Non-touch delay switch

Normally, the transistors VTI~VT3 are in the cut-off state, the thyristor VS is in the off-state, and the lamp H does not light up. At this time,
the light-emitting diode LED lights up to indicate the switch position, making it easier to find the switch at night . When you need to turn on the light, put your finger close to the electrode
piece M (actually touch M through the plastic panel). The voltage induced by the human body is injected into the gate of the field effect transistor VT4, which
reduces the current between the drain and the source. The drain potential , that is, the base potential of VT3, increases. VT3 is turned on, and C2 is charged. When VI2 is due to
the base potential When the voltage rises and turns on, VT1 also turns on, VS obtains the trigger current and turns on, and the lamp lights up. After the human hand
moves away from M, the VT4 gate returns to low level and VT3 is cut off. At this time, the charge stored in C2 will transmit electricity to VT2 through R2
, maintaining the conduction state of VT2, and the lamp will still be lit. When the voltage on C2 drops below 0.7V, Vr2 and VTI
are cut off one after another, vs is turned off, and the lamp H goes out.