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.

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.

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, although I can't imagine it making much difference You could even try 2N2222 transistors in which case you can do away with the resistors all together. They are just used as switches afterall.)

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:

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. You could replace the square-wave generator formed by T1 and T2 for a NE555 square-wave generator if you wish (beware of the input voltage if you do, so you don't blow up the 555 and make sure the duty cycle of the squarewave is 50%). I've tried it with a 555 but on a digital scope the lines are a little bit thicker than with a transistor based square-wave generator. The chip seems to be noisier. (However you won't have this problem if you use an analog scope.) But the plus point is that you can easily make the frequency adjustable with a 555. This gives you a bit more control over how the traces look. Mind you, I only tested it on a bread-board so if you build a print for it, it might yield a better result. 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 you can also use a Schottky Diode if you don't have a Germanium one. I tried it with a 1N5819 and that works just as well. 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. Don't change C5 because that will change the length of the traces. Using a 2N3904 as test transistor, with a 68nF capacitor 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 to go. Believe me, I speak from experience ^____^

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.

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 print:
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:

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.

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 exchange the NPN transistors for PNP's, and exchange 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 print 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.

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.

That concludes this article. Hope you enjoyed it.

You-tuber 'The Tube Roaster' made a cool little video about this circuit too. He's much better at explaining these things so I'll link to the video here, if you'd like to watch it:

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

There's an other, more recent, video made by YouTuber "W4DXZ Signal Shack". He demonstrates this circuit on both an analog scope and a digital Siglent 1102CML+ oscilloscope. A very good 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) HERE:

Transistor curvetracer article (Dutch)

Here's a new link to the original article from Elektor Magazine from September 1980:

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

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.
-- DON'T PUT ANY LINKS IN THE COMMENTS OR I WILL DELETE THEM.--

I see from my statistics that this is the most popular article 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.:)

CZE T251 FM Broadcast transmitter REVIEW

This is an overlook and general impressions review of the Chinese made CZE T251 FM Broadcast transmitter. I ordered mine in 2017 out of China from eBay seller 'Thanksbuyer-hobby' for $219,- including shipping. It took 11 days to arrive which is fast!

Update 1st of November 2020.  Before we start: I haven't been working with FM transmitters in the last 2 years but I had a look lately and it seems they have gone up in price quite a bit. I could also find only one listing of this particular transmitter on eBay. Just so you know, they are getting rarer and are probably being replaced by newer models but I can not give you any opinion on those. As I said I no longer occupy myself with transmitters. It's all synthesizers now =) Therefore the comments for this article are now also closed. The new GD-20155-ALL 15 Watt FM transmitter using the BH1414 chip looks like a good one to buy though.

Normal operation:
You switch on by pressing the on/off button at the back. Then you get either the standby screen or the transmit screen, depending on how the transmitter was set when it was last switched off. If you press the front dial for more than one second the transmitter goes into sleep mode and won't transmit. The button will light up red. A simple short press of the front dial button awakes it and powers it up. The button now lights up blue.
Here are some pictures to illustrate this:

In transmit mode the display shows, from top left to right: the frequency it's transmitting on, the audio volume (goes from 0 to 79, you can see in the picture above the audio is set to the maximum 79) and whether it's stereo or mono (one or two speakers displayed) and the microphone level (also from 0 to 79, M00 means microphone level is set at minimum). The row underneath shows the RF power output, the SWR reading and the temperature of the power transistor. When transmitting, turning the front dial regulates the output audio volume of the transmission. You can access the menu while transmitting and change settings without the transmission being disturbed but if you change the RF frequency, the transmission will stop.

The Menu consists of the following items:
1 - Frequency set
2 - Audio Volume
3 - Microphone Volume
4 - RF Power set
5 - Stereo or Mono
6 - Temperature Alarm set
7 - SWR Alarm set
8 - Mute
9 - Exit Menu
If you turn the dial clockwise you go through the menu from item 1 to 9. If you turn counter-clockwise you go through the menu from item 9 to 1.

Things to be aware of:
First thing to be aware of when you buy this transmitter without a power supply is that it needs a 2.5mm inner diameter/5.5mm outer diameter DC barrel connector. Not the standard 2.1/5.5mm ones that are used with, for instance, the CZE/7C. The correct procedure to switch this unit off is to first press the button at the front to put it in sleep mode. If the fan is running when you switch off like this, it will keep running for a short while (about 20 seconds, give or take) and then, when the fan stops running, you can switch the unit off by pressing the on/off button at the back. If you switch off following this procedure, the unit will always start up in sleep mode. This prevents it from immediately transmitting when switched on. A simple short press of the front dial button will then start the transmission. The RF power will start rising smoothly to the set output power and will reach that power level within a maximum of approximately 10 seconds.
Here are two pictures of the backside:

Also something to be aware of is that this unit has as standard an N-type RF output socket (female). I screwed an adapter plug (N-male to SO239) onto the antenna output socket to change it into an 'SO-239' because I only use 'PL-259' plugs on my coax.
If you look closely at the top two pictures you can see, I made two little extensions (from some Ø15mm copper tubing) for the front feet so the transmitter stands a little higher. The air intake is underneath and at the front so it's best to put the transmitter a bit higher on its feet to give it some space to breathe in air. If you don't do this the front panel almost touches the surface with the main body clearance being 4.5mm. On YouTube I've seen a few other users of this unit do the same thing. You don't HAVE to do this as long as the unit stands on a smooth flat surface.
Before owning this transmitter I had used the CZE-7C, 7 Watt transmitter and I must say I liked it a lot. Even-though many reviews complain about the sound quality of the CZE 7C, on my car radio it sounded pretty good but the T251 sounds even better! Anyway, it's good to have a CZE 7C handy. In my case I use it to test the bandwidth of my antenna and of Low Pass filters I build etc. So I sort of use it as a signal generator, because you can change frequency real easy whilst transmitting. That is something the T251 will not do! If you go into the menu to change the frequency, the RF stage switches off. Btw, I noticed that if you put the T251 in sleep mode after it has been transmitting it still transmits a very weak signal. It can't be more than a few micro Watts but I can still hear the music buried in noise on my radio. Probably the FM chip that still gets power from electrolytic-capacitors that haven't discharged yet.
The FM chip:
I used to think this transmitter was equipped with the BH1414K FM-chip but I inquired with the seller and they informed me that it has the BH1415F chip inside. This was a disappointment for me, because for the price I thought it would have the much better BH1414K, but still, it does a pretty good job. Of course it's not only the chip itself that is important. It's also the circuitry around it, and there is a distinct difference between the signal from this transmitter and the signal from the cheaper ones which also use the BH1415F like the CZE 7C. That's mostly due to the end-stage with it's filters and power transistor. The RD30HVF1 mosfet is an excellent and robust transistor for the end stage. However, it is noted in the comments below that the BH1415F chip can cause some overload issues that result in stations near to the transmitting frequency being overpowered by the CZE's signal for about a maximum of half a mile around the transmitter location. So be aware of that. It's not perfect (obviously).
Frequency stability is rock solid and stays well within the advertised maximum deviation of 10Hz.
Of course you can analyse the signal from this transmitter until you're blue in the face and it's never going to be as good as a professional Broadcast transmitter but that is reflected in the price.
Here is a picture of the output sinewave at 5 Watt RF power on an oscilloscope. As you can see a nice clean signal.

Pre-emphasis delay is 50µSec and can not be changed in the menu. So this transmitter is not aimed at the American market.  You will just have to contact the seller and ask if they have units with 75µSec pre-emphasis delay if you live in the United States. USA Stock is regularly promoted from different sellers on eBay. (I had a link here to some USA stock on eBay but the listing has ended.)
This transmitter has an RS232 connection at the back, underneath the N-Connector but don't think you can control this unit with your PC. That connection is only for debugging and firmware updates.
Don't assume either that the extra 18 Watts of power compared to the CZE 7C is going to increase your range significantly with the same antenna setup. It's not. You want bigger range? Put your antenna up higher. This set gives me just a little bit more range than the CZE 7C with the same open dipole antenna that I use and the signal is a bit stronger within the normal range but it's only a very small improvement. Believe me, like with an audio system, the quality of the speakers is everything, so it is with antennas and especially their height. (I address the point of 'range vs power' at the bottom of this review also.) As a rule, to double your range with the same antenna set-up, you need a 10 fold increase in output power!!

A closer look at the signal:
Alas I don't have expensive spectrum analyzers etc so here's a few screenshots of the signal from my Software Defined Radio (SDR) on my computer (click on the picture to get full screen view):

Stereo transmit signal at 25 Watts:

This is the signal from a professional local FM radio station whose transmitter and antenna are located just 100 meters from my house. Compare this signal to the previous picture of the T251. Pretty similar right?

Btw, if you look at the signal to noise ratio (SNR) you can see that it is a bit lower on the T251 than on the professional transmitter but that changes with the type of music. If there's a low volume bit in the music the SNR on the T251 can go as high as 37dB. Btw, that is SNR measured with this SDR receiver. That's not exactly a precision instrument. The real Signal to noise ratio is advertised as being equal to or above 70dB. And in fact, CubicSDR software indicates a SNR between 65 and 70dB so that is excellent. These numbers I mentioned are for the stereo transmissions. When you transmit in mono, the signal to noise ratio will be even higher and the mono transmissions also have a longer range than the stereo transmissions.

Mono signal:

Stereo signal but no audio input:

Signal comparison between the CZE-7C (7 Watt transmitter) and the CZE-T251 with the same song playing, both in stereo and both audio volumes and output power set to the same level. Note the difference in deviation beyond the 200kHz bandwidth and the more slender signal of the T251:

This is the CZE-7C. You can see in the waterfall display that the audio bleeds over the 200kHz bandwidth limit, causing disruption on the adjacent frequencies. You can also see that the signal is quite wide:

This is the CZE-T251, here you can see the audio stays nice and tight within the 200kHz bandwidth limit, like a good transmitter should do. The signal is also much more slender than that of the 7C:

Here are some screenshots done with "CubicSDR" software:

Signal at 25Watt without modulation. You can see the 19kHz stereo pilot-tone and more in the upper right audio spectrum graph:

Signal with normal music. Again note the audio graph in the upper right-hand corner:

This is an illustration of how the audio signal produced by an FM stereo transmitter is build-up and how it occupies the frequency spectrum:

You can find more on the technicalities of FM Broadcasting on Wikipedia.

More observations about the T251:
The transmitter is very well built. It's all thick Aluminium. There's no plastic on it anywhere except for the on off switch at the back. Even the dial knob at the front is made from Aluminium. It is a lot smaller in size then I expected from the pictures on eBay though. Be prepared for that. The size is: front-panel width: 173mm (6.81") x front-panel height: 58mm (2.28") x depth: 210mm (8.27"). Main-body width: 167mm (6.57"). Main-body height: 53mm (2.09")
The power amplifier part (end stage) is mounted to a big heat sink with ribs that go all the way to the side of the case and with a fan attached to one side which blows air over it. This is more than adequate to keep the temperature down. (More on the fan in the item below.)
The transmitter is equipped with a temperature alarm. If the temperature gets higher than the alarm setting the unit will stop transmitting and an alarm will sound.  I have set mine to 50°C. The power transistor can easily take 120°C according to the spec sheet but the fan keeps the transistor at a maximum of 39°C so if the temperature rises above that, then it's obvious that there's something wrong and in that case the sooner the unit switches off, the better.
The unit also has a build in SWR meter with a programmable SWR alarm setting. If the alarm is triggered the unit stops transmitting immediately and an alarm will sound continuously until you switch the unit off. If you then switch on again the RF power output has automatically changed to 1 Watt. This is obviously a safety measure in case there's still something wrong with your antenna system. If all is well, you can go into the menu and set the power to your desired value. The SWR meter is a bit optimistic. It says 1.0 when it is really 1.1 but that's no problem in normal use.
The power transistor used is the RD30HVF1 mosfet rated at 30 Watts. It has a typical efficiency of 60% (meaning 60% of the energy put in is transferred to RF power, the rest to heat) and can take an SWR mismatch of 20 to 1 without being destroyed (!!!) The temperature sensor is mounted near the mosfet and is of the LM35 centigrade type. Both the sensor and the power transistor are generously covered in a heatsink compound as you can see in the pictures in the link below. In use with a voltage of 12.55 Volts I draw about 3.37 Amps at the full 25 Watt RF-power setting, so that is 42.29 Watts. (this is measured without the fan running) So that results in an efficiency of 59,11%, almost exactly the rated efficiency of the power transistor which was, as I mentioned earlier, 60%.
The chip that controls the display and remembers the settings is the 12C5A.
You can set the input audio volume electronically from 0 to 79 and the same for microphone input. Beware that you need a dynamic microphone and NOT an electret type microphone!! The microphone input is a mono channel (6.3mm jack plug needed). But using a microphone directly connected to the transmitter isn't very practical, unless you're transmitting at a public event or something. Otherwise I would use a microphone connected to an audio mixer and connect the mixer to the audio input of the T251. Audio input sensitivity is a bit less than on the CZE 7C. The input level is rated at ≤-15 dBV. The frequency response is: 50Hz to 15kHz (3dB). Distortion is 0.2% and the channel separation is 45dB (The channel separation of the average tuner/receiver is about 35dB so the transmitter is well above that.) I use a Philips MP3 player and I have its volume set all the way up and the same on the transmitter (volume to 79) and that get's the music exactly to the level of other stations. But it's better to use a pre-amplifier for the music, like a headphone amplifier or the line-out from an audio mixer to give yourself some leeway. I pre-record my programs and then run it through Adobe Audition which has a very good compressor plug-in by iZotope with a 'Broadcast' preset. That's what I use to render the radio show out and then transfer it to my MP3 player for broadcast. This works very well and the music quality is great. This unit has very tight bass and crisp highs. The sound of the 7 Watt CZE-7C is muddier in the lows and just doesn't reach the quality of the T251. Audio input is in the form of RCA connectors at the back of the unit. In the transmitter menu you can choose between stereo and mono transmission and it is my experience that the mono signal gives you a much bigger range.

More on the cooling fan:
The cooling fan kicks in automatically when the temperature reaches 39°C (102 F) and switches off again when the temperature drops to 35°C (95 F). It also starts running if you enter the menu to change settings. I guess this is done as a quick test to see if the fan is working. The fan produces no extra noise in the audio but it does produce a loud noise of itself so you don't want this transmitter near your microphone. Your studio must be in an other room than the transmitter or you'll hear the fan running in the background. You could of course replace the fan by a more silent one. It's not difficult to remove. The dimensions of the fan are: 40mm high x 40mm wide and 15mm thick. The hole spacing is 32mm. The fan get's 11,0 Volts when the transmitter is fed with 12,0 Volts. The fan's power cable has a 3 pin connector on it of which only the outer two are used. The middle pin is not connected. So if you get a replacement fan with a 2 pin connector you could just cut it off and solder the wires directly to the wires of the connector in the transmitter. The fan draws a current of 118 milliAmps at 11 Volts.
Fan Replacement Test:
I ordered some cheap silent fans from eBay and ran some tests. The first fan I tested was 40x40x10mm 12V 0.08 Amp. and this was totally not up to the job. It could not get the temperature down and kept running all the time. The second one I tested had a thickness of 15mm and a current draw of 0.1 Amp. It could get the temperature down enough to make it switch off again but only for the first 15 minutes. After that the heatsink was fully warmed up and too warm for this fan and it couldn't get the temperature down to where the fan would switch off again. So I reinstalled the old fan. So if you plan on replacing the fan you will need a type that is 40x40x15mm and with a minimum current draw of 150 milliAmps at 12 Volts. This is a picture of the fans I tested and which failed the test:

Here's a picture I took after the test, with the original fan again connected but not yet screwed into place. You can see the heat-sink arrangement and the 3 pin connector for the fan:

I couldn't actually find a good silent fan that was up to the job. 

Range vs Power:
With the T251 set to stereo, the full 25 Watts output power and a dipole 7 meters off the ground and in free air but not clearing the rooftops (see picture of antenna below) and an SWR of 1.1 to 1, I get a range of 10 kilometers (6¼ Miles) in a small-town setting without high-rise buildings. (Nothing higher than 5 stories to block the signal). That is a 10 kilometer range in which the audio is noise free received on a car radio driving away from the transmitter location (receive antenna on the rear of the car roof, so the ground-plane formed by the car roof is pointing away from the signal)! The range in which the signal can still be received is about 15 to 20 kilometers (9 to 12½ Miles) depending on terrain and obstructions. If you put the antenna high enough to clear the rooftops of most buildings near you, that can easily double! I also did a test in which I set the transmitter to an output power of 1 Watt and tried what range I had. It was unbelievable. I still had a range of about 5 kilometers (3 Miles). Sure the signal broke up much faster behind obstructions like buildings and bridges but still. 5 Kilometers is not bad. As I mentioned I set the T251 to 1 Watt, but measured on my Diamond SX600 SWR/Power meter an output power of 1.75 Watt. But seeing as it has to go through 7 meters of RG213 and 2 meters of RG58 I recon the Effective Radiated Power (E.R.P.) couldn't have been more than 500mWatts. That's impressive and shows once more that the most important thing about a transmitter set-up is the height and placement (and build quality) of the antenna!! Height is everything!

Here's a picture of my home built dipole antenna:

If you want to have a look at the Printed Circuit Boards inside the transmitter, then I refer you to the link below to my Flickr page where you'll find an album with some closeups of the PCB's:
https://www.flickr.com/photos/ededitz/albums/72157679661456110

Troubleshooting:
Should you encounter any problems when operating this unit then there is an option you can try. If you switch the unit on with the switch at the back and at the same time keep the front dial button pressed in, the unit switches on into Self Test Mode. Here you can automatically check certain functions of the transmitter. Simply turn the dial to select the parameter you want to test and press to confirm. The unit then tests itself to see if it functions correctly. I personally have not tried this option yet because my unit functions perfectly fine and I am not comfortable doing this procedure when it's not necessary so I can't tell you exactly what it does when you select this option but just know that it is available should you need it.

This transmitter has one weird quirk. If you turn the front dial button whilst the unit is in sleep mode, with the button lit up red, or even if the unit is totally switched off, the temperature alarm can go off if you switch it on again! If this happens simply press the front dial button twice or switch off and on and you're good to go. A minor thing. Just don't play with the knob when it's switched off.

I've been getting some feedback that the output power is not really 25 Watt but more like 21 Watt. I myself measured this too on my SX600 SWR/Power meter but I thought it was my meter that was at fault; but I'm hearing from more people that they get the same measuring results. Now 4 Watt isn't a lot when you're already outputting 21 Watts so I shouldn't let it influence you if you're thinking about buying one of these units but I want this to be as honest a review as possible so I thought I would just mention it here.

On that note there is one more thing I must mention. A very small percentage of these units can fail and give a reading of zero output power and the SWR indicates 9.9 even though they go through quality control and are tested for a period of time before being sold. If that happens your only option is to contact the seller and try and get a new unit or a refund. The chances of this happening are extremely small but they are not 0%. Like anything you order from China there is a small gamble involved but usually the seller is very quick to replace the unit for you. It has happened to one person commenting below so you should be aware of this. But it is the first mention I received in the 2 years that this article is now online.

Conclusion:
The unit I bought and tested performed very well. Certainly not a 'toy' as some people on YouTube like to say when they compare this to a $5000.- transmitter. That's like comparing apples to pears.
It simply is NOT a professional FM Broadcast transmitter but listening to it on your car radio no one is going to notice that. Stations that are 0.2 MHz or more away from your frequency won't be interfered with by your signal. And you don't need to be afraid of harmonic signals either. The Butterworth Low Pass Filter in the end stage suppresses that very effectively. But do be aware that around the direct vicinity of the transmitter location, stations can be surpressed by this unit. This is however only within a few hundred meters of the transmitters location and it's not always the case. It depends on the local situation of course and also on the quality of your antenna. Professional broadcast people curse this transmitter, and ones like it, because they say it creates interference on other frequencies. I can not see how it can do that with a lowpass filter behind the powertransistor but then again I am not an expert, just a consumer who reports his own experience with the unit for others to make of what they wish.

The sound quality is amazing. As I mentioned before, the cheaper CZE 7C's sound is phatter and is muddier in the bass region than the T251. This unit sounds very crisp and clear with deep tight bass and crisp highs. Really excellent I thought. Of course you must keep in mind that the perceived sound quality is first of all dependent on the quality of your receiver and audio system. Make sure to check whether your transmitter produces interference on other frequencies before you start using it. Some of these could have fake power transistors in them, although mine was fine.
If you want to buy a Chinese FM transmitter, I would get this one (or the cheaper 15 Watt CZE-15B which does have the BH1414K chip inside and is PC-controllable.)
I would stay away from the 60 to 80 dollar 15 Watt ones like the NIOrfNIO or the ST-15B. They are cheaper for a reason. Their quality is not terrible but comparable with the CZE 7C I mentioned above. But certainly stay away from the cheap FM kits. They use the KT0803L chip which is the worst FM chip you can get. Their signal is a mess, you'll black out the TVs in a full block radius. Spend a few extra dollars and buy better quality.

Here is a link to the USER MANUAL for the CZE T251 in PDF form:
Click here for User Manual PDF

Here is a link to the datasheet (PDF) of the RD30HVF1 Mosfet power transistor:
https://www.mitsubishielectric-mesh.com/products/pdf/rd30hvf1.pdf

The comments below this article are proving to be a great resource. Reading them will give you an idea of other peoples experience with this unit and how it compares with other ones. Unfortunately I had to close the comment section for this article because it became too long and also because I've been out of the FM transmitter hobby for so long now that I don't feel qualified to answer any more questions about it. Sorry. But read the comments below and you'll probably find an answer to your question.

Disclaimer: The author does not accept any responsibility for actions and or alterations to equipment undertaken by anyone after reading this blog post. Anything you do, you carry the responsibility for.
Operating an FM broadcast transmitter without a licence is illegal in most countries, however, owning a transmitter but not using it as such is usually not illegal. Please check the laws of the country you live in to make sure you don't break any laws.

-- SORRY. COMMENTS CLOSED! --

A 'Brute Force' 15 Amp Power (Dummy-) Load. Easy to build!

Power-loads or Dummy-loads are expensive bits of equipment but if you need to test a powersupply or test a current protection circuit and need something that can draw a big current and is preferably regulated so you can dial in the current then this is the easiest and cheapest method.

The circuit below uses a BD182 transistor which I had lying around from an old audio amplifier. It is the equivalent to the well known 2N3055 and you can exchange the one for the other in this circuit without trouble.
The powerload needs no external power supply. It is a passive circuit but do make sure you use a big, and I mean BIG, heat-sink. The transistor will get hot very quickly.
Here is the schematic drawing of the circuit. I don't think you can make them any simpler than this.

I did away with the shunt resistor in the emitter to ground line because we don't need to measure the current at that point. We will use a Digital Multimeter in series with the Dummyload input. A shunt resistor is usually used for creating a feedback voltage to go into an opamp, In this case it only holds back the current so we leave it out.
I made my dummy load switchable. You can draw a big current and then by flipping the switch the current almost doubles. The lowest resistance I got from this circuit was 1.8 Ohms. That means with 12 volts connected you can draw a little under 7 Amps. and this transistor has no troubles with that (provided the heat-sink is up to it). Be sure to use a good heat sink and generous amounts of heat conducting compound around it.

Below are some pictures of how I build my power load. I build mine in an old Droste Chocolate can I had lying around. Previously I had built it in an old wooden cigar case so this is big improvement :)
Beware that the current-rise is exponential so when you turn the potmeter, the first few amps will go nice and smooth but at the end the current rises very quickly and even jumps to maximum just before the potmeter if fully turned. And yes, the potmeter I used is a linear type not a logarithmic one :)
The current fluctuates a little if you put an amp-meter in series with the load. The milliAmp digits will go up and down a bit as you read the amperes. But that's what you get when you keep it simple. The little 50nF capacitor is meant to suppress transients should they occur.
The maximum voltage from Collector to Emitter for the BD182 is 60 Volts so don't go higher then that and the maximum continuous Collector Emitter current is 15 Amps. That's a lot more then most of the circuits for powerloads you'll find online. They mostly go up to just 5 Amps. I try not to exceed 10 Amps with this design just to be on the safe side. I used a CPU heatsink on the 2N3055 which is not ideal but I mounted the transistor on a little copper sheet so that acts as a heat sink too and then I put the CPU heatsink on top of the transistor with heat conducting paste in between so the transistor can dissipate heat from the top of the TO-3 housing aswell. It's a very effective solution and works very well. If you need a power load that can draw even more current you can just put more transistors in parallel over the first one. Just connect base to base, collector to collector and emitter to emitter. (I haven't tested that btw, but it should work.)

That's it for this article. I hope you enjoyed it. If you did then have a look around some of the other articles on my website. There's loads of interesting electronics projects, mostly synthesizer related though. Please leave me a comment, because I always enjoy hearing from likeminded people. If you'd like to support my modest efforts you can do so by subscribing to my YouTube channel EdEditz

THANK YOU!