Dictionary of Electronic Engineering Terms
"A" "B" "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
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IC Monostable Multivibrator Chips
74L121 and 74L122 ICs. The pin out diagram works any any style 74xx121 or 74xx122 DIP chip, but the timing graphs are only valid for the TTL families indicated.

74L121 Monostable Multivibrator IC Chip Schematic
74121 Monostable Multivibrator

TTL One Shot.
The 74121 shown above provides a single Monostable Multivibrator function. While the 74122 shown below provides a retriggerable Monostable Multivibrator function with clear. A Monostable Multivibrator is also called a One Shot.

74L122 Monostable Multivibrator IC Chip Schematic
74122 Monostable Multivibrator

Multivibrator Pin Function:
To use the internal timing resistor connect Rint to Vcc.
Note the 74121 uses a 4k internal resistor, while the 74122 uses a 20k resistor.
An external timing capacitor may be connected between Cext and Rext/Cext. For accurate repeatable pulse widths connect an external resistor between Rext/Cext and Vcc leaving Rint open [unconnected]. To obtain variable pulse widths connect a variable resistance between Rint or Rext/Cext and Vcc.

The 74L121 Truth Table is shown below while the 74L122 Truth Table is shown to the right.
How to Read Logic Truth Tables:
Tn = Time before an input transition
Tn+1 = Time after an input transition
X = Irrelevant, the input may be High or Low
H = A steady state high level
L = A steady state low level
'Up-arrow' = A transition from a low to high level
'Down-arrow' = A transition from a high to low level

74L121 Monostable Multivibrator Truth Table
74121 Truth Table

Monostable Pulse Timing:
The two graphs below show the resultant pulse width for different values of resistors and capacitors.

74L121 Timing Graph
74121 Pulse Width Charts

The first chart works for either the 74L121 or 74L122; however, the second chart only applies to the 74L121. Use this link 74121 / 74122 Pulse Width Charts to find enlarged versions of the two charts above and an external timing capacitor chart for the 74122.

Also refer to Transistor Multivibrator Circuits.

The circuit diagram below is used to test the Switching Time of the 74L121 Monostable Multivibrator. The circuit could be used for any TTL family, such as LS, or H, but the load capacitor might need to be changed to reflect that. For example the 74121, 74LS121, 74ALS121, or 74F121 series of parts could be tested using the circuit.

Test circuit to determine the 74L121 Monostable Multivibrator's switching time
74121 Switching Time Test

The Diodes used in the circuit are 1N3064 or equivalent. The load Capacitance CL is 50pF, including scope probe, wiring, and any stray capacitance.

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