Making a timer with NE555

This is a repetitive timing circuit (also called dual timing circuit or bidirectional timing circuit) composed of 555 time base integrated circuit. The so-called repetitive timing circuit means that it can make the controlled electrical appliance repeatedly work and stop at a fixed time, that is, it can not only make the controlled electrical appliance shut down at a fixed time, but also automatically start at a fixed time after shutting down.

This repetitive timing circuit is mainly composed of a 555 time base integrated circuit. It has the characteristics of simple circuit, stable and reliable operation, easy installation and use. In this circuit, the opening and closing time of the controlled electrical appliances can be adjusted separately without affecting each other, which has high practical value.

The circuit diagram of this repetitive timing circuit is shown in Figure 1. In the figure, IC555, RP1, R1, RP2, R2, VD1, VD2 and C1 form an astable circuit. The high and low level conversion time of its output end (pin 3) is determined by the charging and discharging time of capacitor C1, which is the opening and closing time of the controlled electrical appliance. It can be seen that as long as the charging and discharging time of capacitor C1 is adjusted, the purpose of adjusting the opening and closing time of the controlled electrical appliance can be achieved. In this circuit, in order to make the charging and discharging time of capacitor C1 adjustable independently without affecting each other, diodes VD1 and VD2 are added.

The working process of the circuit is briefly described as follows: After the switch SA is closed, 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 to provide a relatively stable DC voltage to IC555. When SA is just closed, because the voltage across the capacitor C1 is zero and cannot change suddenly, the ② and ⑥ pins of IC are low level, the ③ pin outputs high level, the relay K is closed, the socket XB is powered, and the controlled appliance starts to work. At the same time, because the ③ pin of IC is high level, its ⑦ pin is also high level, the diode VD1 is turned on, VD2 is turned off, and 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 IC become high level, the ③ pin becomes low level accordingly, the relay K1 is released, the socket XB loses power, and the controlled appliance stops working. At the same time, because the IC's pin ③ becomes low level, its pin ⑦ will also become low level, the diode VD1 is cut off, VD2 is turned on, and the 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 IC's pins ② and ⑥ become low level again, and the whole circuit will repeat the above working process.

During the operation of the circuit, the voltage variation process of the ② and ③ pins of the IC is shown in Figure 2. Among them, TH is the working time of the controlled appliance, and T1 is the rest time of the controlled appliance. It should be noted that when the device is first turned on, the working time of the controlled appliance is slightly longer than the subsequent working time. The working time is TH + Ts, but Ts is usually small, so it has little effect on the time calibration of the entire circuit.

In the circuit shown in Figure 1, except for the potentiometers RP1 and RP2, the remaining components can be installed on a printed circuit board. The entire circuit can be powered on and debugged only after installation and inspection. After the
circuit is powered on, RP1 and RP2 should be adjusted to the minimum resistance (zero resistance) first. At this time, relay K should be continuously attracted and released. Its attraction and release time are determined by the resistance values ​​of R1 and R2 respectively. According to the values ​​given in the figure, its attraction and release time is about 5 seconds. If the relay does not attract or does not release after attraction after power is turned on, it means that there is a fault in the circuit. At this time, the relay and VD3 can be temporarily disconnected from the 3rd pin of the IC, and then the voltage of the 3rd pin can be measured with the voltage range of the multimeter to see if it changes continuously between the high level (about 10V) and the low level (0V). If it changes, it means that the astable circuit part is normal, and the fault lies in the relay K and the protection diode VD3. If the voltage of the 3rd pin of the IC does not change (always high or low), it means that the astable circuit part is faulty.
After the circuit works normally, you can calibrate the time at the knobs of RP1 and RP2. If you select according to the data given in Figure 1, the longest timed working and resting time are about 60 minutes respectively.

In this circuit, the integrated circuit IC can be selected from NE555, MA555, LM555, 5C1555 and other time base circuits; the relay K1 can be selected from small and medium power relays with a rated working voltage between 9V and 12V, and the size of its contact power should be selected according to the power size of the controlled electrical appliance; the voltage regulator diode VD6 can be selected with a voltage regulator 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.