Home > Circuit Blox > Intruder Alarm

UPDATED:.21:05 02 January 2019

The PIR will detect any heat source moving in an area. If someone is detected:

  • a siren is acitvated for a fixed period of time and then stops
  • a flashing beacon will activate until reset to show that an intruder was detected

The alarm is reset by pressing #1 on the Infra-Red transmitter.

It is advised to use a paper or mylar cone speaker of at least 1W. Do not use high quality speakers for home entertainment systems.

Transmitter Circuit Voltage: 9 volts
Receiver Circuit Voltage: 6 to 9 volts
Lamp Supply Voltage: as required

Infra-Red Transmitter/Receiver
Select the required channels using the jumper

Link A0 = Ch1,2
Link A1 = Ch3,4
No link = Ch5,6.

+Triangle Astable:
Use VR1 to set Siren modulation, start fully anti-clockwise

Set Range 2 or 3 for audio frequencies, VR1 sets pitch





The PIR output goes low, which connects to the "-Trigger" of the Monostable which begins its timing period and sends its output high.

The Monstable output connects to an Astable, and to a Latch which:

  • enables the Astable to function by sending its "-Reset" high, releasing it to oscillate and be modulated by the Triangle Wave Generator. The output from the Astable is boosted by the Darlington Driver and sent to a speaker.
  • sends the "Set" input high which sets the Latch output high and activates the Relay and Lamp.

The Siren will sound until the Monostable finishes timing, at which point its output will go low, forcing the Astable to reset, preventing further generation of sound. The Lamp will stay activated.


The transmitter sends a coded pulse using Infra-Red light when #1 buttons is pressed (or when #1 input is sent low). The receiver picks up the coded signal. If it is the correct signal for #1, it sends #1 output high.

The IR Receiver #1 output connects to an Inverter and the Latch which:

  • is inverted to a low signal and connected to "-Reset" of the Monostable which resets the timing sending its output low. This disables the Astable and ceases any sound being generated
  • sends the "Reset" input high on resetting the Latch output low, deactivating the Relay and Lamp.

The "Reset" input overides the "Set" input on the Latch so while a signal is received from the transmitter, the alarm will be disabled, even if the PIR is triggered.

This project covers many different blox and can be used when teaching or revising to identify their role within the circuit.

Some other relevant topics could be:

  • Data Transmission
  • Domestic remote controllers
  • Remote control mechanisms
  • Sound Generation and sound waves
  • Modulation of sound waves.
To increase the sound level at high frequency, a 10W or more bullet tweeter can produce some very loud sound with a 12v supply or more.

Beware - it is very loud!

A door or window microswitch can be added as a trigger for this alarm. Connect one side of the switch to the "-Trigger" of the Monostable (as well as the PIR connection), and the other side to 0v. You can parallel as many microswitches or PTM switches as you like.