Capacitive Sensors Circuit

This circuit has 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. Here’s the figure of the circuit;

Parts:
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

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.