It would be hard to find a project with less components than this one. Sometimes infra-red (IR) remote controls develop problems with the keypad. When repairing them, it is hard to tell whether or not it is transmitting because the IR light is invisible. This tiny little project responds to the IR light pulses and blinks a red LED as they are received.
All that is required to complete this project is to solder all the components onto the circuit board. Please be careful to ensure that the red LED is on the left-hand side, and the black phototransistor is on the right. Leave their leads about 5mm long. The "flat" parts of those devices must face each other.
To use it, point the remote control down at the phototransistor from about 50mm away. The LED will blink brightly each time it receives an IR signal. When not in use, keep the project in a dark place and the coin cell will last for many years.