Adapter for sound alarm. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Security devices and object signaling

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Today, any industrially manufactured device can be supplemented, adapted to the specific domestic needs of a radio amateur and give the device a seemingly non-core second life.

For example, a widespread sound alarm device, the appearance of which is shown in the photo, has excellent capabilities at low cost and small size.

Sound signaling unit with 95 dB sound pressure

The node is powered by two AA elements (total voltage 3 V). The design consists of two parts (clearly visible in the photo) - one contains a permanent magnet, the other contains a receiving unit with an AF generator and a reed switch sensor with normally closed contacts. Due to the autonomy of the power supply, the device is placed on doorways and movable doors so that when the door is closed, the permanent magnet is near the receiving unit (the reed switch contacts are open), and when the door is opened, the reed switch contacts close, giving a control signal to the AF generator.

If you open the case of the receiving (main) device, it turns out that inside there is a board with a "flooded" generator chip (which, if desired, is assembled using 2 ... 3 logic elements of the K561 series microcircuits).

The circuit diagram copied from this node is shown in the figure.

Schematic diagram of the device with an additional node (shown in dotted line)

I wonder how such a powerful sound is obtained with a relatively weak power supply. Depot in a few moments: a current amplifier is assembled on the VT1 transistor, thanks to which amplified rectangular pulses (from the GN generator) are supplied to the primary winding of the matching transformer. When the alarm is activated in the base of the transistor VT1 (generator output GN), there is a pulse amplitude of 2,5 V, on the VT1 collector the amplitude is already 4,5 V, and on the secondary winding T1 the amplitude (pulse span) is already slightly more than 20 V.

Sound emitter - piezoelectric primer ZP-22 can be replaced by another one with similar characteristics. The peculiarity of the generator pulses is that they are compatible with the resonant frequency of the HA1 capsule and emit a signal that is not of one tone, but "iridescent", attracting attention with its sharp, loud and, I would say, unusually nasty sound.

In addition, the sound capsule in the body is covered with a shutter, which provides even more resonance and enhances the volume of the sound.

This device, along with the undoubted "pluses", has a couple of drawbacks.

Power autonomy (although in some circumstances this is a positive quality: batteries last for many months, taking into account 1 ... 3 short-term operations per day). This shortcoming can be easily eliminated by connecting a stationary DC voltage source of 3...5 V with a high voltage stabilization and filtering coefficient. You should not increase Un above 5 V - you can disable the generator of the device. There are now a lot of descriptions of circuits for such power supplies (adapters) with the indicated characteristics in amateur radio literature, so I will not give it here. The current consumption from a power source with a voltage of 5 V in signaling mode is 55 mA, and in rest mode it is not fixed at all (due to its smallness). The main thing is good voltage stabilization and filtering by oxide capacitors at the output of a power source with a high (at least 3300 uF) capacitance. With insufficient filtering of the stationary voltage source, the sound signal will give off a network "background" with a frequency of 50 Hz and characteristic interference in the form of hoarseness and crackles. When a stationary power source is connected, of course, the device’s mobility and autonomy are lost (it’s bad if the 220 V voltage is turned off), but this moment depends on the radio amateur’s plans for using the structure.

The ability to turn off the sound alarm (the device has a standard SA1 switch) can also be considered both a plus and a minus of the design - it all depends on its specific application. So, for example, in the author's version, the node provides protection for a safe with civilian and hunting weapons. Therefore, for critical cases (as well as for the protection of apartments), it is necessary to introduce a node with self-locking, which, once turned on, will signal with a sound until power is supplied to the node. And to provide a secret alarm switch is also easy and anyone can do it. The changes relate to the introduction of a thyristor VS1 into the circuit (as shown in the diagram by a dotted line), a break in the conductor marked with an "X" and short-circuiting the contacts of the SA1 power switch.

Now, when disconnecting the two parts of the structure, installed, for example, on the doors of the safe, the positive potential will go to the control electrode of the thyristor VS1, open it and, due to this, the supply voltage will be applied to the generator assembly.

The alarm will be activated until the power is turned off (at least for a short time).

The sound pressure power of the device is so high that the signal is clearly audible during the day (and even more so at night) with different strengths on all floors of the entrance of an apartment building (in case of unauthorized opening of the door of a protected apartment).

Author: A.Kashkarov

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