A simple analog contact switch circuit design

After learning circuit knowledge for a period of time, I believe many friends hope to make circuits by themselves. However, due to the limitations of knowledge and experience, overly complex designs are not suitable for beginners, and some simpler designs are better choices. This article will introduce you to a simple analog contact switch circuit diagram. If you are interested, please take a look.

As shown in Figure 1, it is a simple analog touch shape circuit using the switching characteristics of a transistor.

When the power is turned on, VTl-VT4 are all in the cut-off state, no current flows through the relay coil, the normally open contact of the relay is disconnected, and the light-emitting diode VD2 does not light up. When the "on" switch is touched with a finger, the power supply is injected into the base of VT3 through finger resistance (approximately several hundred to several thousand ohms), R2 is injected into the base of VT3, the composite tubes VT3 and VT4 are turned on, current flows through the relay coil, and the normally open contact is closed. , VD2 lights up. Because 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 maintain the conduction state. When it needs to be extinguished, you can touch the "off" electrode sheet. Because the positive power is injected into the base of VT1 through finger resistance and Rl, VT1 and VT2 are turned on, and the collector potential of VT1 and VT2 drops.

That is, the base potential of VT3 drops, VT3 and VT4 turn from the on state to the off state, no current flows in the relay coil, the normally open contact is disconnected, and VD2 is extinguished.

See the table below for component selection.

Table 1

Installation, debugging and testing

1. Install the components according to the assembly drawing.

2. It is better to use anti-corrosion metal sheets to touch the switch electrode sheet with fingers.

3. After the check is correct, turn on the power supply.

4. Measure the voltage at both ends of C with a multimeter in the DC voltage range. Normally it is about 9V.

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 turned on, the relay is closed, and VD2 becomes brighter.

6. Touch the "off" electrode sheet with your finger. VT3 base voltage drops. VT3 and VT4 are cut off, the relay is released, and VD2 goes out.

figure 2

This article combines graphics and text to provide beginners with a simple and easy-to-use analog touch switch circuit diagram. Interested friends can try to design according to the circuit and related instructions given in the article, while learning new knowledge , To consolidate their own hands-on ability. In detail, after reading this article, you will surely have unexpected gains.

Grounding Transformer


The grounding transformer is a kind of auxiliary transformer that used to offer an earth point for the Delta-Star system. They are typically used for providing a path to ground for unbalanced load current and for fault currents on system where a suitable ground is otherwise not available.


The grounding transformers are generally designed with one of the two configurations: Zig-Zag (Zn) with or without an auxiliary winding or a Wye (Ynd) with a delta connected secondary. The Zig-Zag connection is preferred as it is more cost effective and have smaller size than the Delta-Wye unit. For more safety, neutral earthing resistors (NER) are often used in conjunction with the grounding transformer to limit neutral ground fault current magnitude. The rated voltage of the NER should be equal to the line to ground voltage of the grounding transformer. The current rating and duration should match the grounding transformer


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