Capacitive Sensor

Posted in Other sensor, Switches and relays, on 2015-09-12

Circuit diagram:

Capacitive Sensor


  • R1,R2_____1M 1/4W Resistors
  • R3,R4____47K 1/4W Resistors
  • C1_______10µF 25V Electrolytic Capacitor
  • C2______470pF 630V Ceramic or Polyester Capacitor
  • D1-D3____1N4002 100V 1A Diodes
  • Q1-Q3_____BC337 45V 800mA NPN Transistors
  • RL1_______Relay with SPDT 2A @ 220V switch Coil Voltage 12V. Coil resistance 200-300 Ohm
  • J1________Two ways output socket
  • Sensor____Aluminium or copper thin sheet with the dimensions of a post-card, glued at the rear of the same (about 15x10.5 cm.) Thin screened cable

Circuit description:

The purpose of this circuit is to animate shop-windows by means of a capacitive sensor placed behind a post-card-like banner. The card is placed against the glass inside the shop-window, and the visitor can activate the relay placing his hand on the card, from the outside. Especially suited for toy-shops, the circuit can activate model trains, small electric racing cars, lights etc. Further applications are left at user's imagination. Adopt it to increase the impact of your shop-window on next Christmas season!

Q1, Q2 & Q3 form a high impedance super-Darlington that drives the relay, amplifying the 50 or 60Hz alternate mains-supply frequency induced in the sensor by the human body. C1, D2 & D3 ensure a clean switching of the relay. Power supply can be any commercial wall plug-in transformer adapter with rectifier and smoothing capacitor, capable of supplying the voltage and current necessary to power the relay you intend to use.


For proper operation, circuit ground must be connected via a small value, high voltage-rating capacitor to one side of the mains supply socket. The "Live" side is the right one.

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