My workmate has this nifty little EMI-detecting wand he built back in 2000. It senses EMI fields using an approx. 200 turn air-core coil, and converts them to audio signal. He uses it to detect e.g. when a microcontroller comes out of power saving mode.

I wish to built a similar device, but unfortunately he didn't have complete schematics for it. Also part of the PCB of the working device is covered in hot glue, so I couldn't get the schematic down 100% perfectly.

Half of the circuit is just LM386 audio amplifier, which is quite easy to get working based on the datasheet. What I'm having trouble with is the preamp stage consisting of 2 NPN transistors:

Preamplifier circuit consisting of 2 NPN transistors

What I have drawn above is what I'm quite certain about. There are also some biasing resistors, which I think connect to Q2 emitter. It seems no resistors connect to Q2 collector, which seems strange to me.

It looks like the first transistor is in common-base configuration, and the second one is supposed to bias the first one so that it will detect AC currents. However I've been unable to find any similar circuits online.

Does this preamplifier circuit have a name that I could search for?

  • \$\begingroup\$ You're going to have to melt the glue off and get a better schematic. It can't work as shown. For one thing, there's no DC path to Q2's collector. \$\endgroup\$
    – Dave Tweed
    Commented Mar 28, 2015 at 13:53
  • 1
    \$\begingroup\$ Hot melt glue goes very runny with a rework station set to 100°C. \$\endgroup\$
    – Majenko
    Commented Mar 28, 2015 at 14:17

1 Answer 1


The preamp-circuit was from here: http://www.4qdtec.com/preamp.html

A few biasing resistors were all that was missing and it works after adding them in proper places.


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