Under the action of external pulse, monostable trigger can change from steady state to transient state.

Due to the function of RC delay link in the circuit, the transient state returns to the original steady state after a period of time, and the time to maintain the transient state depends on the parameter value of RC.

Circuit composition:

R and C are timing elements of monostable flip-flop, and their connection point Vc is connected with the threshold input terminal (pin 6) and the output terminal VO' (pin 7) of the timer. Output pulse width of monostable trigger tpo= 1. 1RC. Ri and Ci constitute the differential link of the input loop, which is used to limit the negative pulse width tpi of the input signal Vi within the allowable range. 5RiCi, through the differential link, the sharp pulse width of Vi' can be made smaller than the output pulse width tpo of monostable flip-flop. If the negative pulse width tpi of the input signal is less than tpo, the differential link can be omitted. The reset input (pin 4) of the timer is connected to a high level, the control input Vm is grounded through 0.0 1uF, and the timer output VO (pin 3) is used as the monostable signal output of the monostable trigger.

Edit how this paragraph works:

When input Vi remains high, Ci is equivalent to disconnection. Due to the existence of Ri, the input Vi' is a high level Vcc. At this time, ① If the original state of the timer is 0, the collector output (pin 7) is grounded, and the capacitor C is discharged, and Vc=0, that is, the signal input at pin 6 is lower than 2/3 VDC, and the timer is kept at 0 at this time. (2) If the original state of the timer is 1, the collector output (pin 7) is disconnected from the ground, and Vcc is charged to C through R, which makes the potential of Vc rise. When the value of Vc is higher than 2/3 VDC, the timer will go to 0. Conclusion: When the monostable trigger works normally, if no negative input pulse is applied, that is, Vi keeps high level, the output Vo of the monostable trigger must be low level. The working process of monostable trigger is divided into the following three stages to analyze, and Figure 6-8 shows its working waveform diagram:?

① Trigger flip phase: When the input negative pulse Vi arrives, the falling edge will generate a negative sharp pulse jumping down at Vi' end through RiCi differential link, and its value is lower than the negative threshold (1/3Vcc). Because Vc is lower than the forward threshold (2/3Vcc) at steady state, the fixed timer turns to 1, the output Vo is at high level, and the collector output is disconnected from the ground. At this time, the monostable trigger enters a transient state. ② Transient maintenance stage: Because the open collector output terminal (pin 7) is disconnected from the ground, Vcc charges C through R, and Vc rises exponentially and tends to Vcc. The time from the start of the transient to the time before the Vc value reaches the positive threshold (2/3Vcc) is the transient maintenance time tpo. ③ Return to the recovery stage: when the charging of C makes the Vc value higher than the positive threshold (2/3Vcc), because the negative spike pulse at the Vi' terminal has disappeared and the Vi' value is higher than the negative threshold (1/3Vcc), the timer turns to 0, the output level is low, and the collector output terminal (pin 7) is grounded, and the transient stage is ended. C is discharged through pin 7, and the value of Vc is lower than the forward threshold (2/3Vcc), and the monostable trigger returns to the steady state.

Edit this monostable trigger application example:

Using the characteristics of monostable flip-flop can realize pulse shaping, pulse timing and other functions.

1. Pulse shaping takes advantage of the characteristic that monostable flip-flops can generate pulses with a certain width, and can shape an input pulse that is too narrow or too wide into a pulse output with a fixed width. The irregular input waveform shown in Figure 6-9, after being processed by monostable trigger, can obtain regular rectangular wave output with fixed width, fixed amplitude and steep rising and falling edges. ?

2. Pulse timing: If the output Vo of the monostable trigger is connected to one input pin of the AND gate, the other input pin of the AND gate inputs the high-frequency pulse sequence Vf. When the input negative narrow pulse arrives, the monostable trigger starts to flip and the AND gate opens, allowing the high-frequency pulse sequence to be output from its output terminal VAND through the AND gate. After the tpo timing time, the monostable trigger returns to the steady state, and the AND gate is closed to prohibit the output of high-frequency pulse sequence. So as to realize the timing gating function of the high-frequency pulse sequence.