Electro-Static Detector, not working as designed

Discussion in 'The Projects Forum' started by Rendar1970, Jan 14, 2011.

  1. Rendar1970

    Thread Starter New Member

    Jul 1, 2010
    Hey all, so im making a simple (or so I thought) electrostatic detector based off a mpf 102.

    The schmatic looks like this. I got it from someone else, and there circuit is working great, they actually already built 3 with the same design, however mine doesnt work.


    The Idea is the light and buzzer are off. And when the coil picks up a static charge, the light and buzzer will turn on.

    However mine is reverse. As soon as you turn on the circuit. The light and buzzer are already going off. When you introduce a static field, the light and buzzer turn on.

    Is there a reason mine is working in reverse to how it should and all the others.

    I already ruled out environment as im testing mine right next to one wired the same way that works.

  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    Your circuit is missing a power supply. Did you forget to include it?

    Senior Member

    Jul 1, 2008
    Besides the missing power supply, I see no limiting resistor for the led and your LED is wired across the Drain - Source terminals (Parallel), where it should normally be in series. Also; the MPF102 is a 'Depletion mode' JFET. Meaning, they are on (conducting) until the Gate voltage reaches 'Pinch Off Voltage'. I think the pinch off voltage for a MPF102 is about -2V to -3V with respect to the Source but it's been a while since I worked with one. As I remember,.. this FET was popular as a small signal RF Amp. Come to think of it, I have a box of over a thousand of them.... somewhere!:confused: