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Nov 24, 2021
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Simple autonomous security alarm system with your own hands.

Sometimes there is a need for an autonomous, inexpensive security alarm, for example, for protection in the country. The use of industrial designs in such cases may be economically impractical.

What do we need from an alarm?
– Reaction to intrusion, for example by passive infrared motion sensor
– Siren intrusion alert. The notification should work for a short time (for example, 5 minutes) and then turn off.
– After triggering, the system should switch to standby mode again. If necessary, it should be triggered multiple times.
– low current consumption for long-term (6 months) operation in standby mode.

To make such an alarm, we need:

Passive infrared motion sensor. For example, a sensor purchased from OBI is a light switch. The price is about 300 rubles.
Siren for 12 V. In particular, a model with 105dB was used, you can use any other. The price is not more than 200 rubles.
Other little things: Battery holder, 6 V relay, insulating tubes, wires.

So. We need to remake the motion sensor by switching it from 220V to 12V. A superficial analysis of the circuit showed that the circuit can operate at supply voltages from 7–8 V to 30 V. When using 12V, it is necessary to set the relay to 6V. (12 volt does not work). Let’s open the sensor. The ball-shaped part is removed by bending one of the supports. The halves are held by snaps.

We remove the board. As you can see, the sensor is a passive IR receiver that reacts to changes in the amount of IR radiation that falls on it and a simple optical system. The sensor’s viewing angle is 180 degrees.

Power must be applied to the points on the left. On “+” the positive pole and on “-” negative from the power supply. We will connect the relay coil to the points on the right. Dismantle the standard relay (black box).

Due to the lack of space inside the ball-shaped part of the sensor, it was decided to bring the relay into the base of the case through the wires.

Power is supplied to the sensor through the switch. When the sensor is triggered, it energizes the relay coil. The relay is activated and its closing contacts will turn on the siren. Thanks to the relay, a large number of sirens can be connected.

Bottom view. Siren and batteries are connected via terminals. On the bottom left is the relay. Above right is a switch.

Complete system. ATTENTION! Do not turn on the siren without protecting your ears, despite its small size, it is very loud and may damage your hearing.

In the end it turned out. In accordance with the regulator on the sensor, you can set the operating time of the siren after triggering. 10 sec to 8 min. Install the sensor indoors and bring the siren outside. Unfortunately, after turning on the power, the sensor is triggered, so it is reasonable to bring the siren switch to a secret place and turn it on 5 minutes after turning on the sensor. The switch can be switched with a key, like a car ignition lock.

The sensor turned out to be quite economical. Judging by the ammeter:
Standby current 700 μA
Operating current 1.1 mA
Siren current 200mA
A simple calculation suggests that 3.1 A * h is needed to work for 6 months. The capacity of an alkaline battery is about 2.5 A * h. Therefore, for the winter you need 16 mixed alkaline batteries.

A freezer test showed that the system works even at -32

Added later: given our turbulent times and the autonomy of the system, it can be used to build a security perimeter in nature around the tent, for example.

Tests in the country have shown:
1. Glare from water causes false alarms
2. Segments of insulating tape can be used to glue the sensor so that it would have blind spots.

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