PIR Sensor Alarm


This simple alarm can be used to detect the movement of human beings at a distance of 10 meters.It works both day and night and beeps shortly when there is a human movement through its detecting range. It is ideal to monitor the entry passages, stair case, corridors etc. It is highly useful to monitor the human movements in Flats, Multi storied buildings, Shops etc. A relay can also be connected for activating lights, electric alarms etc. It can be used as a Personal alarm during journey to protect luggage.

The PIR Sensor module is readily available in the market  for Rs.250 ( You can also purchase online through ebay). It is an assembled unit with PIR sensor, signal amplifier, presets for adjusting sensitivity range, duration of output etc. There are three connecting pins for Vcc, Ground and Output. It is marked on the module if the filter cap is removed ( pull it to remove). It works off 5-12 volt DC and the output is positive to drive the transistor. In the standby mode, the sensor sets its sensitivity by using the ambient temperature around it and the surface on which it is fixed. When a human being passes across its detecting range, the passive infrared (Heat) from the body changes the sensitivity of the sensor and its output turns high. This output is used to drive the transistor to switch on the buzzer.



For better sensitivity, mount the unit on the wall at an angle of 45 degree and a height of 2-3 meters. Avoid sunlight falling on the sensor.

To drive AC Load

The same sensor can be used to drive AC loads like Bulb, CFL, AC Bell etc.

1. Driving load using Triac

Triac can be connected to the output of sensor through a 100 ohms resistor. Sensor output is 3.3 V. Load should be a 60-100 watts bulb.


2. Driving load using Relay

Relay can be operated using the output of the sensor using a relay driver transistor. Load can be Bulb, CFL, AC Bell etc.


Warning ! The AC operated circuits should be constructed only by a person experienced in handling high volt AC. The circuits give fatal shock, if handled carelessly. Do not touch or troubleshoot when connected to mains.

3 responses to “PIR Sensor Alarm

  1. Sir, I want to ask you about circuit no 1(driving load using triac). Is it okay to replace CFL load with LED bulb? and, if I want to make it dim in open circuit (no motion) then turn on full light in close circuit (when there is motion), where should I put variable resistor to adjust its dimming? Thanks

  2. Thank you for your alarm project its very wonderful and encouraging more especially to someone like me in the field of Electronic Repair.