Set timing circuit when power is on

When the power is turned on, within the time determined by R2 and C2, the trigger level of pin 2 of NE555 is lower than Vcc/3, and pin 3 outputs a pulse ; then, Vcc charges C2 through R2 and VD1, and the charging voltage rises to close to Vcc . Due to the isolation effect of VD1, it does not affect the negative trigger pulse differentiated by R1 and C1 used during normal operation. VD1 is a diode that is quickly discharged by C2 when the power is turned off. This ensures reliable operation of the circuit even when the power is turned on and off repeatedly for a short period of time. The high level of the NE555 output pulse T is determined by R3 and C3, that is, T=R3C3.