Capacitive proximity sensing detection circuit

Capacitive proximity controllers usually consist of a radio frequency oscillation circuit and a detection board. Figure 6-56 is a capacitive
proximity sensing detection circuit made with discrete components .
    In the figure, transistor VT1 and surrounding components form a
radio frequency oscillation circuit. The metal sensing electrode piece is connected to
the collector of VT1 as a detector. When no other conductor is close to the sensing electrode piece , the oscillation circuit composed of VT1 oscillates normally. At this time , the radio frequency voltage signal output by the emitter of
VT1 becomes a DC control signal after being detected by VD1 and VD2. This signal causes the switch tube VTZ to conduct the wine. The relay is electrically closed and turns on the power of the controlled circuit; when a conductor is close to the sensing electrode sheet, any conductor close to the sensing electrode sheet will induce the capacitance between the electrode sheet and the "ground", and the capacitance The increase of will reduce the positive feedback amount of the oscillator until the oscillator stops oscillating . If the oscillator stops vibrating, the radio frequency detection circuit will no longer output the DC control signal, and the switch VT will be cut off at this time. The relay is de-energized and disconnected. After the relay is disconnected, the switch S needs to be released and then closed again before the circuit can enter the next oscillation state. Otherwise, the relay will remain disconnected.   G is the sensitivity adjustment capacitor . Adjusting its size can adjust the critical value of the oscillator to start and stop oscillation, thereby adjusting the distance and size of the object controlled by the controller. In Figure 6-56, inductor L1 can use any color-coded inductor with an inductance of 1-6mH. If the inductance is greater than 4mH, the value of G needs to be appropriately increased so that the circuit can start to oscillate smoothly.