The timer circuit is driven by a 555 timer. The circuit itself was originally borrowed from a classic Forrest Mims book, though it was slightly modified to accommodate the PIR sensor.

This is a very simple touch sensor switch that is build with the 555 timer IC. You just need to touch the metal plate and the relay gets energised and is kept in this state for about 100 seconds, then ...

This motion detector alarm can detect a moving person from a distance of 1 meter. It uses a dual IR Transmitter-Receiver module HOA1405. When the sensor detects the reflected IR rays, alarm will sound ...

The project is aimed at Arduino-based circuits, but any chip with an ADC will work. The 555 timer is used as a free running oscillator.