Posted on Jun 22, 2012 8648
Capacitor Cl and body capacitance (C2>of the operator form the voltage divider from the hot side of the ac line to ground. The voltage across Cl is determined by the ratio of Cl to C2. The higher voltage is developed across the smaller capacitor. When no one is close to the touch button, C2 is smaller than Cl. When a hand is brought close to the button, C2 is many times larger than Cl and the major portion of the line voltage appears across Cl. This voltage fires the neon lamp, Cl and C2 discharge through the SCR gate, causing it to trigger and pass current through the load
The sensitivity of the circuit depends on the area of the touch plate. When the area is large enough, the circuit responds to the proximity of an object rather than to touch. Cl may be made variable so sensitivity can be adjusted.