7/5/2019 74121 or 555 Timer as a One-Shot
https://www.utm.edu/staff/leeb/3b3.htm 1/3
74121 or 555 Timer
As a One-Shot
74121 Monostable Multivibrator
Shown below is the pin out diagram for the 74121. There are three trigger inputs from
the device, two negative edge-triggering (A) inputs, one positive edge Schmitt-triggering
(B) input. If A1 or A2 is LOW and B goes HIGH, OR if B is HIGH and A1 or A2 go
LOW, the one-shot will fire.
Notice also that pins 2, 8, 12, and 13 are not connected to anything inside the IC. Therefore,
these pins are labeled "NC" for "No Connection".
The time of the pulse is determined by an external resistor and capacitor connected to
REXT, and REXT/CEXT. If the value of capacitor connected to CEXT is greater than 1uF the
pulse width (tw) is equal to .33 * REXT * CEXT The 74121 is a non -retrigerable one-shot.
The schematic diagram below shows how a 74121 may be wired. Here we are using pin
5 (B) for the input trigger signal which will fire on a high input pulse. When a trigger
input activates the device, the Q output will go HIGH as illustrated in the timing diagram.
The length of the pulse is determined by the formula Tw = .33 * Cext * Rext.
