Home > Circuit Blox > PIR Sentry

UPDATED:.00:58 03 January 2019

This is a basic PIR controlled siren, ideal for warding off uninvited visitors at a place you don't often visit, a barn or remote building perhaps. This one was put on the landing of a property being renovated with the sensor aimed at the downstairs hallway.

The PIR sensor enables the siren to function when it detects body heat from someone. After a set amount of time, the siren ceases until the PIR makes another detection.

The speaker used is a piezo horn of about 10Watts. When run at 12 volts, it is very very loud, beware!

Circuit Voltage: 6 to 12 volts




Adjust the warble rate using VR1. Start with PR1 set at minimum, fully anti-clockwise.

Set Range 2 - (31Hz-850Hz) and use potentiometer to change frequency.
Or Range 3 - (700Hz-18KHz, can also produce some very piercing sounds)

Set Range 3 - 2 seconds or more sounding the siren, use potentiometer to change duration.

The PIR sensor output goes low when a presence is detected.

The PIR connects to the trigger input of the monostable. When triggered low, the monostabl starts its timing by sending its output high.

The output from the monostable connects to the Reset input on the AStable. When the input is high, the astable starts to oscillate.

The Triangle Wave astable connects to the FM (Frequency Modulation) input of the Astable which causes the output frequency to vary in proportion with the voltage. The more voltage, the more deviation from the original frequency creating a rapidly rising and falling tone.

The output from the Astable is connected to the Darlington Driver which boosts current for the speaker.

By turning VR1 on the Triangle Wave Astable gradully clockwise, the speed of the warbling will increase to a point where it matches the frequency of the second astable. This can produce some very unusual sounds. For a standard siren, turn VR1 fully anti-clockwise


Topics counld include:

  • Sensors in potential dividers.
  • Frequency Modulation and associated terminology
  • Principle of radio transmission
  • Sound generation

    Can be a stepping point to more complex alarm systems such as:

    Intruder Alarm

    Panic Alarm

    The circuit makes an alarm system or warning system for many applications auch as animals or houssehold pets to prevent them from entering an area or getting to close to a road.