Sooner or later you’ll encounter an unknown transistor to which you cannot find a datasheet for. Here’s a quick and simple tester you can build on perfboard or you can simply wire it up temporarily on your breadboard to identify those pesky pins!

The circuit:

Explanation:

Test procedure:

NPN

PNP

Conclusion:

Current PCB layout for the ESR Meter

Continuing with the ESR project, I’d like to enumerate the various issues I encountered during layout design, fabrication and finishing of the project.

I believe some people only show their “good side” and bury deep down each and every failure they suffered, however you don’t learn from winning and that’s why I thought it would be a good idea to write this.

These are some of the problems diagnosed during troubleshooting in no particular order:

1. PCB Layout error, first revision circuit had a trace that was mistakenly connected to another component, this rendered the detector phase useless.
   
2. Bad ceramic capacitor, the 470pF was damaged. Replaced with two 1nF in series to approximate the value.
3. The transformer displayed losses at high frequencies, it was replaced with another transformer. (I still worry about the output level, it’s way under 100mV)
   
4. Negative supply “biasing” potentiometer had one of the wires broken by the PCB end, most likely due to the amount of handling involved during the troubleshooting phase: always hotglue your wires!!
5. Power supply wasn’t providing enough voltage for the opamp used, turns out 5v input is a no-go for TL082 due to manufacturing tolerances, etc. I had to go for 12V. The final unit uses 13.8V — I would like to go up to 15V but that calls for a different regulator.
   
6. Deflection issues due to the 100uA meter, feedback gain was changed and a potentiometer was added for convenience.
7. One of the test leads had a high resistance due to severed wires by the connector, this was a cheap DMM set of leads and I regret using them!
   
8. The only TL082s I had were in SMD packages, so I had to etch a dip adapter board!
9. My PCB layout was lost, well — the source file at least, this meant I had to redraw the entire layout from scratch, based on screenshots.
   
10. Several placement and power issues during the last phase of the project were encountered, all of which have been fixed ever since — Although I’m not entirely happy with the placement of the potentiometers, they’re at a slight angle and this means their values will shift due to the slight pressure they’re under.

So, I wasn’t kidding when I said this wasn’t a simple project! — Probably the most problematic one thus far. However I didn’t give up and I could probably say I succeeded.

Special thanks to Lee and everyone in ##electronics @ freenode  for the invaluable help and support!

Have fun!
Cheers.

A simple FM Bug for beginners

For months I’ve been looking for a simple FM BUG project, the ones online require inductors which you either have to acquire or build, if you don’t have a LCR meter it becomes rather hard to get the circuit working, specially if you’re a beginner without an oscilloscope! – Sometimes they don’t even tell you which inductance is required and you have to calculate an estimate, which is the main reason why many high frequency RF projects fail in the first place.

From all the projects out there I’ve only seen one which didn’t require an external inductor since  it simply used a pcb / trace inductor, however the board was big and the circuit itself had lots of stability issues, etc. I wasn’t going to waste my time with it.

My first FM BUG was based on one of the many schematics out there, it seldom worked. It was microphonic (due to the air core inductor) but the electret capsule itself did not modulate the output at all, needless to say it was very unstable and it never worked properly.

This circuit on the other hand performs pretty well, even if you’re manipulating the board or touching the coax it will stay within the tuned frequency (unless you touch the transistor or timing capacitor!). The power is very low, so don’t expect great distances, specially in populated areas!

This is the schematic:

No surprises there, it’s very similar to other schematics so therefore I won’t go into the working details. Actually I might argue this one is a bit on the low efficiency side, but at the moment I don’t have the equipment to further improve it’s design.

But let’s not focus on that and instead let’s talk about the inductor / antenna, it’s based on a 12cm long 50-75Ohms coaxial cable with one of it’s ends soldered (mesh and core are joined together) this is a big plus, we don’t have to use a flimsy air core inductor and we don’t need a lengthy wire antenna either – Great!

This tiny ghetto transmitter is guaranteed to work first time, as long as you double check all connections and you make sure your transistor is properly placed and in working order. I used the BF199 because it’s got a low capacitance and it’s ideal for this type of application, but you may use a 2n2222 or similar general purpose NPN BJT. I know, the schematic calls for a BF259 which is what I used in my simulation, but believe me it works beautifully with the BF199.

I didn’t have a 8.2pF capacitor so I used a smaller one, which naturally led the transmitter to work on the upper FM broadcast band, but hey — better than nothing!

The sound quality is not the best, this FM Bug could clearly benefit from an audio amplification stage. Another idea would be to scrap the electret and use an mp3 player instead. I reckon R4 could be ignored in that case but you might want to use a resistor in series with the input capacitor.

Another important component is the variable capacitor which forms the tuning circuit, I used a 4.5-20pF trimmer type capacitor, it’s the first one I found in the junk box, it was a bit flimsy and it required lots of patience to tune but I eventually got it to the point I wanted.

I recommend you get a ceramic screwdriver for the tuning or you can improvise one with a piece of plastic, it’s important because otherwise your body capacitance will affect the transmitter!

For those interested I also included the PCB layout, it’s very easy to etch your own PCBs so you should definitely give it a try! – Remember to keep the component leads to a minimum, we’re working with high frequencies, any parasitic capacitance, etc. will modify the behaviour of the circuit one way or another!

Click here to download all pertinent files for this project: fmbug_docs. The exports are not in perfect quality due to the PDF printer driver I used, but hey at least I’m providing the PCB layout, other projects leave this as an “exercise for the reader” (that’s because they never actually built the damn thing!)

One improvement over the current layout would be to get rid of the header for the power connection and use a button cell with a holder instead, another one would be to use a 3.5mm connector instead of the electret being soldered directly to the board.

And in case you’re wondering, here’s the BOM:

• 1x BF199 or similar RF NPN BJT.
• 1x 8.2pF Capacitor.
• 1x 2.2nF Capacitor.
• 1x 0.1uF Capacitor.
• 1x 50nF Capacitor.
• 1x 100 ohm resistor.
• 2x 4.7k resistor.
• 1x 5.6k resistor.
• 1x 3-30pF variable capacitor.
• 1x 3V source (I used 2x 1.5V AAs with a battery holder).
• 1x 12cm long piece of 50-75Ohms coaxial cable to serve as the inductor/antenna.
• 1x electret microphone (double check the polarity!)

Standby power consumption is ~5mA. During normal operation you should expect peaks of 10mA but on average 6mA is about right. L1 would be lucky to see peaks higher than 15mW!

If you want to increase the transmission distances you may use a metallic enclosure and solder the negative of the battery to the enclosure itself.

Also keep in mind my design was not meant to be used as a concealed spy bug, if you really wanted to build a proper spy bug you’d have to use SMT parts, these are generally not available to the beginner and therefore I went with through-hole components instead. Plus I’m against spying people without their consent…

That’s all for now, hopefully you’ll build and enjoy this tiny FM “BUG”, I know I did

Cheers.

Oh by the way… check your local laws regarding this type of device. I can’t be held responsible if you interrupt your neighbours favourite station!

It just makes no sense whatsoever! they’re trying to get a piece of this huge market by introducing a ridiculously cheap product and yet theres no way anyone outside the US without a CC would be able to get one!.

Therefore TI’s product is as appealing to me as sucking a rusty nail while watching a match of cricket under severe chronic depression! It’s going to end one way and it won’t be pretty.

The worst part is the fact that once you’ve got the kit and you want more ICs, you’d expect them to be cheap but they really aren’t, unless you’re buying bulk that is! — Do you know any student who buys bulk quantities of ICs (10.000 units) ? I didn’t think so, unless they’re running a meth lab.

Well TI, heres a launchpad you can use to launch this lie out of orbit! But wait, you’ll need a CC to use it.

At least I know they won’t be making a profit out of this and due to the fact that they didn’t try to compete directly with the Arduino, they won’t be doing well since it’s an established, properly supported product with a plethora of options to choose from. TI’s Launchpad just isn’t.

So yeah, there you go.

And by the way, I’m not an Arduino fanboy. In fact PICs are pretty much the norm here in Argentina (not a fan either) so I’m in no way biased, this is just my point of view. Hopefully other weasels will start reselling them on eBay, giving us a chance to give it a try…

Remember: If it sounds too good to be true, it probably isn’t.

As to avoid frustration, you should ignore them for now and take a look at simpler oscillators like the one that follows:

( This schematic is also available in PDF: Very_Simple_Oscillator )

The output wave:

If you want something even simpler, you may take a look at “negistors” — however you will need a much higher voltage than 3V so it may not be possible to operate it in a portable device, unless you were to use two 9V batteries (and this to me wouldn’t be cheap nor practical!)

List of materials:

• 1x 100K Resistor (1/4W)
• 1x 10nF Capacitor (Ceramic or any other type)
• 1x 3V Cell (use a button cell or two 1.5V batteries)
• 1x BC548 (Or any other general purpose NPN transistor)
• 1x BC558 (Or any other general purpose PNP transistor)
• 1x 8 Ohm Speaker (any low power speaker will do)
• 1x SPST Switch or Push button (optional, highly recommended)

Note: this oscillator will generally work reliably with voltages down to ~1.0V

If you want to operate the oscillator at higher voltages, you’ll have to replace the transistors with higher power devices such as the BC337 and the BD140. These should allow you to run it up to 12V (although the BC337 may run hot!)

The total cost is just pennies, considering you may already have a suitable speaker, switch and button cell which are arguably the most expensive components in the list.

Have fun building it!

Cheers.