A sensitive detector for static electricity based around a single Field Effect Transistor It can also be used to detect the presence of negative ions, or test a negative ion generator.
This circuit utilizes the extra high input impedance of a FET, and also demonstrates the gate terminals sensitivity to changes in voltage. The gate terminal here is left open circuit,connected only to the “probe”. The probe is just a few inches of bare copper wire. With no fixed DC biasing, the gate terminal will respond to micro changes in voltage. This causes a current change in the drain circuit and the meter will indicate a charge or “field strength”.
As static electricity can have either a positive or negative charge, the meter used should be a centre zero type. Full scale deflection can be 1mA or 250uA for greater sensitivity. Remove the meter and use a multimeter to measure the voltage between FET drain and the preset resistor. Adjust the preset for 0 volts and then replace the meter. This will avoid “bending” the needle.
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