Use NE555 to make push-button delay energy-saving lamps (2)

The delay energy- saving lamp circuit is shown in Figure 3-6. The circuit is mainly
composed of a time base integrated circuit, a transistor drive circuit, a diode rectifier circuit and a thyristor switching circuit.
    VDI, VD2, C3, C4 and R5 form a simple diode rectifier and voltage stabilizing circuit. After the power is turned on, the two ends of C3 will output
a DC voltage of about 12V to supply power to the entire control circuit. The time base integrated circuit A is connected to a monostable operating mode. Normally,
because the capacitor CI is fully charged, the threshold terminal of A, that is, pin 6, is at a high level. The time base circuit is in a reset state, and its pin 3
outputs a low level. The transistor VT is cut off, the thyristor VS is in the off state without triggering current, and the bulb H does not light up. At this time,
the light-emitting diode LED lights up to indicate the position of the switch, making it easier to find the switch at night.
    When the switch SB is pressed, both ends of the capacitor CI are short-circuited by SB, causing the trigger end of the integrated block, pin 2, to be at low level. The time
base circuit is set and enters the transient state. Pin 3 of the output end outputs high level and the LED goes out. , the transistor VT is turned on, 12V DC
is injected into the gate of the thyristor vs through VT and R4, vs is turned on, and the lamp H lights up. After releasing SB, since
the voltage across the capacitor Cl cannot change suddenly, pin 2 of the time base circuit still remains low, the state of pin 3 of the output terminal remains unchanged, and the lamp H still
lights up. At this time, the power supply charges the capacitor Cl through the potentiometer RP and the resistor Rl, so the voltage across Cl continues to rise.
When approximately t=ll(RP+Rl)x CI, the voltage across C1 rises to 2/3 VDD or 8V. When the time base circuit is reset and returns to the original stable
state, pin 3 outputs low level, the transistor VT is cut off, the thyristor VS loses the trigger voltage, and when the alternating current crosses zero, it is turned off and
the light H goes out.
    From the above analysis, it can be seen that the delay time of the circuit, that is, the transient time of the base circuit, is mainly
determined by the values ​​of the resistor (RP+R1) and capacitor CI, and has nothing to do with the power supply voltage and other parameters of the circuit, thus ensuring the accuracy of the delay time.