The IR-based safety alarm circuit can detect any movement and activate the alarm. This circuit is very useful in homes, banks, shops, restricted areas where an alert alarm is needed in any movement. This circuit is based on an infrared sensor where an infrared beam falls continuously on a photodiode, and when this infrared beam is broken, by any type of movement, an alarm is triggered.
The IR sensor consists of an IR LED and a photodiode, in which the IR LED emits IR radiation and the photodiode detects the radiation. The photodiode conducts the current in the reverse direction, whenever light falls on it, and the voltage across it changes, this voltage change is detected by the voltage comparator (as LM358) and generates the output accordingly.
In this IR-based safety alarm circuit, we have placed IR LED in front of the photodiode, so that the IR light can fall directly on the photodiode. Each time someone moves through this beam, the IR rays stop falling on the photodiode and the buzzer emits a beep. The buzzer automatically stops after some time, since the buzzer is connected to timer 555 in monostable mode.