repetitive timing circuit

This is a repetitive timing circuit (also called a dual timing circuit or a bidirectional timing circuit) composed of a 555 time base integrated circuit. The so-called repetitive timing circuit means that it can make the controlled electrical appliances work and stop repeatedly at regular intervals. That is to say, it can not only make the controlled electrical appliances turn off at regular intervals, but also automatically turn them on at regular intervals after being turned off. This repetitive timing circuit is mainly composed of a 555 time base integrated circuit. It has the characteristics of simple wiring, stable and reliable operation, and easy installation and use. In this circuit, the opening and closing times of the controlled electrical appliances can be adjusted separately without affecting each other, which has high practical value. working principle　　

The circuit schematic diagram of this repetitive timing circuit is shown in Figure 1. In the figure, IC555 and RP1, R1, RP2, R2, VD1, VD2 and C1 form an astable circuit. The high and low level conversion time of its output terminal (pin 3) is determined by the charge and discharge time of capacitor C1. This The time is respectively the opening and closing time of the controlled electrical appliance. It can be seen that as long as the charging and discharging time of the capacitor C1 is adjusted, the opening and closing time of the controlled electrical appliance can be adjusted. In this circuit, in order to enable the charging and discharging times of capacitor C1 to be adjusted independently without affecting each other, diodes VD1 and VD2 are added. The working process of the circuit is briefly described as follows: After closing the switch SA, the 220V AC voltage is stepped down by C5 and R4 (discharge resistor of C5), rectified by VD4 and VD5, and stabilized and filtered by VD6, R3, C3 and C4, and then provided to IC555. A relatively stable DC voltage. When SA is just closed, because the voltage across the capacitor C1 is zero and cannot mutate, the ② and ⑥ pins of the IC are low level at this time, and the ③ pin outputs a high level. The relay K is closed, and the socket XB is energized. The electrical control unit starts working. At the same time, because the ③ pin of the IC is at high level, its ⑦ pin is also at high level. The diode VD1 is turned on and VD2 is turned off. The power supply charges C1 through RP1 and R1, and the charging speed is adjusted by RP1. When the voltage on C1 is charged to 2/3 of the power supply voltage (Ucc), the ② and ⑥ pins of the IC become high level, and the ③ pin correspondingly becomes low level, the relay K1 is released, the socket XB loses power, and the controlled electrical appliance stop working. At the same time, because the IC's ③ pin becomes low level, its ⑦ pin will also become low level, diode VD1 cuts off, VD2 turns on, capacitor C1 discharges through R2 and RP2, and the discharge speed is adjusted by RP2. When the voltage on C1 is reduced to 1/3 of the power supply voltage, the ② and ⑥ pins of the IC become low level again, and the entire circuit will repeat the above-mentioned working process. 　　During the operation of this circuit, the voltage change process of pin ② and pin ③ of the IC is shown in Figure 2. Among them, TH is the working time of the controlled electrical appliance, and TL is the rest time of the controlled electrical appliance. It should be noted here that when the power is first turned on, the working time of the controlled electrical appliance is slightly longer than the subsequent working time. The working time is TH + Ts, but usually Ts is small, so it has little impact on the time calibration of the entire circuit. . Selection of components In this circuit, the integrated circuit IC can use NE555, MA555, LM555, 5C1555 and other time base circuits; the relay K1 can use a small and medium power relay with a rated operating voltage between 9V and 12V, and its contact power should be Select according to the power of the controlled electrical appliance; the voltage stabilizing diode VD6 can be selected with a stabilizing value of about 12V, such as 2DW21, etc.; the capacitor C5 should be a non-polar capacitor with a withstand voltage of more than 400V. There are no special requirements for other components, as long as they are selected according to the parameters marked in Figure 1. Installation and debugging In the circuit shown in Figure 1, except for the potentiometers RP1 and RP2, all other components can be installed on a printed circuit board. The entire circuit can be powered on and debugged after the installation is completed and checked to ensure that it is correct. After the circuit is energized, RP1 and RP2 should first be adjusted to the minimum resistance (resistance is zero). At this time, the relay K should continuously pull in and release. Its pull-in and release time are determined by the resistance values ​​of R1 and R2 respectively. It is determined that according to the values ​​​​given in the picture, the pickup and release time is about 5 seconds. If the relay does not close or releases after being powered on, it indicates that there is a fault in the circuit. At this time, you can temporarily disconnect the relay and VD3 from pin 3 of the IC, and then use the voltage range of a multimeter to measure the voltage of pin 3. , see if it continuously changes between high level (about 10V) and low level (OV). If it changes, it means that the astable circuit part is normal, and the fault lies in relay K and protection diode VD3. If the voltage at pin 3 of the IC does not change (always high or low), it means there is a fault in the astable circuit part. After the circuit is working normally, time calibration can be performed at the knobs of RP1 and RP2. If selected according to the data given in Figure 1, the maximum scheduled working and resting times are about 60 minutes respectively.