A simple do-it-yourself autonomous security alarm system.
Sometimes there is a need for an autonomous, inexpensive security alarm, for example, for protection in a country house. The use of industrial designs in such cases may not be economically feasible.
What do we need from an alarm system?
- Reaction to intrusion, for example with a passive IR motion sensor
- Intrusion alert with siren. The notification should work for a short time (for example, 5 minutes) and then turn off.
- After triggering, the system should go into standby mode again. If necessary, it should operate multiple times.
- low current consumption for long-term (6 months) operation in standby mode.
To make such an alarm we will need: 
What do we need from an alarm system?
- Reaction to intrusion, for example with a passive IR motion sensor
- Intrusion alert with siren. The notification should work for a short time (for example, 5 minutes) and then turn off.
- After triggering, the system should go into standby mode again. If necessary, it should operate multiple times.
- low current consumption for long-term (6 months) operation in standby mode.
To make such an alarm we will need:
Passive infrared motion sensor. For example, a sensor purchased from OBI is a light switch. Price about 300 rub.
12 V siren. In particular, a 105dB model was used, you can use any other. Price no more than 200 rub.
Other small items: Battery holder, 6 V relay, insulating tubes, wires.
So. We need to remake the motion sensor, switching it from 220V to 12V.A superficial analysis of the circuit showed that the circuit can operate with supply voltages from 7–8 V to 30 V. With a 12V supply, it is necessary to set the relay to a voltage of 6V. (12 volt does not work). Let's open the sensor. The spherical part is removed if you bend one of the supports. The halves are held on by latches.
12 V siren. In particular, a 105dB model was used, you can use any other. Price no more than 200 rub.
Other small items: Battery holder, 6 V relay, insulating tubes, wires.
So. We need to remake the motion sensor, switching it from 220V to 12V.A superficial analysis of the circuit showed that the circuit can operate with supply voltages from 7–8 V to 30 V. With a 12V supply, it is necessary to set the relay to a voltage of 6V. (12 volt does not work). Let's open the sensor. The spherical part is removed if you bend one of the supports. The halves are held on by latches.
We remove the board. As you can see, the sensor is a passive IR receiver that responds to changes in the amount of IR radiation incident on it and a simple optical system. Sensor viewing angle is 180 degrees.
The points on the left need to be powered. The “+” is the positive pole and the “-” is the negative pole from the power source. We will connect the relay coil to the points on the right. Remove the standard relay (black box).
Due to lack of space inside the spherical part of the sensor, it was decided to bring the relay through the wires into the base of the housing.
Power is supplied to the sensor through a switch. When the sensor is triggered, it supplies voltage to the relay coil. The relay is triggered and its closing contacts turn on the siren. Thanks to the relay, you can connect a large number of sirens.
Power is supplied to the sensor through a switch. When the sensor is triggered, it supplies voltage to the relay coil. The relay is triggered and its closing contacts turn on the siren. Thanks to the relay, you can connect a large number of sirens.
View from below. The siren and batteries are connected via terminals. Bottom left relay. Top right switch.
Complete system. ATTENTION! Do not use the siren without protecting your ears; despite its small size, it is very loud and can cause hearing damage.
In the end it worked out. In accordance with the regulator on the sensor, you can set the operating time of the siren after activation. From 10 sec to 8 min. Install the sensor indoors and take the siren outside. Unfortunately, after power is applied, the sensor is triggered, so it is reasonable to move the siren switch to a secret place and turn it on 5 minutes after turning on the sensor.The switch can be operated with a key, like the ignition switch of a car.
The sensor turned out to be quite economical. According to the ammeter:
Standby current 700 µA
Trigger current 1.1 mA
Siren current 200mA
A simple calculation suggests that 3.1 Ah is needed to operate for 6 months. The capacity of the alkaline battery is about 2.5 Ah. Therefore, for the winter you need 16 alkaline batteries mixed together.
Testing 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 build a security perimeter in nature around a tent, for example.
Tests at the dacha showed:
1. Glare from water causes false alarms
2. You can seal the sensor with pieces of isolette tape so that it forms blind spots.
In the end it worked out. In accordance with the regulator on the sensor, you can set the operating time of the siren after activation. From 10 sec to 8 min. Install the sensor indoors and take the siren outside. Unfortunately, after power is applied, the sensor is triggered, so it is reasonable to move the siren switch to a secret place and turn it on 5 minutes after turning on the sensor.The switch can be operated with a key, like the ignition switch of a car.
The sensor turned out to be quite economical. According to the ammeter:
Standby current 700 µA
Trigger current 1.1 mA
Siren current 200mA
A simple calculation suggests that 3.1 Ah is needed to operate for 6 months. The capacity of the alkaline battery is about 2.5 Ah. Therefore, for the winter you need 16 alkaline batteries mixed together.
Testing 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 build a security perimeter in nature around a tent, for example.
Tests at the dacha showed:
1. Glare from water causes false alarms
2. You can seal the sensor with pieces of isolette tape so that it forms blind spots.
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