Wiparat Circuit Electronics

This 1 channel infrared transmitter/receiver remote control is the cheapest and simplest you can find. The transmitter transmits a sequence of pulses on 36 KHz frequency carrier. The diodes are Schottky type because of their low voltage drop (only 0.2V). Link

Beeps if you leave open the door over 20 seconds
3V battery operation, simple circuitry
The track, closed into a small box, placed in the refrigerator in the vicinity of the light (if any) or open. With the closed doors inside the refrigerator is in the dark, the photo resistor R2 made the anti-high ( “200,000), thus [...]

This circuit turns-off an amplifier or any other device when a low level audio signal fed to its input is absent for 15 minutes at least.
Pushing P1 the device is switched-on feeding any appliance connected to SK1. Input audio signal is boosted and squared by IC2A & IC2B and monitored by LED D4. When D4 [...]