In this proximity detector an NE567 tone decoder provides a signal of about 100 kHz that is fed to one sensor. The senso
rs are copper or aluminum wires or plates, or any other suitable conductive material. When another object is near the sensors, it causes an increase in capacitance between the sensors. Ql and Q2 amplify the signal and feed it to Ul. Notice that C8 and R4 phase shift the VCO signal from the NE567 so that Ul can detect its own output signal.