The bistable flip-flop described earlier has two steady-state output states, and these two states are always opposite. While monostable flip-flops have only one steady state. Before the trigger signal is added, the trigger is in a stable state. After being triggered, the trigger changes from a stable state to a transient state. After the transient state is maintained for a period of time, it will automatically turn back to the original stable state. Monostable flip-flops are usually used in delay and pulse shaping circuits.

After turning on the power supply, there is no negative pulse, but C charges and Vc rises. When Vc=2Vcc/3, the output of RS circuit is low, the discharge tube T is turned on, and Vc discharges rapidly, so that Vc= 0. In this way, before the negative pulse is applied, the output is low, that is, Vo= 0, which is the steady state of the circuit. When t = t0, Vi jumps negatively (the level at Vi terminal is less than Vcc/3) and VC = 0 (the level at TH terminal is less than 2Vcc/3), so the output Vo turns to high level, T turns off and VC is charged. Exponential growth. When t = t 1, negative pulse disappears. When t = t2, Vc rises to 2Vcc/3 (at this time, the level at TH terminal is greater than 2Vcc/3, and the level at TR terminal is greater than Vcc/3), and Vo automatically turns to low level again. During tw, the circuit is in a transient state. T & gtT2, T turns on, C discharges rapidly, and the circuit returns to steady state. From the analysis, we can get:

Output positive pulse width tW = 1. 1RC.

Note: The circuit in Figure 6-3 (a) can only be triggered by a narrow negative pulse, that is, the trigger pulse width ti must be less than tW.

Examples of 555 timer in practical application are: the circuit that can send out "ding, dong" doorbell and the control circuit that rotates lantern.

555 timer monostable trigger Figure 8-2 555 constitutes a monostable trigger Figure 8-2 shows a monostable trigger composed of 555 timer and external timing elements R and C, and D is a clamping diode. In the steady state, the input terminal of 555 circuit is at the power level, the internal discharge switch T is turned on, and the output terminal Vo outputs a low level. When the external negative pulse trigger signal is applied to the Vi terminal. When the potential at both ends is instantly lower than 1/3VcC, the low-level comparator acts, the monostable circuit starts the steady-state process, the capacitor C begins to charge, and VC increases exponentially. When Vc is charged to 2/3VCC, the high-level comparator acts, the comparator A 1 flips, the output Vo returns from high level to low level, the discharge switch T is turned on again, and the charge on the capacitor C is discharged rapidly through the discharge switch, and the transient ends, and the stability is restored, so as to prepare for the next trigger pulse. See Figure 8-3 for the waveform diagram. Figure 8-3 waveform diagram of monostable trigger

The duration Tw (that is, the delay time) of the transient depends on the size of the external components R and C. ..

Tw= 1. 1RC

By changing the sizes of r and c, the delay time can be changed from a few microseconds to tens of minutes. When this monostable circuit is used as a timer, it can directly drive a small relay, and the reset terminal can be grounded to terminate the transient and re-time. In addition, the freewheeling diode should be connected in parallel with the relay coil to prevent the back electromotive force of the relay coil from damaging the internal power tube. Multivibrator, also known as unstable trigger, has no stable output state and only two transients. After the circuit is in a certain transient state, it can trigger itself to flip to another transient state after a period of time. Two transients are converted to each other, and a series of rectangular waves are output. Multi-oscillators can be used as square wave generators.

After turning on the power supply, assuming that the output is at a high level, T is turned off and capacitor C is charged. The charging loop is VcC-r 1-R2-C- ground, which rises exponentially. When it rises to 2Vcc/3 (TH, the terminal level is greater than VC), the output flips to low level. Vo is low, T is on, C is discharged, and the discharge circuit is C-R2-T- ground, which decreases exponentially. When it drops to Vcc/3 (TH, the terminal level is less than Vc), the output turns to high level, the discharge tube T is turned off, and the capacitor is charged again. This cycle produces oscillation, which can be obtained by analysis.

Output high time T=(R 1+R2)Cln2.

Output low time T=R2Cln2

The oscillation period T=(R 1+2R2)Cln2.

The duty cycle of the output square wave is: