Tag: Electronics

How to quickly identify bipolar transistors.

Posted by on May 8, 2011


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:


  • R2 pulls the base down so it’s not left floating whilst SW1 remains open.
  • If the transistor is correctly connected and it’s of NPN type, when pressing the button the base would be forward biased and the LED should turn on brightly. Otherwise it may not turn on or it will be barely visible, this means C and E have been swapped.
  • For PNP once power is applied the LED should turn on by itself if C and E are correct, because the base is pulled to ground by R2, if the button is pressed the base would be lifted from ground and the LED should turn off.
  • If swapping C and E on both cases does not yield a conclusive result, you can assume this BJT  is damaged or you incorrectly identified the base.
Perform your initial  tests with known good devices. Make sure you swap C and E on purpose to visualize the outcome.

You may place an ammeter between VCC and C instead of an LED. You should normally get around ~10mA across it with a general purpose NPN as long as R1 is 330 Ohms, with a PNP this value would be nearly halved due to the pull-down resistor being an order of magnitude bigger than the pull-up.

However due to the nature of this circuit the resistor values are not critical, just respect (roughly) their ratios and you’ll be fine.


Test procedure:


  1. Identify the transistor base using your DMM in DIODE mode. The base is going to be the anode of both “diodes”. (See Fig:1) – If you can’t identify both diodes, this may not be a BJT or it’s been damaged; throw it away.
  2. Connect base to the connector B from the transistor-tester and the rest to C and E (doesn’t matter which)
  3. Apply power, nothing should happen. Press the button, if the LED barely lights up or it doesn’t light up at all then the pin you connected on C may actually be E, if it lights up brightly then your assumption of C may have been correct.
  4. Turn C and E around even if you’ve got a result and go back to 3) to rule out a faulty device. If the transistor is shorted it would conduct either way around. With a good and properly connected transistor the LED should only turn on when the button is closed and should remain OFF for the rest of the time.


  1. Identify the transistor base using your DMM in DIODE mode. The base is going to be the cathode of both “diodes”. (See Fig:2) – If you can’t identify both diodes, this may not be a BJT or it’s been damaged; throw it away.
  2. Connect base to the connector B from the transistor-tester and the rest to C and E (doesn’t matter which)
  3. Apply power, if the LED lights up brightly then your assumption of C and E may have been correct.
  4. Press the button, the LED should go OFF, otherwise swap C and E and go back to 3) — If the results are inconclusive you may have a defective transistor. With a good and properly connected transistor the LED should only turn on when the button is  open and should remain OFF for the rest of the time.



With just a few parts from the junk box you can identify your unknown BJTs and test known ones for YES/NO fault finding. This circuit won’t identify damaged junctions; for that you’d need a curve tracer.
However it may come in handy if you’re constantly recycling components from old devices!

Have fun and don’t forget to share your findings with the community! — Likewise if you have any old datasheets don’t hesitate to scan them!


ESR Meter – What went wrong.

Posted by on November 6, 2010

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!

A simple Coil-Less FM Bug

Posted by on September 22, 2010

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! More…

What is LaunchPad?

Posted by on June 25, 2010

Bullshit, that’s what it is. They targeted the “Launchpad”  toward the maker/hacker market and yet they don’t provide the according payment methods, these corporate weasels will only take your credit card, tell me how many students actually own a credit card? only the rich kids, who are probably more interested in getting more free money from their parents and living la vida loca than spending their time hacking away on a TI development board alone in their basement.

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.


Very Simple Oscillator

Posted by on April 17, 2010

This is the first oscillator I ever built. I remember looking at books and magazines, schematics on the net, etc. They were all either colpitt or hartley oscillators. Generally speaking while they’re simple enough, they require the use of proper inductors which is something that usually you don’t have when you’re just starting out or you can’t measure them because you don’t yet own the proper equipment (unless you’re lucky).

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