repetitive timing circuit

(1) Introduction to circuit principle In Figure 3-8, Ic555 and RP1, Ri, RP2, VD1, VD2 and cl form an astable
circuit with high and low levels at its output end (④ pin). The conversion time is determined by the charging and discharging time of the capacitor cl, which is
the opening and closing time of the controlled electrical appliance respectively . It can be seen that it is necessary to adjust the charging and discharging time of capacitor C1, so as to achieve
the purpose of adjusting the opening and closing time of the controlled electrical appliance. In this circuit, diodes
VD1 and VDZ are added in order to enable the charging and discharging times of the capacitor Ci to be adjusted independently without affecting each other.
    The working process of the circuit is briefly described as follows: After closing the switch SA, the 2ZOV AC voltage is stepped down by G and pin (discharge resistor of G),
rectified by VD4 and VD, and stabilized and filtered by VD6, R3, c3, and 6 before being supplied to IC 555 Provide a relatively stable DC voltage. When SA is just closed
, because the voltage across the capacitor cj is zero and cannot mutate, the ② and ⑥ pins of IC555 are at low level at this time, and the ③ pin outputs high level. The relay
K is closed, and the socket X is powered. The electrical control unit starts working. At the same time, because the ③ pin of IC555 is high level, its ⑦ pin is also high level,
the diode VD1 is turned on, VD2 is turned off, and the power is given to c through RP1 and R1. Charging, the charging speed is adjusted by RP1. When the voltage on c] is charged to
2/3 of the power supply voltage (Ucc), pins ② and ⑥ of IC555 become high level, and pin ③ correspondingly becomes low level. Relay K} is released, and socket XB
loses power and is The electrical appliance stopped working. At the same time, because the ③ pin of IC555 becomes low level, its ⑦ pin will also become low level, the diode
VD1 is cut off, VD2 is turned on, the capacitor cl is discharged through Rz and RP2, and the discharge speed is adjusted by RP2. When the voltage on Cl is reduced by
1/S of the power supply voltage, pins ② and ⑥ of IC 555 become low level again, and the entire circuit will repeat the above working process.