|ne555 monostable multivibrator|
Monostable multivibrator realized with ne555 timer circuit is given in figure bellow.
NE555 is very handy for monostable operation too. Generally, monostable multivibrator must be triggered in order to operate. Once when it is triggered, it suppose to hold output to certain logic level (high or low, depending on particular circuit) for fixed time interval, independently of triggering signal duration. How is this feature achieved with NE555? For stable state (lasts as long as there is no triggering signal and circuit is supplied with electric power), whole scheme looks like in figure bellow.
Output is "low", and discharging transistor T2 is saturated so current through resistor R1 bypassing capacitor C1 through transistor T2, keeping capacitor voltage on low potential.
When triggering signal on point 2 change its value from VCC to 0V, comparator CP1 goes "high", setting S-R latch.
In this change, discharging transistor T2 is blocked, letting resistor current to charge capacitor C1.
Time necessary for the capacitor to charge from 0V to 2/3 of VCC is
This time defines fixed time interval necessary for monostable operation. After reaching stable state, monostable multivibrator will remain stable until next triggering signal.