The most economical indicators. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Indicators, sensors, detectors

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Many portable radios do not have dial illumination or even a power-on indicator. The scheme in Fig. 1 allows you to control the inclusion and illuminate the scale. It is enough to touch the touch sensor with your finger and the LED will flash brightly. An amplifier is assembled on transistors, the DC resistance of which depends on the signal level on the sensor. The signal is formed by the mains voltage pickup, which is received by the user's body and transmitted to the sensor.

If the supply voltage is too high, other LEDs or a resistor should be introduced in series with the diode, the resistance of which must be selected.

The circuit was designed taking into account that most silicon transistors are npn and most germanium transistors are pnp. The touch sensor is connected to the base of a silicon transistor (for example, KT315), and a germanium transistor (for example, MP20) plays the role of a power sensor. The disadvantage of the circuit is that it does not work "in nature", when the receiver is powered by an internal power source (chemical cells and batteries). Savings from the use of this indicator is achieved by the fact that it works as long as necessary.

Sometimes it becomes necessary to operate the indication at very low voltages [1]. To operate LEDs from one or less volts, you can use one of the two circuits in Fig. 2, created on the basis of a low-voltage reed relay.

The relay can be made independently by winding the coil around the switching reed switch. You can also use the more common reed switch with a normally open contact by bringing the magnet closer to it (Fig. 3). The magnet is designed to counteract the magnetic field of the coil, its position is determined empirically.

Voltage multipliers can also be made non-mechanical, on transistors. The circuit of Fig. 4 on a complementary pair of transistors forming a generator is very popular.

Energy is stored in an inductance (in a coil). As a result of transient processes (the addition of induction and self-induction currents), voltage surges are obtained that exceed the voltage of the power source and are quite sufficient for the operation of the LED. Interestingly, the connection of the LED in this circuit does not increase, but reduces the current consumption. If we use a headphone with a resistance of about 100 ohms (for example, TK-67-NT) as a storage choke, we will also get an audible alarm without increasing current consumption.

The LED can be moved to the earpiece (Fig. 5), which again will not increase the current consumption. And if you turn on a low-resistance dynamic head in series with the signaling device (for example, 0,1 ... 0,5 W), then the volume of the signaling device will increase approximately 4 times. The circuit consumes only 1 mA, since it operates in a pulsed mode.

A variable resistor sets the optimal mode of operation of the circuit. It is desirable that its resistance exceeds 10 kOhm. The pnp structure transistor (marked with one) must be germanium (silicon will not work). With a low-voltage supply of this generator, the second transistor must also be germanium.

You can also assemble the converter on one germanium transistor (Fig. 6), using a ready-made blocking transformer (frame) from an old tube TV. The resistance of the resistor depends on the supply voltage and approaches zero at 0,2...0,3 V [2].

The idea of ​​an intermittent glow of the indicator, which is not only more economical, but also more noticeable to the eye at certain frequencies, can be used to power lamp indicators. The circuit in Fig. 7 allows the MTX-90 lamp or a conventional phase indicator to flicker.

Details. A rectifier diode of the D226 type, etc., a capacitor of 0,5 μFCh100 V. A 100 kΩ resistor in the lamp power circuit (marked with a dotted line) serves to extend the duration of the light flashes, but the circuit works without it. The frequency of flashes depends on the capacitance of the capacitor.

References:

1. Proksin I.N. Indicators for connecting electrical appliances to the 220 V network//Electric. - 2001. - No. 2. - P.10.
2. Minster V. Schemes "not according to the rules" // Young technician. - 1985. - No. 5. - P.47.

Author: Y. Bearded

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