Retriggerable monstable 74LS123

Thread Starter


Joined Apr 6, 2021
Hello, I am trying to understand how the Retriggerable monstable 74LS123 work in the inside, what kind of latch does it use, and how it can be retriggerable. What is the connection of the R and R / C pins in the latch? I do understand how it can be use, and in fact it work, but i want to know whats happening inside.

Appreciate any help.


Joined Feb 24, 2006
All the monostables you will ever see, besides all the ones you won't, work by allowing a source to charge a capacitor through a resistor. The time it takes to do this is fairly well know with some degree of precision. The remaining logic has to do with allowing the capacitor voltage to be reset in the retriggerable case, or locking it out in the non-retriggerable case. Since the state of a triggered monostable is known, this is a fairly simple piece of logic which can be implemented in a variety of ways.


Joined Mar 14, 2008
Basically, in the monostable mode, a trigger pulse sets the output high and it then detects how long it takes the capacitor to charge to some voltage through a resistor, (probably to about 2/3 of the supply voltage)
This triggers the output to go low and discharge the cap for the next trigger.