Making an Induction Electroscope Using CD4069

　　1. Circuit Principle
　　The inductive electrometer is also constructed with the CMOS digital integrated circuit CD4069 six NOT gates as the core component, as shown in the figure below. The circuit consists of three parts.
　　
　　(1) Inductive probe
　　It consists of two NOT gates on the left side of the figure below. R1 is the working point adjustment resistor, which makes the (1) foot probe have a higher sensitivity (high input resistance and strong amplification ability). A copper core insulated wire with a length of about 5cm is welded at the (1) foot. When it is close to the 220V live wire, it is affected by the electric field and induces an alternating voltage. After two-stage amplification, it is output from the (4) foot and rectified by VD1 and filtered by C1 to generate a high level voltage on the resistor R2. Capacitor C3 has an anti-interference effect.

(2) Level detection and light display
　　It is composed of the two NOT gates in the middle of Figure 1. When the sensor head receives the 220V live wire electric field induction, a high level signal is generated at the (13) pin of the NOT gate. After two stages of NOT gate inversion and amplification, the high level voltage output by the (10) pin is sufficient to drive the light-emitting diode VD3 to emit light, indicating that a 220V voltage has been detected.
　　If the probe has no induced electric field, VD1 is cut off, and both the (13) and (10) pins are low level, and VD3 will not emit light.
　　(3) Sound alarm
　　When the probe has a 220V sensing signal, since the (3) pin is high level, the diode VD2 is forced to cut off, and the positive feedback multivibrator composed of the two NOT gates on the left and R3, R4 and C2 starts to oscillate. The oscillation signal is amplified by the transistor VT, driving the speaker BL to emit an audio oscillation sound, reminding people that a 220V voltage has been detected at the probe.

2. Component selection and installation and debugging
　　Select components according to the table below. The printed circuit board is shown in the figure on the right. As long as the circuit components are installed and wired correctly, the circuit can work normally. The resistance value of R1 at the input end of the circuit can be adjusted within the range of 2 to 10MΩ to obtain a more ideal sensitivity. If the two input ends of the NOT gate have too high a series working gain and produce "self-excitation" (the speaker also beeps when there is no signal), a 51 to 100 kΩ resistor can be connected in parallel across the capacitor C3 to eliminate vibration.