The key component of the IR detector is a sensitive photo transistor 2N5777. All infrared remote controls transmit pulse bursts. The photo transistor detects the modulated IR signal from the remote control handset under test and its output is processed by next BC557 and fed to the final LED driver transistor BC557.
As a result the red LED in the circuit starts blinking in tune with the received signal. Please note that the LED lights up constantly when day light or other source of continuos IR light is detected.
Infrared Detector Circuit Diagram
The infrared detector circuit may be powered by just about any standard 9V compact battery or a wall adaptor capable of supplying 9VDC at about 100mA.The only setting up required is to adjust the 100K variable resistor.The construction drawing shows how the circuit may be cased in a small ABS case.
Block Diagram of the IR detector
source : electroschematics