|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.