Simple watchdog alarm. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Safety and security

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The proposed system is designed to perform the functions of security and apartment bells. It is designed for installation in apartments with two doors in one opening (for example, metal and wooden) and allows you to control whether the doors are tightly closed, what condition their locks are in.

In my apartment, when installing steel entrance doors, the old (wooden) doors that open inside the apartment remained in the same place. At the same time, situations began to repeat themselves, especially through the fault of the children, when the new outer doors turned out to be ajar, but not locked, while the wooden ones were closed. Such a situation could lead to undesirable consequences. To warn them, there was a need for constant monitoring of the state of the locks of the outer doors.

It was decided to develop and install a simple sound alarm system that notifies of an unlocked door, at the same time combining it with an apartment entrance bell. The alarm signal should be triggered after some time required to enter the apartment.

The diagram of the guard alarm system is shown in the figure.

(click to enlarge)

The device is assembled from available parts based on designs published in the journal "Radio" over the past years [1; 2]. Power is supplied through a small-sized T2 transformer permanently connected to the network, which provides galvanic isolation and, therefore, safe operation. The voltage on the secondary winding of the transformer must be between 10 and 20 V.

A sound generator is assembled on the VT1 transistor, which generates signals for the apartment bell modes and the door lock protection signal. The call itself is turned on by the SB1 button, which is located, as usual, outside the front door. Limit switch SA1 is mounted on a bracket in the recess of the front door jamb in such a way that when the doors are closed, the bolt of the lock fully presses and closes its contact.

The purpose of the node on the transistor VT2 is to delay the activation of the alarm signal. When the lock is opened, switch SA1 opens, capacitor C3 starts charging through resistor R10. Zener diode VD3 is used to delay the opening of the transistor VT2. The delay time depends on the ratings of these parts. If the lock is not closed after 12 seconds (and this is quite enough to pass through the door), the key on the transistor VT2 will open, supplying power through the decoupling diode VD2 to the sound generator. At the same time, the key on transistor VT4 will open, changing the bias for transistor VT1 of the sound generator. This changes the tone of the call. Such a signal informs about the open lock of the front door.

If it is necessary to open the door for a long time, for example, to load furniture, the bell can be de-energized with the SA2 switch. At the same time, the multivibrator on transistors VT3, VT5, loaded on the HL1 LED, starts working. The frequency of its flashes is selected by the capacitor C8. The blinking of the HL1 LED reminds you of turning off the alarm and the bell.

Transformer T1 can be any output from a small-sized radio receiver.

The design is mounted by volumetric mounting in a metal case and is installed high above the front door. On the front panel there is a SA2 switch, an HL1 LED and a BA1 dynamic head.

Literature

1. Yakovlev V. Electronic bell on one transistor. - Radio, 1991, No. 2, p. 81.
2. Lomakin L. Generators of light impulses. - Radio, 1974, No. 4, S. 44.

Author: S. Stashkov, Perm

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