Collection Schematic

Take a square wave signal with a given frequency and integrate it. This gives a stable continuous average voltage.
By changing the existing frequency of the signal the average integrated value remains the same but, at the instant when the frequency is changed, a positive or negative voltage peak will appear due to the momentary change in the average waveform of the signal. This is the principle upon which our switch is based. The 555 or 7555 timers will oscillate in a stable manner. However, if we add an external capacitive sensor it becomes possible to vary the oscillation frequency. ln this circuit the square wave is integrated by the triple RC network, while IC2, used as a comparator (with a variable reference value), uses the changes in the integrated voltage to alternately make and break the relay.

Thus when you e move close to C the relay makes; if you remain stationary the relay breaks. It may seem a blt basic but it is a valid idea and it is worth l looking at it in greater detail. To obtain better results you could take the signal after integration and differentiate between negative pulses (the frequency decreases as the value of C increases: when the sensor is approached) and positive pulses (the frequency increases again if the sensor is no longer affected) and compare them. Without this l refinement the size of the sensitive plate must be such that the frequency of oscillation be at least several kHz. Failing this the operation of the circuit would often be disrupted by false detections. Coarse and fine adjustment is provided, using P1 and P2, to reduce the risk of incorrect switching. Note: The numbers in parentheses are the pins if an LM3l1 is used in place of the CA3130