capacitive switch

elektorjuly/august 1983 101 capacitive switch 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 fre- quency is changed, a positive or negative voltage peak will appear due to the momentart" change in the average waveform of the signal. This is the principle upon which our switch is based. The 555 or 7555 timers wilt oscil- late in a stable manner. However, if we add an external capacitive sensor it becomes possible to vary the oscillation frequency. In this circuit the square wave is integrated by the triple RC network, while 1C2, 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 move close to C the relay makes; if you remain stationary the relay breaks. It may seem a bit basic but it is a val...