A very simple presence detector can be made using this electronic scheme. This electronic circuit reacts to the presence of a conductive object within a particular field. Circuit sensitivity can be adjusted with potentiometer P1 for the desired distance. The electronic circuit consists essentially of a monostable oscillator and a tipping stage. Oscillator is built with transistor T1. This is why a flap that oscillator frequency is particularly stable, because the capacity of the transistor is in parallel with relatively large capacities C1 and C2 of the oscillating circuit. Surface acts as a sensor oscillator circuit capacity of, capacity may be small in comparison with C1 and C2. in this configuration, the oscillator operates at about 1 MHz.
High frequency signal is amplified and eventually rectified. In this way receives a control pulse to monostable tipping stage consisting of T3 and T4. By using VFET's possible to order the relay switching by tipping stage. Switching time is determined by adjusting potentiometer P2.