The PIR burglar alarm explained here will detect a human intruder within the stipulated range and sound an alarm. Thus, the system could be effectively use for detecting trespassing, theft, intrusion, burglars, or any form of unlawful entry inside a restricted area or zone.
The operating principle of the proposed burglar alarm circuit can be understood by referring to the following block diagram. The design looks very simple, since most of the complex infrared detection is carried out by the advanced PIR module itself.
The PIR module converts the IR radiation from human body into corresponding electrical signal. These electrical signals are amplified by a single transistor amplifier which also works like a relay driver controller stage.
When a human intrusion from a burglar is detected, the PIR comverts the heat map of the intruders body into tiny electrical pulses.
The transistor receives these pulses and amplifies it sufficiently to drive a relay.
The relay switches ON, and sounds the connected alarm through its contacts.
How the Circuit Works
In the above paragraph we learned about the basic block diagram and the working principle of the burglar alarm using a PIR sensor.
Now let's try to understand the precise details of the circuit configuration.
The design is based on a PIR motion sensor circuit which can be easily built using the following basic set up and applied as a burglar alarm circuit.
Here we can see, the system can be assembled using just 5 fundamental components, which are: 1) The PIR Module 2) A 1 k resistor, 3) An NPN transistor, 4) A Relay, 5) An Alarm Siren unit.
The PIR has 3 terminals labelled as the Vcc, OUT, and the GND or ground. The Vcc is supplied with a +12V via the 1k resistor, the GND with the 0V from a DC supply which could be from any standard 12 V AC to DC adapter.
Although the PIR is rated to work with a 3.3 V from a 5 V DC supply, for the sake of simplicity a 5 V regulator is avoided. Instead a 1 k limiting resistor is used for dropping the 12 V supply to the required 3.3 V. The 3.3 V is achieved through a 3.3 V regulator which is provided inside the PIR module PCB. Thus, the PIR works safely from a 12 V supply even without external voltage regulator circuitry.
The 1 K resistor also allows the circuit to eliminate the transistor base resistor, which further simplifies the shown PIR burglar alarm circuit concept.
The Relay Driver And Alarm Stage
The base of the relay driver transistor is directly attached with the OUT pin of the PIR for receiving the detected signal from the PIR module.
The transistor acts like a small signal amplifier as well as an effective relay driver stage.
Whenever the PIR detects a human presence which may be from an intrusion from a possible burglar or an attempt of theft, it converts the infrared heat map detection from the human body into corresponding 3.3V electrical pulses.
This DC pulse being higher than the 0.7 V switching bias for the transistor, the transistor instantly switches ON and activates the relay.
The relay contacts move from its initial N/C to N/O position which triggers ON the the connected alarm system.
Using an External Alarm
The burglar alarm system or the siren unit attached with the relay contact can be a homemade alarm circuit, or simply an external ready made siren, depending on the user preference.
The explained simple PIR burglar alarm circuit looks too simple to build yet provides an outstanding protection from all forms of human interventions, or intrusion inside a restricted zone, such as shops, homes, offices, lands, etc. The system could be wired with a wireless transmission also for sending the detected intrusion to another destination such as police station or the owners residence.