Jeff Duntemann's Contrapositive Diary Rotating Header Image

February 21st, 2013:

The Five Minute Electrostatic Charge Detector


I’ve been building electronic gadgets for almost fifty years, and I don’t think I’ve ever managed a working five-minute project before. If the photo above shows a slightly random assembly style (I’m typically more artful, whether or not the circuit requires it) that’s because I wanted to complete it in five minutes.

I did. And it works. When I snapped the picture, I was holding my hand about six inches from the gate lead of the MPF102. When I pulled my hand back about a foot or so, the LED went out. If I scuff around my static generator of an office, it “sees” me even two or three feet away. The sensing “antenna” is nothing more than the gate lead of the FET, left loose in mid-air. A short scrap of wire three or four inches long would have increased the range greatly.

The circuit came to me from Bill Beaty, who posted a comment with the link to my February 18, 2013 Odd Lots entry. It consists of an MPF102 JFET, a generic red LED, and a 9V battery plus its associated connector. I lashed it up on a scrap of perfboard with a total of three solder joints. Bill found me because of this post, to an old Popular Electronics electroscope project using a 6J7 tube instead of an FET. (They didn’t have MPF102s in 1961.)

Sure, it’s a stunt, but I was in a stunt mood. Remember, if you try this and it takes a little more than five minutes, that I’m good at this stuff, and had gathered all the parts to my workbench before starting the clock. And clock it I did: the stopwatch said 4:39 when I snapped in the battery, leaned toward the gate lead, and saw the light come on.

I’m still considering a tube-based device using some sort of neon light more exotic than an NE-2. I’m considering an NE-34, and would have had one by now if they didn’t cost so much. Or an Aerolux bulb, if I’m feeling Art Nouveau-ish. Those cost even more.

I can feel my resistance crumbling. Remember: It’s futile.