Touch switch for backlight on transistors. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Touch switch for backlight on transistors. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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In the article by I. Alexandrov "Touch switch for backlight" in "Radio" 2001, No. 10, p. 56, a device was described for illuminating scales and function buttons of various radio equipment, made on CMOS chips.

I propose an analogue of such a device on widely used transistors (Fig. 1). As in the mentioned device, a piece of one-sided foil fiberglass 1x10 mm in size with a foil cut in the middle is used as sensor E10.

Transistor backlight touch switch

A composite transistor is assembled on transistors VT1 and VT2, which has a large current transfer coefficient, which provides the necessary sensitivity of the device. And so that the device does not respond to pickups, the base of the transistor is shunted by resistor R1. The transistor load is resistor R2, which, together with capacitor C1, form a timing chain.

The transistor VT3 has an electronic key that controls the on/off of the HL1 LED. Resistor R3 limits the current through the emitter junction of the transistor, and R4 limits the current through the LED (the resistance of the resistor is indicated for a supply voltage of 12 V).

The device works like this. In the initial state, the capacitor is charged, there is no bias based on the VT3 transistor, the LED is off. If you now touch the contacts of the sensor plate with your finger, then a bias voltage will appear at the base of the composite transistor. The transistor will open, discharging the capacitor. At the same time, the transistor VT3 will open, the LED will light up. It will glow as long as your finger touches the touch plate.

After releasing the finger, the composite transistor will close, the capacitor will begin to charge through the resistor R2, and the bias voltage at the base of the transistor will gradually decrease. After a few seconds, the LED will turn off.

It is permissible to use other transistors of the specified series in the device, for example, with the letter index G, E. The LED can be of any glow color, but with the highest possible brightness, say, AL307BM (red), AL307EM (yellow). Within certain limits, the brightness of the LED can be changed by selecting the resistor R4. Capacitor - K50-35 or another small-sized one with a minimum leakage current (the author used a foreign oxide capacitor with a diameter of 8 and a length of 14 mm). Resistors - MLT-0,125

The details of the device, except for the LED and the sensor plate, are mounted on a board (Fig. 2) made of one-sided foil fiberglass. The device is powered from the source of the structure in which it will be installed, or from a separate power supply with an output voltage of about 12V at a load current of up to 50 mA. In standby mode, the device consumes a current that is practically determined by the leakage current of the capacitor.

Transistor backlight touch switch

If the device is supposed to be used in mains-operated radio equipment, a screw screwed into the front panel and connected to the base of the transistor VT1 with a piece of mounting wire will do instead of the sensor plate. In this case, the composite transistor will open due to network pickups on the human body.

Author: E. Mukhutdinov, p. New Tikhonov, Volgograd region

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