Home > Circuit Blox > Infra-Red Door Unlock

UPDATED:.20:51 02 January 2019

This circuit activates a door solenoid for a fixed amount of time, when the receiver picks up a specific coded infra-red signal. The range is up to 8 metres!

The length of time the the solenoid is active can be adjusted using VR1 on the monostable.

Transmitter Circuit Voltage: 9 volts
Receiver Circuit Voltage: 6 to 9 volts
Solenoid Supply Voltage: as required, or Receiver supply

Infra-Red Transmitter/Receiver
Select the required channels using the jumpers on the Transmitter and Receiver - they must be the same.

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

Set Range to 2, (0.05-2.5s) with VR1 midway.



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 high signal is inverted to a low signal by the Inverter.

This low signal triggers the Monostable which activates a Relay for a preset period of time - up to 2.5 seconds.
Alter VR1 to change length of the unlock time. If the button is pressed for longer than the Monstable tine, the relay will stay active until the button is released.

Infra-Red communication:
  • Data Transmission
  • Domestic remote controllers

Topics could include:

  • Monostables - generating timed events
  • Calculating time using T=1.1CR
  • Relays and protecting from EMF
  • How coils can generate high voltages
  • Home Security control systems.
The Relay can be replaced with a Darlington Driver if the load is less than 3 Amps. This will simplify wiring and use the circuit supply to drive the load.

The solenoid can be replaced with any low voltage DC device up to 5 Amps with the relay. This can be applied to a variety of situations.


  • Remote controlled cat flap
  • Remote controlled garage door
  • Remote controlled lamp.