Many years ago I bought a multi-function testing screwdriver, the instruction sheet (which I have mislaid) claimed you could use it for a surprising number of tasks:

  • Continuity of fuses
  • Mains voltage present (barrel marked "70-240V" and, amusingly, "CE")
  • Locating a conductor break inside a mains-lead
  • Checking your microwave oven for microwave leakage

If you hold the blade and place the top near a wall it lights up near buried 230V wiring (e.g. for light switches) but isn't very reliable.

If you touch the blade to an earthed condcutor (e.g. radiator or water-pipes) and press your finger on the metal cap, it lights up.

I took it apart, there is a 3-pin device in a TO-92 package labelled


I guess this is some sort of transistor.

There is also what seems to be a very high value resistor (too high for my cheap multimeter to measure)

Inside the capsule are also a LED, a small resistor and two small button batteries.

Here are some photos


Surprisingly, it still works after I reassembled it.

I don't use it to test for 230VAC (I plan to get a non-contact tester for that)

I assume it relies somehow on AC and human capacitative coupling to earth? How does it work?


4 Answers 4


It's an LED turned on by a transistor! The base (gate?) lead connects to the screwdriver blade. Yes, all sorts of tricks and useful applications are possible with something this simple.

The resistor is a few-KVs high-volt type, probably chosen out of extreme safety paranoia (since customers might use it on a 440V breaker or worse. Can't have resistor arcing over internally!)

If the transistor is a JFET or an hfe~5000 Darlington, the ohms value can be extremely high: turn on 1mA LED using 50v signal input, Base resistor can be

50v/(1mA/5000) = 250meg

and 250v/250meg = 1uA, so it's pretty safe, electrocution-wise, plus offering lots of ESD protection to the transistor.

Try sweeping the driver blade across cloth or plastic sheet while touching the button. If the LED flickers, then the transistor is probably a JFET.


Traditionally such devices used a Neon bulb with a ~~~~ 100V strike voltage, a HIGH value resistor and capacitive body coupling to ground.

This one MAY instead use a MOSFET. AC capacitive couple signal turns on MOSFET by driving gate high enough and LED is driven via batteries. Gate sensitivity can be 1V or less if desired. If you can get enough voltage via capacitive coupling to strike a Neon then driving a MOSFET gate should be easy. And, apparently, is.


circuit diagram

This should give you all you need to understand this useful little tool!


The transistor is a ksp13 Darlington. Your picture is unclear, but you can decode the resistor code here. Circuit diagram here. Hope this helps...


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