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

What do we need from signaling?
- Reaction to intrusion, eg passive infrared motion sensor
- Notification of an invasion by a siren. The notification should work for a short time (for example 5 minutes) and then turn off.
- After operation, the system must again enter standby mode. If necessary, it must operate repeatedly.
- Low current consumption for a long (6 months) operation in standby mode.

To make such an alarm, we need:

Passive infrared motion sensor . For example, in the sensor purchased in the OBI - the light switch. The price is about 300 rub.
Siren at 12V . In particular, the 105dB model was used, you can use any other. The price does not exceed 200 rubles.
Other fines: Battery holder, 6 V relay, insulating ducts, wires.

So. We need to remake the motion sensor , moving it from a power supply of 220V to 12V. Surface analysis of the circuit showed that the circuit can work at supply voltages from 7-8 V to 30 V. With 12 V power supply, it is necessary to install a relay to voltage 6V. (12 volt does not work). Let's open the sensor. The spherical part is removed if one of the supports is bent. The halves are snap-fastened.

We are extracting the fee. As you can see, the sensor is a passive IR receiver , which reacts to the change in the amount of IR radiation to the incoming and simple optical system. The viewing angle of the sensor is 180 degrees.

You need to supply power to the points on the left. On the "+" positive pole and on "-" negative from the power source. To the points on the right we connect the relay winding. Establish the relay (black box) to dismantle.

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

A switch is supplied to the sensor through the switch. When the sensor is triggered it supplies voltage to the relay winding. The relay operates and with its closing contacts the siren will turn on. Thanks to the relay, you can connect a large number of sirens.

Bottom view. The siren and batteries are connected via terminals. At the bottom left of the relay. Top right switch.

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

In the end, it turned out. In accordance with the regulator on the sensor, you can set the operating time of the siren after the trigger. From 10 seconds to 8 minutes. The sensor should be installed indoors and the siren led to the street. Unfortunately, after the power is applied, the sensor is triggered, so it is reasonable to put the siren switch in a hiding place and turn it on after 5 minutes after the sensor is turned on. The switch can be switched by a key, like the car's ignition switch.

The sensor turned out to be quite economical. Judging by the ammeter:
Current in standby mode 700 μA
Current in the operating mode 1.1 mA
Siren current 200mA
An uncomplicated count indicates that 3,1 A * h is needed to work for 6 months. The capacity of the alkaline battery is about 2.5 Ah. Consequently, for winter, you need 16 alkaline batteries connected mixed.

The check in the freezer 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 erect a security perimeter in the nature around the tent for example.

Tests at the dacha showed:
1. Glare from water cause false triggering
2. It is possible to seal the sensor segments so that it can form blind zones.