Five examples of touch switch circuits

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 the "on" touch chip. When a human hand touches it, the clutter signal induced by the human body is added to the ② pin of the low-level trigger terminal IC of the time base circuit. The circuit is set, and the ③ pin outputs a high level. Relay K is electrically closed, its normally open contact is closed, and the controlled electrical appliance is energized to work. M2 is the "off" touch piece. Once touched, the clutter signal induced by the human body is added to the threshold terminal IC⑥ of the 555, the circuit is reset, pin ③ outputs 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 block is connected as a monostable trigger. It is usually in the reset state and the relay K does not operate. When M is touched, the circuit is triggered into a transient state, pin ③ outputs high level, relay K is closed, and the controlled electrical appliance works. The transient time t=1.1R2 　　Figure 3 is a touch switch with a specially designed power circuit made of a 555 time base circuit. It has only two external lead wires, so it can directly replace an ordinary switch without changing the power wiring. EL is an incandescent lamp or AC contactor not larger than 25W. The left part of the dotted line is the general lighting circuit, and the right part is the touch switch circuit. When the IC is in reset state, pin ③ outputs low level, and the gate of thyristor VS is clamped at low level through resistor R3, so VS is turned off and EL does not light up. At this time, the working power of 555 is provided by 220V AC through the lamp EL. , diodes VD1 ~ VD4 rectifier, resistor R2 current limiting, VD5 voltage stabilization and IC1 filtering to obtain a DC operating voltage supply of about 6V. When the ② pin of the 555 time base circuit is triggered and is set, the IC pin ③ outputs a high level, VS is turned on, and the EL lights up. After VS is turned on, the 555 working power supply directly consists of the lamp EL, diodes VD1 ~ VD4, thyristor VS and voltage regulator VD5 to form a loop. The 6V DC working voltage can still be obtained at both ends of C1, but the resistor R2 does not work at this time. 　　The time base circuit part of the right part of the circuit is the same as Figure 1. If the power supply on the left part of Figure 2 is modified 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 determined by VD1 ~ VD4, VS and VD5. The weak link is VD5. VD5 of this circuit uses a 1W, 6V voltage regulator tube, and its maximum on-state current is 0.16 A. To ensure reliable operation of the circuit, EL should use incandescent lamps no larger than 25W. 　　Figure 4 is a touch switch made with a double D flip-flop. CD4013 is a double D flip-flop, which is connected into a monostable circuit and a bistable circuit. The function of the monostable circuit is to perform pulse broadening and shaping of the touch signal to ensure that each touch action is reliable. A bistable circuit is used to drive the thyristor VS. When a human hand touches M, the voltage drop of the alternating current leaked by the human body on the resistor R2, and its positive half-cycle signal enters the ③ pin CP1, causing the monostable circuit to flip into a transient state. Its output terminal Q1, or pin ①, jumps to 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 low level. So every time M is touched, pin ① outputs a positive pulse of fixed width. This positive pulse will be directly added to the CP2 terminal of pin 11, causing the bistable circuit to flip once, and the level of its output terminal Q2, that is, pin 13, will change once. When pin 13 is high level. VS is turned on and the electric lamp EL is on. At this time, the voltage across the capacitor C3 will drop to about 3V, and the LED VD6 will go out. Due to the micro-power consumption characteristics of the CMOS circuit, the integrated block can still work normally. When pin 13 outputs low level. VS loses the trigger current, turns off when the AC crosses zero, and EL goes out. At this time, the voltage across C3 returns to the regulated value of VD5, 12V, and VD6 lights up 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 external lead wires, so it is very convenient to install and use. Figure 5 is a touch switch made using 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 music integrated circuits have extremely high input impedance. When a human hand touches the "open" electrode sheet M1, the leakage current of the human body is injected from R1 into the trigger terminal TR1 of the music doorbell chip, causing the bistable trigger inside the chip to flip. In order to make the entire doorbell chip show low resistance, the transistor VT1 is turned on, the relay K is closed, and the normally open contact can connect the controlled electrical appliance. When you need to turn off the appliance, just touch the "off" electrode sheet M2. The human body leakage current is injected into the Vss terminal of the chip through R2, forcing the bistable trigger inside the integrated block to flip again, making the entire chip have a high resistance, and VT enters. In the human cut-off state, K is released when power is lost, and the controlled electrical appliance is powered off. 　　To ensure safety, connect a high-quality high-resistance metal film resistor with a resistance of not less than 4.7MΩ to the touch electrode.