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.
Tw = .33 * 20uF * 10K = .066 Seconds
Data Sheet for 74121