: 6 seconds with 4,7µF cap.  9 seconds with 5,3µF (which is what I installed)
Maximum Decay/Release time: ±30 sec.
Maximum input pulse frequency: ±60Hz

Here's a link to the Electro-Music Forum page that deals with this design:
http://electro-music.com/forum/topic-61297.html

Here's a little demo video of this AD/AR in action:

This second video shows one way of using the AD setting (trigger mode) of the AD/AR to control the cutt-off frequency of the ARP2600 filter. The Attack is fully closed so the instant a key is pressed the envelope voltage opens up the filter and then the Decay sets in and slowly closes the filter off as the envelope voltage fades down to zero. Watch the big blue light and listen to the effect on the sound.

The LED inside the Arcade push-button is connected to the +10V envelope output with a 4K7 resistor. It shines nice and bright. There's also a yellow LED on the panel between the input and the output. That one is connected to the output jack with a 1K resistor. It shines normally when you use +5 V out and extra bright when you use the +10 V output level. This is just a handy indication of how the output switch is set. It also reacts faster to pulses than the LED inside the push-button so it's a better indicator for that too. The LED was already built in so I thought I might aswel use it like this. :)
The Arcade push-button switch, which is the manual trigger, is fed with half the positive rail voltage (+7,5V) by means of the voltage devider formed by the two 68K resistors. I thought that was better than giving it the full whack of the +15V rail voltage. You can of course use other values for these as long as they are both the same. If you want to feed the switch with a different voltage then you can calculate that voltage as follows: Say R1 is the resistor coming from +15 V and R2 is the resistor going to ground. V = 15/(R1+R2)*R2
The arcade push button will not work as long as a Gate signal is present!!
Gate takes priority over manual trigger, just so you know that.

Okay, conclusion time: This design is a big improvement over the LMNC simple AD/AR and I can highly recommend using it. It works very well with patches where you feed it a fast trigger signal to control drum modules for instance. The switch which lets you choose between +5V or +10V output works perfectly fine but if you want more control just build it with the output controlled by a potmeter like I mentioned before. I do recommend you include the extra buffers at the end. They will insure that this AD/AR works perfectly under any condition. The only tiny little down point is that in Trigger mode the Attack needs to be a tiny bit opened to get a full envelope pulse. With Attack fully closed in Trigger mode, the pulse you get on the envelope output stops so fast that is doesn't have time to reach the full voltage potential. You could say it's too fast for its own good. You can see this happening on the oscilloscope. You get really fast pulses that don't reach the full voltage before they're cut off again. In Gate mode you won't have this issue and it works just perfectly. I really like this design and I highly recommend building it.

Here's a picture of how I added the buffer stages by glueing on a little print with a single TL072. This saved me from having to rebuild the whole thing.

Finally, for my own record keeping purposes, here's two pictures of how the finished synthesizer now looks, with two new VCO's and the Envelope Follower and the little oscilloscope of course:

Okay that's it for this article.
This article isn't really part of the synthesizer build itself so I named it 'Synthesizer Extra's'.  That's the header I will use for articles describing enhancements and changes to the original synthesizer that I build in the past 19 articles.
If you have any questions please leave them in the comments or post them on the special Facebook Group for this website. Okay, see you on the next one.

Synthesizer Build part-17: MIXER and PASSIVE ATTENUATOR in one.

It's a mixer and a passive attenuator in one with added Clipping Indicator LED and a Gain potmeter. Very useful module for the DIY Synthesizer.

This is a very simple project to build yourself and it will be a module you will use a lot in your synthesizer. I built two of them so far and they are in constant use. You can use this mixer for both Control Voltages (CV) and audio signals. 

I wanted to have a mixer in my synth but also a passive attenuator that I could use for signals that have no level control. So I decided to put both functions into one panel. I later added a clipping indicator and a potmeter to add more Gain to the output signal. Then I streamlined the layout and made it much more compact so if you want the latest version then scroll down and check the last layout in this article.

Passive attenuator is nothing more than a fancy word for a volume knob. It's just a potmeter inbetween the in- and output of the signal. It's called 'passive' because it doesn't require any power source. But by flipping the switch from passive to mix, the signal is now split in two and also lead into the mixer and becomes part of the signal coming out of the 'Mix' output while still being available at the original output too. This can also be useful for side-chaining for instance if you lead the original signal into the Envelope Follower and then use that to trigger the Lopass Gate. Just a thought :)

I used the super simple mixer circuit that Sam from LookMumNoComputer also uses and I added toggle switches for the signal to be added to the mixer.

LAYOUTS:

There are four different variations on the mixer theme represented in this article:

- Right beneath here we have just the mixer/attenuator stripboard.

- Below that we have just the clipping indicator stripboard.

- Below that is a spread out version of the mixer with clipping indicator which will work for the Kosmo sized modules.

- And finally we have a Eurorack sized stripboard project which includes the clipping indicator but is only 24 strips by 26 holes, so very compact. That's the version I would advise you to build. All the previous ones are older projects, made very early on in my synthesizer building career.

PROJECT 1: THE MIXER/PASSIVE ATTENUATOR.

Here's the first stripboard layout that I made of just the complete Mixer/Attenuator. All potmeters viewed from the front.  You can easily add a passive output to the first channel too, like with the other three channels, by adding a switch and extra socket, but I didn't do that to save space on my panel. The switches also function as Mute switches for the mixer. A very versatile and useful circuit.  

(There's a layout of the mixer with the clipping indicator, further down the article):

(Last revised: 05-Feb.-2020)

Stripboard only:

Here's the schematic for this module:

You can use other dual opamp chips for the mixer. It doesn't have to be TL072. As long as the pinout is the same (which it almost always is) and make sure they are not fakes, otherwise it won't mix the audio correctly.

Like it says in the schematic, it doesn't really matter what value potmeters you use as attenuators but it's probably advisable not to go above 1M Ohm and not below 47K. In the panel wiring diagram, on the drawing, you can see how I combined the mixer with the passive attenuator function by simply adding switches that lead the output signal coming from the wiper of the potentiometer into the mixer. Only the top output jack carries the mixed signal out, and the other three always carry an attenuated version of their respective input signal. That line is never interrupted. Those other three outputs are all for passive attenuation. So you could get a mixed signal out of the top output jack, and 1 to 3 original signals on the other outputs. You can also split a signal into two parts by putting it on, for instance, input 2 and sending it into the mixer. Then you can tap the signal from the mixer output and from the output of channel 2. (Of course no other signal must go into the mixer otherwise the signal at the mix output will contain the mix of the multiple inputs but you can use the other passive attenuators at the same time.) 

A very good idea was posted in the comments below and that is to use the socket switches of the attenuator outputs instead of using toggle switches. If no cable is attached to the output the signal will go into the mixer and if you attach a cable the signal to the mixer will be cut and you just have attenuation. The downside of this approach is of course that it is not possible to split the signal in two by having it go into the mixer and still be available at one of the other outputs but it will save space on your panel not having to include toggle switches. It's up to you as to how you want to use this mixer.

So a very versatile mixer design! The mixer takes + and - 12 Volt but will work just as well on +/- 15 Volt. 

Because it's a tiny little circuitboard and weighs next to nothing, I glued the circuit board straight to the back of two of the potmeters in the panel with hot glue. Works great! :)
You can add as many channels to this mixer as you like by simply adding more potmeters with input jacks and 100K resistors to the input of the opamp. You can even add an inverted output by tapping off the signal from pin 1 of the chip (I've marked the place in the schematic on the drawing and the layout) with a 1K resistor going to an output jack. I didn't include that in my build because inversion doesn't do much for audio signals and my LFO already has an inversion option so there's really no need for it. There is also the possibility of adding a 'Gain Control' potmeter by replacing the 100K resistor over pins 1 and 2 for a 500K potmeter in series with a 50K resistor. It is drawn in the schematic in dotted lines. This will give a gain of x 0.5 to x 5.5
The inputs of the 4 channels can never be shorted out because they are connected to pin 3 of the potmeters so the input impedance stays the same as the value of the potmeters you used. If one or more of the potmeters switched to the mixer is turned to zero, there still is a 100K resistor in series with the wiper(s) so there's never a short circuit possible. You won't even hear the slightest drop in volume, this is a very simple but very good working mixer. Like I mentioned earlier, I've built two of them so far and they are used all the time!

Sam Battle of LookMumNoComputer fame has a similar setup with the switches choosing between 'Mix' and 'Individual Output' on his quad VCA module, which you can see in this video (5m33 into the video). I noticed this only recently when I watched the linked video and when he talked about the VCAs it reminded me of this mixer.

You might wonder why the input signal is connected to the negative (inverting) input of the opamp. Why not just on the positive (non-inverting) input and do away with the second opamp? Well that's because the opamp's summing function only works on the negative (inverting) input. The way it's shown in the schematic is the right (and only) way to do it.

You can also ad a 'Mute' function by simply putting an extra single pole double throw switch into the mixer output. Some people find that a handy function to have but it's not included in this project.

Here's a picture of the finished panel with the blue clipping indicator LED.

PROJECT 2: THE AUDIO CLIPPING INDICATOR ADDITION.
When I first made this mixer, the blue LED was just there so I knew the mixer had power but it served no real function other than that it looked cool. (I had actually drilled the 3mm hole by accident so I put a LED in it to fill it up.) But later I decided to give it a useful function and to use that LED as a clipping indicator. So I started looking for schematics of clipping detectors and I found some low resolution circuit images. I used one of those to draw my own schematic in a better readable higher resolution:

Here is a Falstad simulation of the clipping circuit:  --- CLICK HERE ---

I made this stripboard layout for it which is verified, I used this for my build:

I had already built the mixer previously so I built the clipping indicator on a separate piece of stripboard and glued it to the mixer board with hot-glue, using a spacer in between so the boards wouldn't touch each others copper strips. Since then I have made a new layout combining the mixer with the indicator which is of course much more convenient. The layout below is verified, I recently built a 2nd mixer using this layout and it works fine. 

You can use a variety of (dual) opamp chips for this circuit. The TL072, TL082, NE5532 or even a 4558 or an LM358 will work for both the clipping indicator and the mixer. It doesn't matter which you use where, they all work fine. As long as the dual opamps are suitable for audio circuits and are pin for pin compatible with the TL072 (see datasheets) and make sure they are not fakes from AliExpress or other dubious sources. Note the opamp in the clipping indicator circuit has it's minus pole (pin 4) connected to ground. It only needs a positive voltage source not a dual one. 

Btw, you can use other value potmeters for the attenuator/mixing pots. I used 1M but 100K or 50K will work too. I'd recommend not to go lower than 47K though.

All potmeters are viewed from the front with shaft facing you.

PROJECT 3: FOUR CHANNEL MIXER w. CLIPPING INDICATOR for KOSMO SIZE.
Wiring diagram:

Stripboard only:

(Last revised: 27-April-2020: Corrected mistake with 10K resistor to pin 6 of IC2, it was connected to ground when it should be connected to V+. 30-March-2021: Cosmetic changes to make layout clearer.)

ADDING A GAIN POTMETER:

Here's the layout for if you want to add a 'Gain' potmeter. With a 500K potmeter the gain will be between 0.5 and 5.5 times and with a 1M pot it'll be between 0.5 and 10.5 times. 500K is really enough because with gain set to above 3 times the audio will start to clip anyway but it's up to you.

Note the 100K resistor over pins 1 and 2 of the mixer IC (to the left) is now changed for a 47K in series with the Gain potmeter. The 47K resistor is there to make sure the feedback resistance can't go all the way down to zero. I've highlighted the potmeter connection in the square on the layout. The rest is the same as the previous layout (although this is a somewhat older version but it's all correct.)

PROJ. 4: MAKING IT SMALLER AND EURORACK FRIENDLY. (BUILD THIS VERSION)

Below is a new layout which I made in Dec. 2024 which puts the whole mixer with all the extra's on a much smaller piece of stripboard. It's only 24 strips by 26 holes wide. 

This is the version I advise you to build if you need a mixer. I added a Eurorack connector because there was room left to put one in. You can build this mixer in an afternoon. It took me two hours to build one; re-using the old panel of course. The layout is verified, I used it to update one of my mixers with this new design and it worked like a charm. A day later I also updated the second mixer in my synth and again it was done in two to three hours and it worked rightaway.

Here's the new layout:

(Last revised: 19 March 2025. Updated layout and removed jumpwire.)

If you don't want to include the gain potmeter then you can leave it out and also the 47K resistor. Instead you need to connect a 100K resistor between pins 1 and 2 of the mixer IC (IC-1).

Stripboard only:

Cuts and wirebridges seen from component side. As always, mark the cuts on the component side with a Sharpie or Edding pen and then stick a pin through the marked holes and mark them again on the copper side. Then cut the copper strips at the marked positions with a sharp hand held 6- or 7mm drill bit.

Bill of Materials:

The end result; a neat and tidy looking mixer module.

There is one other function you can add to this mixer and that is an Offset function or Bias control. This will shift the whole audio signal up or down in voltage without changing the dynamic range of the signal. In other words the audio won't change in amplitude or volume only a positive or negative offset voltage will be added to the signal to counteract any DC voltage coming in at the audio inputs..

This is very easy to do as you can see in the layout below. I just summed the voltage from the wiper of the Bias potmeter through a 100K resistor to pin 6 of IC-1 and the other potmeter lugs go to positive and negative voltage as shown in the layout below.

Calibrating the clipping circuit:

This clipping circuit works on anything from 9 to 15 Volt. It happily takes 10V peak-to-peak audio input signals (if powered from 12-15V). You can set the sensitivity, or the clipping threshold, with the 5K trimmer potmeter. I just fed it the normal signals from the two VCO's on channels 1 and 2 and slightly mistuned one VCO so you get that frequency beating effect where the two signals amplify eachother when they are in phase and subtract when they are in opposite phase. With the mixer-level pots turned to maximum I set the trimmer in such a way that the LED would just come on when the combined signals from the two VCO's would be at their highest amplitude. That way any signal louder than the VCO's will trigger the clipping light and you also get a visual indication of the 'frequency beating effect' because the LED will blink in time with this effect. So at the level I set it, the audio isn't actually clipping yet but the volume is louder than the normal 10Vpp. You can of course choose any threshold level you like and calibrate the circuit with whatever method you wish.

Here's the mixer panel with the newly designated clipping LED just under the top potmeter:

DISTORTION:

If you want to add some grit to this mixer in the form of variable distortion, then you can. It's a matter of adding two diodes and a potmeter. This video by Moritz Klein explains how you can do that and how it works.

Okay, that's an other one done. If you have any questions or remarks please put them in the comments below or post them in the special Facebook group for this website.

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.