Making an Induction Electroscope Using CMOS Integrated Circuit

Taking advantage of the extremely high input impedance of CMOS integrated circuits, you can make your own inductive electroscope.

1. Circuit Principle

The inductive electrometer is also constructed with the CMOS digital integrated circuit CD4069 six-inverter gate as the core component, as shown in the figure below. The circuit consists of three parts.

(1) Induction probe

It is composed 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 two NOT gates in the middle of Figure 1. When the induction head is inductively acted upon by the 220V live wire electric field, a high level signal is generated at the NOT gate's (13) pin. After being inverted and amplified by two stages of NOT gates, the high level voltage outputted 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, pins (13) and (10) are both at low level, and VD3 will not emit light.

(3) Audible alarm

When the probe has a 220V sensing signal, the (3) pin is at a high level, forcing the diode VD2 to be cut off, and the positive feedback multivibrator composed of the two non-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, 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.