Here is a circuit of capacitance beeper which uses a two-transistor flasher in conjunction with a Darlington transistor. When the probe is touched to a capacitor, the project beeps at a frequency that varies with capacitance. The frequency change is so noticeable that small capacitors can be precisely matched or an exact fixed value can be selected to replace a trimmer in a prototype.
When the beeper is properly adjusted it draws only 10uA with nothing touching the probe excluding the LED current. This design is optimized for capacitors less than about 100n. Large capacitors give a low frequency clicking sound and small capacitors sound a tone that increases as the capacitance decreases. Many decades of frequency change occur over the beepers range.
The probe should be built into a metal box so that one hand makes good
contact to 0v.
The resistor values are selected to barely turn on the transistors to conserve battery power. The transistors must have very high gain and good low current properties. The MPS-A18 is a very high gain transistor with excellent gain at very low currents. The capacitors are not particularly critical but the trimmer might require a little care. The trimmer is adjusted until the beeping just stops and only a very weak squeal is heard when a 2.2p is touched to the probe.
When the beeper is properly adjusted it draws only 10uA with nothing touching the probe excluding the LED current. This design is optimized for capacitors less than about 100n. Large capacitors give a low frequency clicking sound and small capacitors sound a tone that increases as the capacitance decreases. Many decades of frequency change occur over the beepers range.
The probe should be built into a metal box so that one hand makes good
contact to 0v.
The resistor values are selected to barely turn on the transistors to conserve battery power. The transistors must have very high gain and good low current properties. The MPS-A18 is a very high gain transistor with excellent gain at very low currents. The capacitors are not particularly critical but the trimmer might require a little care. The trimmer is adjusted until the beeping just stops and only a very weak squeal is heard when a 2.2p is touched to the probe.
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