Sound signaling device to the power supply unit. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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A portable radio, tape recorder or other device in stationary conditions receives electricity from a power supply connected to the lighting network. In this case, there is a risk, leaving the house and turning off the receiver, tape recorder with their own switch, leaving the power supply energized. If damage happened in it in your absence, the damage could be very great. An audible signaling device, the scheme of which is shown in fig. 1.

On the transistor VT2 and transformer T1, an audio frequency oscillation generator is assembled according to the inductive three-point circuit. Capacitor C1, together with the primary winding of the transformer, determines the pitch of the tone reproduced by the dynamic head BA1. The operation of the oscillator is controlled by a cascade on the transistor VT1, in which the collector-emitter junction is connected between the base of the transistor VT2 and the plus of the power supply. The diode VD2, connected in parallel to the emitter junction of the transistor VT1, is in the load power circuit. Another diode-VD1 is in the emitter circuit of the transistor VT2.

When the load is turned on, its current creates a voltage drop across the diode VD2, which opens the transistor VT1. As a result, the voltage at the base of the transistor VT2 is positive relative to the emitter, determined by the voltage drop across the diode VD1. Due to this, the transistor VT2 is closed and the signaling device is "silent".

If the load is turned off by its switch (SA1), the current through the diode VD2 and the voltage drop across it disappear, the transistor VT1 closes. The oscillator comes into operation, a sound signal is heard from the dynamic head BA1, reminding you to disconnect the power supply from the network.

Fig. 1

Fig. 2

In the signaling device, in addition to those indicated in the diagram, other transistors of the MP39-MP42 series can also be used. Transformer T1 - output from Selga-404 receivers, dynamic head - 0.1GD-8, 0.25GD-19 or other small-sized with a voice coil with a resistance of 8 ... 10 Ohm. Instead of a VD1 diode, any silicon one (D226, D206 series) is suitable, instead of VD2 it is better to use germanium, for example, the D312 series with a load current of up to 50 mA, D311A with a load current of up to 80 mA or D310 with a higher current. Capacitor C1 can be MBM, KLS, resistors - MLT, MT with a power of 0,125 W or more.

It is good to mount the parts of the signaling device on a printed circuit board made of foil fiberglass (Fig. 2). The thick lines in the drawing indicate the slots in the foil that isolate the printed conductors from each other. Of course, the board can be made by etching, but then you need ferric chloride.

Establishing the generator is reduced to the selection of capacitor C1, which determines the tone of the signal. It may also be necessary to refine the resistance of the resistor R1 in order to ensure a clear start of the generator.

If you do not have the required transformer, the signaling device can be assembled according to the diagram shown in fig. 3. On transistors VT1, VT3 of different structures, an asymmetric multivibrator is made, the generation frequency of which is changed by selecting capacitor C1. The sound emitter is a dynamic head or an electromagnetic capsule.

Transistor VT1 - any of the MP35-MP38 series, the rest of the details are similar to those of the previous version. They are located on the printed circuit board in accordance with Fig. 4.

Any of the signaling devices should be mounted on a common base with a power supply.

Fig. 3

Fig. 4

Author: Yu. Prokoptsev, Moscow; Publication: cxem.net

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