Capacitive proximity control circuit design

In Figure 2, transistor VT1 and surrounding components form a radio frequency oscillation circuit; the metal sensing electrode is connected to the collector of VT1 as a detector. When no other conductor approaches the sensing electrode, 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 detection by VD1 and VD2. This signal turns on the switch tube VT2, the relay is energized and closes, and the power supply of the controlled circuit is turned on: when a conductor approaches the sensing electrode, any conductor close to the sensing electrode will sense the capacitance between the electrode and the "ground", and the increase in capacitance will reduce the positive feedback of the oscillator until the oscillator stops oscillating. If the oscillator stops oscillating, the radio frequency detection circuit will no longer output a DC control signal. At this time, the switch tube VT2 will be cut off, causing the relay to lose power and disconnect. After the relay is disconnected, the switch S needs to be released and closed again before the circuit can enter the next oscillation state, otherwise the relay will be disconnected.
C0 is the sensitivity adjustment capacitor. Adjusting its size can adjust the critical value of the oscillator's start and stop oscillation, thereby adjusting the distance, size and other items of the object controlled by the controller. In Figure 2, the inductor L1 can use any color code inductor with an inductance of 1mH-6mH. If the inductance is greater than 4mH, the value of C0 needs to be appropriately increased so that the circuit can oscillate smoothly. The other component parameters are shown in Figure 2.
Figure 3 is a capacitive induction control circuit made of integrated circuits.

The circuit in Figure 3 is mainly composed of a four-input "NAND" gate integrated circuit CD4093 (see Figure 4 for the internal circuit). R1, C1 and a NAND gate of CD4093 (pins ①, ②, ③, IC1) form a 400Hz square wave oscillator. The square wave output by the oscillator is divided into two paths: one path is directly sent to a NAND gate circuit (pins ④, ⑤, ⑥, IC2); the other path is sent to an input end of a NAND gate (pins ⑧, ⑨, ⑩, IC3) through capacitor C2. Since IC2 is connected in the form of a non-gate, the potential difference between its input and output ends is 180°. The signal output by IC2 is coupled to the other input end of IC3 through C3 and C4.
Since the levels of the two input ends of IC3 are the same and the phases are opposite, as long as IC1 oscillates normally, at least one of the two input ends of IC3 is at a low level, so the output end of IC3 is a stable high level. Due to the effect of C6, VT1 is cut off. However, if the input signal of any input terminal of IC3 is cancelled or the amplitude of the signal is reduced to a level lower than the input threshold level of the gate circuit, IC3 will output a square wave signal.
When a conductor approaches the sensing electrode, part of the signal coupled to the input terminal 9 of IC3 by C2 will be shunted to the ground. If the amplitude of the shunted signal is lower than the threshold level of the input terminal of the NAND gate, IC3 will output a square wave signal. After the signal is rectified by VD1 and VD2, the switch tube VT1 will be turned on, and the power supply of the relay will be turned on to make it close. Capacitor
C4 is a sensitivity adjustment capacitor. If the circuit needs to work at the maximum sensitivity, you can first adjust C4 to make the relay just close, then adjust C4 to make the relay just disconnect, and then seal C4 with high-frequency wax or insulating paint. The components
used in the circuit of Figure 3 are all ordinary components. Since CD4093 is a CMOS integrated circuit, it is easy to be broken down by the static electricity brought by the electric soldering iron. Therefore, when making it, it is best to solder an integrated circuit socket first, and then insert CD4093 into the socket after the circuit is checked. The sensing electrode sheet can be cut from a metal can.