In the timer circuit designed by using the 555 time base circuit, the 555 time base circuit is generally connected into a monostable trigger, which makes the circuit design simple, and only needs a few resistors and capacitors to realize the trigger function , but at the same time, there is also the problem of external interference to pin 2 of the 555 time base circuit. This circuit cleverly uses the forced reset function of pin 4 of the 555 time base circuit to realize an anti-jamming timer circuit.
1. The working principle of the circuit
The circuit principle is shown in Figure 17.
When SB is disconnected, pin 4 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 received on pin 2, the 555 time base circuit will not work. When the button B is pressed, the power is added to a high level at pin 4 through the diode VD1, and the forced reset function of the time base circuit is released. C2 outputs a low level to the pin 2 of the IC circuit by connecting with the collector of VT1, the IC turns over and sets, the pin 3 outputs a high level, the light-emitting diode lights up, the relay K is energized, the contact K-1 is closed, and the socket External power supply, at the same time the high level of pin 3 outputs a high level to pin 4 through VD2 to make the circuit self-locking. When the transient state is over, the circuit returns to a steady state, pin 3 outputs a low level, the relay K loses power, the contact K-1 is disconnected, and the circuit returns to the initial state.
2. Selection of components
IC1 555 time base circuit selects NE555, μ A555, SL555 and other time base integrated circuits; R1-R7 selects RTX- 1/4W carbon film resistors; RP can use WSW type organic solid fine-tuning variable resistors ; C2, C4, C5, and C6 use CT1 ceramic capacitors, C1, C3, and C7 use CD11-16V electrolytic capacitors; diodes VD1, VD2, and VD3 use IN4 148 silicon-type switching diodes, and VD4~VD7 use IN4001 silicon-type ordinary rectifiers Diode; Relay K can be selected according to the needs of electrical equipment; the three-terminal integrated voltage regulator uses the 7809 type three-terminal integrated voltage regulator circuit.
3. Production and debugging method
In the debugging stage of the circuit, the timing time of the circuit can be estimated by T=1.1 (Rrp+Rs) X C3, so it is necessary to
change the timing time by adjusting the variable resistor.