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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Very loud alarm. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches At present, electronic TRAVEL CLOCK and similar watches with a miniature buzzer as a sound source are widely used - reminders of the due date. However, the low volume of the standard signal, as practice has shown, is often insufficient to wake up, for example, a soundly sleeping person. I also developed the connection of an external sound emitter, which can significantly increase the effectiveness of an electronic alarm clock. Such a device was assembled on the basis of an electromechanical doorbell (GOST 7220-66), which, in addition to the network, has a standard low-voltage winding for connecting a button, which was equipped with an electronic transistor key and a relay with sealed contacts (reed switch) in the load circuit. Moreover, the design of the relay used here is special, with the so-called magnetic fixation. The fact is that in this device, in addition to the coil with the reed switch, there is also a source of the polarizing field - a permanent magnet. The magnetodynamic force (MMF) of the latter, in its absolute value, although not sufficient to operate the relay itself, is quite acceptable for keeping the contacts in a position attracted to each other. As a result, there is no need for a constant flow of current through the coil while holding the relay in the closed state. The relay is switched on from a sufficiently intense pulse, at which the MMF created by the coil is added to the magnet field so that the total result exceeds the reed switch threshold. After that, the sealed contacts remain attracted to each other (closed) for an arbitrarily long time without being supplied with energy from the outside. To release such a relay, the MMF of the opposite sign is required with respect to the magnet field, when the resulting force becomes less than the reed switch release threshold. This situation can be created in various ways. For example, by turning on an additional coil, wound, as in two-position relays of the RPS series, in the opposite direction. In the recommended design, everything is solved much easier - using an external permanent magnet brought to the relay. In standby mode (when the signal from the base clock is not applied), the ends of the secondary winding of the bell are open and there is about 12 V AC voltage on them. Rectified by the VD3 diode, it is used to power the circuit. That is, the electromagnetic "stuffing" of the bell works as a power transformer with a half-wave rectifier in the secondary winding.
When the alarm goes off, a rectangular pulse voltage with a frequency of about 1 kHz appears on the standard buzzer. Through the VD1R1C1 chain, it is fed to the VT1 base, thereby opening the electronic key circuit. As a result, the collector current of the transistor increases. This leads to the operation of the relay K1, the contacts K 1.1 of which, like a bell button, close the low-voltage winding. And along with the weak buzzer of the standard clock, the door bell starts ringing loudly, automatically connected to this home-made prefix. Well, the diode VD2 prevents the "breakdown" of the transistor when the relay coil is turned off. With the closing of the reed switch, the voltage at the rectifier input becomes equal to zero, that is, the circuit is actually disconnected from the power source. However, the sealed contacts remain strongly attracted to each other, because no external power supply is required to keep the relay in this state. The bell will continue to ring with might and main until the awakened user unlocks the relay with an external magnet, bringing the reed switch to its original open state. Important note. If the signal from the base clock does not end, then the circuit starts working again after unlocking. Given this circumstance, the user has to take his time with the magnet. You can resort to his help only after thoroughly waking up from sleep. And such an awakening inevitably occurs, as practice shows, only after the end of the long regular buzzer. The scheme is not critical to the choice of most of the details indicated in it. Diodes VD2 and VD3 can be with a forward current of at least 30-50 mA and a reverse voltage of about 50 V (for example, KD102, KD103, KD509, KD510, KD521A, KD521V, KD522). KD105, D7, D226, D206-D211, D220, D223, D310 and other semiconductor valves with suitable parameters are also acceptable, but their use entails the need to remake the printed circuit board. The approach to VD1 is more rigorous. It is desirable that the selected diode has a small forward voltage (for example, it is germanium, type D9). Otherwise, it is difficult to guarantee reliable operation of the device, especially when the battery of the base clock is partially discharged. Transistor VT1 must have h21e > > 40 ... 50, Ik max >= 25 mA, Uke max> = 25 V. These are the parameters that, for example, KT315, KT361, KT312, MP20, MP21, MP25, MP26, MP37A, MP37B, MP40A. When using p-p-p transistors, it is necessary to reverse the polarity of both switching on the diodes and connecting the device input to the base clock buzzer. The relay is self-made, assembled on the basis of a serial reed switch KEM-2. The coil consists of 1500 turns of PEV2-0.1. It is wound on a frame made of thick paper and put on a reed switch. To create a polarizing field, a small-sized permanent magnet is used (for example, from a reed button). The main part of the device is mounted on a printed circuit board, which is made in advance from 1,5 mm foiled getinax or textolite by cutting the corresponding insulating grooves on the conductive layer. Resistors ULM, diodes KD509 and D9 are installed vertically. The conclusions of the KT315 transistor and the KM capacitor are soldered, trying not to deviate from the recommended topology. Well, when using parts of other sizes, they make appropriate adjustments to the board. The cutout on the board is for the relay coil. In the place circled on the sketch with a dashed line, a magnet is glued. A correctly mounted board is placed in a case, which can be a plastic box of suitable dimensions; through the holes in one of the side walls, four flexible wires of the MGSHV-1 type are output. A pair of terminals, which goes in accordance with the electrical and wiring diagrams to the base clock, is connected to the buzzer contacts in compliance with the polarity, which it is useful to clarify in advance with the help of a tester. And all because manufacturers of cheap "electronics" often make mistakes in the symbols of parts on their products. The true polarity of the voltage on the contacts of the buzzer is determined by the deviation of the arrow of the voltmeter connected to them during the arrival of the sound signal from the alarm clock, which has not yet been docked with the prefix. The docking itself can be done one-piece, connecting the wires to the buzzer contacts, or detachable, for which a micro-socket is installed on the watch case, and the set-top box input is equipped with a micro-plug (the plug-and-plug option is not shown in the illustrations). Setting up a homemade device comes down to selecting the optimal placement of the magnet, which is found by gradually moving the latter towards the reed switch. At the moment of "sticking" of sealed contacts mark the location of the magnet. Then they begin to slowly move it away to a distance sufficient to open the reed switch. Equidistant from these "critical places" is the optimum, where the magnet is glued to the board. The value of the resistor R2 is selected according to the reliable operation of the relay. However, one should not get carried away looking for too little resistance, because this can lead to the appearance of an extremely high collector current during the operation of the VT1 transistor. Author: D.Volkov
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