Mains-operated Capacitive Sensor

Easily adjustable, high sensitivity circuit

Minimum parts counting

Circuit diagram:

Capacitive Sensor


R1_____________220K  1/4W Resistor (See Notes)
R2______________47K  1/2W Trimmer or Potentiometer (See Notes)
R3_______________1K  1/4W Resistor

D1__________TIC106M  600V 5A SCR

LP1_____________Any small Neon Bulb
LP2_____________230V Lamp (Any wattage up to 500W) (Optional)

BZ1_____________230V Buzzer (Optional)

SW1_____________SPST Mains Switch (Optional)

PL1_________Male Mains plug & cable

Sensor______(See Notes)

Device purpose:

This very simple circuit acts as a high sensitivity capacitive sensor. Lamps and/or Buzzers are operated at half the mains supply voltage when a part of the human body comes in contact with the sensor or approaches it at a close distance.
The circuit can be used as an alternative to the Door Alarm or Capacitive Sensor circuits, already available on this website.
When used as a Door Alarm, the Lamp and/or the Buzzer are activated when someone touches the door-handle from the outside. The alarm is not self-latching, therefore the Lamp or Buzzer will switch-off as soon as the hand will be taken off.
The wide-range sensitivity control (R2) allows the use of the circuit over a wide variety of door types, handles and locks. The device had proven reliable even when part of the lock comes in contact with the wall (bricks, stones, reinforced concrete), but does not work with all-metal doors.
The circuit can be used as a simple touch control as it will operate when the left-side lead of R1 will be touched. In this case you can do without the sensor plate.
On the other hand, using e.g. a thin copper or aluminum sheet measuring 30 x 20cm for the sensor, a part of the human body can be detected at a distance of about 20cm.