This capacitive sensor that operates with a high frequency oscillator was found in Radio Electronics from the 90's. The circuit operates around 300 kHz.
This circuit is from a 1979 documentation. The critical component is the CA3130 operational...
This circuit can be used with 3 receivers and is intended for low frequencies (medium wave range). We...
This circuit shows how to control a TTL logic gate with a phototransistor. The phototransistor supports an...