Set timing circuit when power is on

(1) Introduction to the circuit principle. Figure 3-42 shows the set timing circuit when the power is turned on. When the power is turned on, within
the time , the trigger level of pin ② of NE555 is lower than ( 1/3)
Ur, ③ pin output pulse . Then. +Ucc
charges C2 via R and VD1, and the charging voltage rises to close to
+Ucc:. Due to the isolation effect of VD1, it does not affect the
negative
trigger pulse differentiated by R, and C used during normal operation. VD1 is a diode used to quickly
discharge 0 when the power is turned off. Even if the power supply is repeatedly turned on and
off for a short period of time, it can ensure reliable operation of the circuit. The high level of
the NE555 output pulse T is determined by R3 and G
, that is, T-Y C3.
    (2) Component parameter selection The selection of components is
shown in Figure 3-42. There are no special requirements.
    (3) The set timing circuit when the power is turned on is shown in Figure 3-42 .