This is an infrared gate with two sensors planned to use in the wall in the way behind a door. It can be applied in a toilet to keep track of that someone is inside exceeding a certain amount of time. After that time elapsed, the circuit triggers the digital output wich can turn on a ventillator. The time period the output is turned on can be separately controlled by a second timer.source

This is a lamp timer capable of operating two separate relay switches. Outputs can be in three (or restricted to two) states: OFF, delayed ON and constant ON. Delayed ON mode is indicated by the LEDs. The source code is easily customizable for your needs: set your desired delay time or disable constant ON state.source

The circuit detects a sudden shadow falling on the light-sensor and sounds the bleeper when this happens. The circuit will not respond to gradual changes in brightness to avoid false alarms. The bleeper sounds for only a short time to prevent the battery running flat. Normal lighting can be used, but the circuit will work best if a beam of light is arranged to fall on the light-sensor. Breaking this beam will then cause the bleeper to sound. source

These circuits use a CdS photocell to simulate motion detection, by sensing shadows and other small disturbances in ambient lighting caused by nearby people.Reverse engineered circuit diagram of the sensor circuitry out of a very sensitive children’s photoelectric Room Gaurd toy. The original toy would talk and make alarm sounds, whenever an intruder walked by or cast any kind of shadow onto the photocell. This circuit has immunity to battery voltage transients. source

This circuit allows you to construct a electronic switch which will automatically turn on your tape recorder when you pick up the handset of your telephone.So that your tape recorder must have both a MIC socket and a REMOTE socket on it so that circuit can plug into the recorder and control it.This circuit will allow you to record both sides of your telephone conversations.source