Simple and practical household appliance delayed shutdown circuit

After cooking, there are still a lot of harmful gases in the kitchen, and it takes three or four minutes to turn off the exhaust fan. After using the computer, some programs still need to run, and it takes a few minutes to turn off the power. The simple circuit introduced below, after the relevant operations are completed, just press the delayed stop button, people can leave immediately, and after a few minutes, the fan or computer and the control circuit are automatically disconnected from the city power grid, thus eliminating the trouble of not being able to shut down synchronously.

The working principle of the circuit is shown in the figure. Transformer T, D1~D4, C1 form a 9V power supply circuit, BG1, BG2, R2, R3, R4, R5, R6, C2, C3, J, etc. form a bistable circuit, and NE555, R7, C5, C6, BG3, R8, R9 form a remote control shutdown switch (this switch includes a manual switch K and a trigger reset electromagnetic coil DL) to form an automatic delayed shutdown circuit. Press K. K1-1 and K1-2 are closed and self-locked, and the fan or computer is connected to the city power grid together with the control circuit. When the fan motor or computer starts working, the bistable part in the control circuit is also powered and enters the working state. Since the β value of BG1 is greater than BG2, BG1 is turned on and BG2 is turned off at the moment of power-on. There is no current passing through the relay J coil, and J1-1 does not absorb the station. The automatic delayed shutdown circuit part cannot enter the working state due to no current, and the fan or computer can work as usual. When the delay stop button AN is pressed, the bistable circuit flips, changing from BG1 on and BG2 off to BG1 off and BG2 on. Then J1-1 is energized, the automatic delay shutdown circuit is connected to the power circuit, and C5 starts to charge. At the beginning of charging, since the potential of NE555's ② and ⑥ pins is close to vcc, the ③ pin outputs a low level, BG3 is off, and no current flows through the coil DL of the TV remote control shutdown switch, K cannot be triggered to reset, and the contacts of K1-1 and K1-2 are still closed, and the fan or computer works as usual. As the charging time increases, the potential of ② and ⑥ pins gradually decreases. When the potential on ② and ⑥ pins drops to 1/3 Vcc, NE555 flips, the ③ pin outputs a high level, BG3 is on, and a current of about 400mA flows through the TV AC shutdown switch coil DL. Then K is triggered to reset, K1-1 and K1-2 are disconnected, and the electrical appliance is disconnected from the control circuit and the city power grid.

Component selection and adjustment precautions. The transformer can be a 5w, secondary 9v power transformer. The β values ​​of BG1 and BG2 should be about 300 and 200 respectively. The β value of BG3 should not be less than 200. The TV remote control off switch can be any model with a coil DC resistance of 26Ω (such as KDC-A04-6Y, etc., each model only has different installation dimensions, and the electrical parameters are the same). J can use high-sensitivity relays with a nominal voltage of 12v such as JZC22 and 4123. There are no special requirements for other components. The values ​​of C5 and R8 in the figure can delay about 3.5 minutes. Changing the values ​​of C5 and R8 can change the delay time, and the specific data can be determined through experiments. Note that C4 in the figure must not be omitted or replaced by a diode, otherwise, it cannot be guaranteed that BG1 will be turned on first when the power circuit is turned on.

This circuit has been working properly for more than 4 years and has never failed.