Design of anti-interference timer circuit using 555 time base

　　In the timer circuit designed using the 555 time base circuit, the 555 time base circuit is generally connected to a monostable trigger. This connection makes the circuit design simple and only requires a few resistors and capacitors to realize the trigger function. However, there is also the problem of external interference to the 2nd pin of the 555 time base circuit. This circuit cleverly uses the forced reset function of the 4th pin of the 555 time base circuit to realize an anti-interference timer circuit.

　　Working principle of the circuit: When SB is disconnected, the 4th pin of the 555 time base circuit is connected to the ground through the resistor R6, and the 555 time base circuit is forced to reset. At this time, no matter how much interference is caused to the 2nd pin, the 555 time base circuit does not work. When the button B is pressed, the power supply is added to the 4th pin through the diode VD1 to form a high level, and the forced reset function of the time base circuit is released. At the same time, the power supply is added to the base of the transistor VT1 through the resistor R1, so that VT1 is turned on, and the capacitor C2 outputs a low level to the 2nd pin of the IC circuit after being connected to the collector of VT1. The IC flips and sets, and the 3rd pin outputs a high level, the light-emitting diode lights up, the relay K is energized, the contact K-1 is closed, and the socket is powered externally. At the same time, the high level of the 3rd pin outputs a high level to the 4th pin through VD2, making the circuit self-locking. When the transient state ends, the circuit flips back to the steady state, the 3rd pin outputs a low level, the relay K loses power, the contact K-1 is disconnected, and the circuit returns to the initial state.

　　Selection of components: IC1555 timing circuit uses NE555, μA555, SL555 and other timing integrated circuits; R1~R7 use RTX-1/4W carbon film resistors; RP can use WSW organic solid fine-tuning variable resistor; C2, C4, C5, C6 use CT1 ceramic capacitors, C1, C3, C7 use CD11-16V electrolytic capacitors; diodes VD1, VD2, VD3 use IN4148 silicon switching diodes, VD4~VD7 use IN4001 silicon ordinary rectifier diodes; relay K can be selected according to the needs of electrical equipment; three-terminal integrated voltage regulator uses 7809 three-terminal integrated voltage regulator circuit.