1. Working principle

The following figure is a simple analog touch shape circuit using the switching characteristics of crystal triode.

When the power is connected, vtl-vt4 is in the cut-off state, no current passes through the relay coil, the normally open contact of the relay is disconnected, and the LED VD2 is not on. When you touch the “on” switch with your finger, the power supply is injected into the VT3 base through the finger resistance (about hundreds to thousands of ohms), the composite tubes VT3 and vt4 are connected, there is current in the relay coil, the normally open contact is closed, and VD2 is on. Since the normally open switch is closed, R3 is connected to the VT3 base circuit. After the finger leaves the electrode, VT3 and vt4 can still remain on. When it is necessary to turn off, you can touch the “off” electrode piece. Since the positive power supply is injected into the VT1 base through the finger resistance and RL, VT1 and vt2 are turned on, and the collector potential of VT1 and vt2 decreases.

That is, the base potential of VT3 decreases, VT3 and vt4 change from on state to off state, no current passes through the relay coil, the normally open contact is disconnected, and VD2 is extinguished.

2. See the following table for component selection

3. Installation, commissioning and testing

(1) Install all components according to the assembly drawing.

(2) It is better to use antirust metal sheet when touching the switch electrode with fingers.

(3) After checking, turn on the power supply.

(4) Use the DC voltage gear of the multimeter to measure the voltage at both ends of C, which is about 9V normally.

(5) Touch the “on” electrode sheet with your finger, the base voltage of VT3 increases from 0 to about 1.4V, VT3 and vt4 are on, the relay is closed, and VD2 lights up.

(6) Touch the “off” electrode with your finger. The base voltage of VT3 drops. VT3 and vt4 are cut off, the relay is released and VD2 is off.

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