This presence detector or proximity sensor circuit reacts in presence of any conductor object including humans. Sensitivity is adjustable with P1 which will be located at a great distance from the rest of the presence detector circuit. This circuit does not detect object movement but can act as an proximity sensor!
The sensitivity of the proximity sensor circuit can be adjusted with P1 for the desired “distance”. One of its obvious uses is to open a door automatically. For this the sensor must be placed on the front of the door.
The presence detector is made of one oscillator with T1 and a monostable. The oscillator is a Clapp one, well known for its frequency stability. The surface of the sensor acts as a capacitor for the oscillator circuit and in this configuration the frequency is around 1 MHz.
The switching time can be adjusted with P2. Do not bring metallic objects in the proximity of the circuit because if doing so the relay will stay closed! This circuit can be used as a detector of aggressive liquids, the advantage being that the surface of the sensor will not come in contact with the liquid.