Five examples of touch switches

Figures 1 and 2 show a double-key touch switch and a single-key touch delay switch made using a 555 time base circuit. In Figure 1, M1 is an "on" touch piece. When a person touches it, the noise signal induced by the human body is added to the low-level trigger terminal IC of the time base circuit, pin ②, the circuit is reset, pin ③ outputs a high level, relay K is energized and attracted, its normally open contacts are closed, and the controlled electrical appliance is powered on. M2 is an "off" touch piece. Once touched, the noise signal induced by the human body is added to the threshold terminal IC⑥ of the 555, the circuit is reset, pin ③ outputs a low level, the relay loses power and trips, and the controlled electrical appliance stops working.

Figure 2 is a delay switch circuit. The 555 integrated circuit is connected as a monostable trigger. It is usually in the reset state, and the relay K does not work. When M is touched, the circuit is triggered to enter the transient state, the ③ foot outputs a high level, the relay K is energized, and the controlled appliance works. The transient time t=1.1R2 X C4, the transient time ends, the circuit flips to the steady state, the relay K is released, and the controlled appliance stops working.

Figure 3 is a touch switch made of a 555 time-base circuit with a specially designed power supply circuit. It has only two lead wires to the outside, so it can directly replace ordinary switches without changing the power supply wiring. EL is an incandescent lamp or AC contactor of no more than 25W. The left part of the dotted line is the ordinary lighting circuit, and the right part is the touch switch circuit. When the IC is in the reset state, the 3rd pin outputs a low level, and the gate of the thyristor VS is clamped at a low level through the resistor R3, so VS is turned off and EL is not lit. At this time, the working power supply of 555 is supplied by 220V AC through the lamp EL, diodes VD1~VD4 rectification, resistor R2 current limiting, VD5 voltage stabilization and IC1 filtering to obtain a DC working voltage of about 6V. When the 2nd pin of the 555 time-base circuit is triggered and in the set position, the 3rd pin of IC outputs a high level, VS is turned on, and EL lights up. After VS is turned on, the 555 working power supply directly forms a loop through the lamp EL, diodes VD1~VD4, thyristor VS and voltage regulator VD5. Both ends of C1 can still obtain a 6V DC working voltage, but the resistor R2 does not work at this time.

The right time base circuit of the circuit is the same as Figure 1. If the left power supply of Figure 2 is changed according to Figure 3, a touch delay switch with only two external leads can be easily made. One thing that needs special attention is that the load capacity of this circuit is jointly determined by VD1~VD4, VS and VD5, among which the weak link is VD5. In this circuit, VD5 uses a 1W, 6V voltage regulator tube, and its maximum on-state current is 0.16A. To ensure reliable operation of the circuit, EL should use an incandescent lamp of no more than 25W.

Figure 4 is a touch switch made of dual D flip-flops. CD4013 is a dual D flip-flop, which is connected to a monostable circuit and a bistable circuit. The function of the monostable circuit is to stretch and shape the touch signal to ensure that each touch action is reliable. The bistable circuit is used to drive the thyristor VS. When a person touches M, the voltage drop of the AC leaked by the human body on the resistor R2, and its positive half-cycle signal enters the CP1 end of pin ③, causing the monostable circuit to flip into a transient state. Its output terminal Q1, i.e., pin ①, jumps to a high level. This high level charges C1 through R3, causing the potential of ④ to rise. When it rises to the reset level, the monostable circuit is reset and pin ① returns to a low level. Therefore, every time M is touched, pin ① outputs a positive pulse of a fixed width. This positive pulse will be directly added to the CP2 end of pin 11, causing the bistable circuit to flip once, and its output terminal Q2, i.e., pin 13, will change its level once. When pin 13 is at a high level, VS is turned on and the light EL lights up. At this time, the voltage across capacitor C3 will drop to about 3V, and the light-emitting tube VD6 will go out. Due to the micro-power consumption characteristics of the CMOS circuit, the integrated circuit can still work normally. When pin 13 outputs a low level, VS loses the trigger current, and when the AC power passes through zero, it is turned off, and EL goes out. At this time, the voltage across C3 returns to the regulated voltage value of VD5, 12V, and VD6 emits light to indicate the position of the switch. It can be seen that every time M is touched, the light can be turned "on" or "off". It has only two lead wires to the outside, so it is very convenient to install and use.
Figure 5 is a touch switch made of a music doorbell chip KD-9300. The circuit is novel and interesting. It cleverly uses the function of the internal circuit of the doorbell chip and uses the leakage current of the human body to control the bistable trigger inside the chip. We know that the music integrated circuit has a very high input impedance. When a person touches the "open" electrode M1, the leakage current of the human body is injected into the trigger terminal TR1 of the music doorbell chip through R1, causing the bistable trigger inside the chip to flip, so that the entire doorbell chip presents a low resistance, the transistor VT1 is turned on, the relay K is energized, and the normally open contact can connect the controlled appliance. When you need to turn off the appliance, just touch the "off" electrode M2, and the leakage current of the human body is injected into the Vss terminal of the chip through R2, forcing the bistable trigger inside the integrated circuit to flip again, so that the entire chip presents a high resistance, VT enters the cut-off state, K loses power and releases, and the controlled appliance is powered off.

To ensure safety, the resistance connecting the touch electrode sheet must not be lower than 4.7MΩ high-quality high-resistance metal film resistor.