Non-touch touch delay light circuit

The touch delay lamp introduced in this example is very interesting. Its switch panel is an all-plastic structure without any metal
parts. When using it, just touch the plastic panel with your hand and the lamp will light up. After a delay, the lamp will
turn off on its own. The circuit is shown in Figure 127.
    Normally, the three alarm tubes VT1-VT3 are all in the cut-off state, the thyristor VT4 is in the off-state, and the light E
does not light up. At this time, the light-emitting tube LFD emits weak light 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 telegraph piece M (actually r is touching the metal piece Mh through the plastic panel). The voltage induced by the human body is injected into
the gate of the field effect transistor VF,
so that the current between the drain and the source
is small, and the drain potential is
VT3. The base potential is
high, and VT3 is turned on, (1),
that is, it is charged. When
VT2 is turned on due to the increase in base potential
, VT1 is also
turned on, and the thyristor vr4 gets the trigger current and is turned on, and the lamp lights up and emits light. After the human hand moves away from M,
the VF gate returns to low level and wn is cut off. At this time, the charge stored in c2 will pass through R and discharge to the VT2 emission junction,
maintaining the conduction state of VT2, and the lamp is still lit. When the above voltage drops When it reaches below 0 7V, VT2 and
VTI are cut off one after another, VT4 is turned off, and the electric lamp E goes out.
    During production, the 86 series plastic panel can be used, and a small round hole is placed at the appropriate position on the upper part to embed
the light-emitting diode. The electric trigger can be cut into a disc shape of ≠30mm from tinned iron sheet. Use non-F tape to stick it
on the back of the switch panel, and then connect it to the grid of VF with a shielded wire. The shielded wire should not be too long. Generally, it should be Control
it within 40mm, and then position the entire movement on the back of the panel. The delay time of this circuit mainly
depends on the values ​​of R:, C, and the data delay time is about 1min.