Proximity Switch

A 3-x3-inch piece of circuit board, or similar size metal object which functions as the pick-up sensor, is connected to the gate of Q1. A 100-MW resistor, R2, isolates Q1's gate from R1, allowing the input impedance to remain very high. If a 100-MW resistor cannot be located, just tie five 22-MW resistors in series and use that combination for R2. In fact, R2 can be made even higher in value for added sensitivity. Potentiometer R1 is adjusted to where the piezo buzzer just begins to sound off and then carefully backs off to where the sound ceases. Experimenting with the setting of R1 will help in obtaining the best sensitivity adjustment for the circuit. Resistor R1 can be set to where the pick-up must be contacted to set off the alarm sounder. A relay or other current-hungry component can take the place of the piezo sounder to control most any external circuit. (Hands-On Electronics/Popular Electronics 5/89, p. 85) From figure FC80-3 from Encyclopedia of Electronic Circuits Vol. 1.)