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7400 Series Guide: 74HC122/74LS122 (Monostable multivibrator)

The 74×122 (ex 74LS122) is a chip that contains a monostable multivibrator, also known as a one-shot pulse generator. It is designed to produce a single output pulse of a specified duration in response to an input trigger event.

In this guide, you’ll learn the things you need to know about this chip in order to use monostable multivibrators in your own projects.

74HC122 / 74LS122 Package And Pinout

What does the 74HC122 / 74LS122 do?

The 74×122 is a monostable multivibrator, which is a type of circuit that generates a single output pulse when triggered. This chip takes a trigger input and, upon receiving a trigger, gives a HIGH pulse for a specific time duration at its output.

You can control the length of this pulse with an external resistor and capacitor that you connect to the chip. There’s also a clear input that can stop the pulse early, resetting the output to LOW if needed.

This chip is handy in creating precise timing delays or generating one-time pulses in electronic projects.

How To Use This Chip

The 74HC122 comes in a 14-pin package, and you need to connect it to power before you can use it. Most 7400 ICs support a VCC voltage of 5V. One difference between the HC and LS version of the chip is that the 74HC122 supports 2V to 6V, while the 74LS122 only supports 5V.

7400 series IC connected to 5V on a breadboard
The 74HC122/74LS122 IC connected to power

Configuring Your Timing Pulse

Once you’ve connected it to power, you need to configure the timing pulse. If you want to use the internal timing resistor, connect Rint to VCC:

Configuring the 74ls122 with internal resistor

If you instead want to use your own resistor (which will improve the accuracy), leave Rint disconnected and connect a resistor from RCext to VCC:

Configuring the 74ls122 with external resistor

Calculating Pulse Length:

You can calculate the resulting pulse length in nanoseconds from this formula:

t(ns) = 0.32 * RT * Cext * (1 + 0.7/RT)

Where Cext is in pF and RT is in kΩ.

So with C = 1000 pF and using the internal timing resistor of 10 kΩ, the pulse width becomes:

t(ns) = 0.32 * 10 * 1000 * (1 + 0.7/10) = 3424 ns

Output Current

74HC chips can normally supply a maximum of 4 mA from the output pin. If you’re using the 74LS version, the maximum current you can pull out of one output pin is 0.4 mA when the pin is high (sourcing) or 8 mA when the pin is low (sinking).

But these values can differ between models, so check the datasheet of your model to verify.

74×122 Pinout

The 74×122 has 14 pins and contains Retriggerable monostable multivibrator with overriding reset and integrated 10k timing resistor laid out as shown in the pinout diagram below:

74122 Pinout
Pin NamePin #TypeDescription
A11InputNon-retriggerable input for the first section.
A22InputNon-retriggerable input for the second section.
B13InputActive high transition input (retriggerable) for first section.
B24InputActive high transition input (retriggerable) for second section.
CLR5InputClear input (active low), clears the output and turns off the one-shot.
Q6OutputInverted output of the one-shot.
GND7PowerConnect to ground (GND).
Q8OutputNon-inverted output of the one-shot.
Rint9InputConnect to VCC to use internal resistor.
NC10NCNot connected.
Cext11InputExternal timing capacitor connection.
NC12NCNot connected.
RCext13InputConnection for external resistor-capacitor network for timing.
VCC14PowerPositive power supply. Connect to +5V power.
Pin overview for the 74×122 IC

Alternatives and Equivalents for 74HC122 / 74LS122

There are many versions of the 74×122 chip. They all have the same functionality, but with different specifications such as supported voltages and maximum current output.

Here’s a list of a few equivalents of this chip:

  • 74HC122 (High-speed CMOS)
  • 74HCT122 (High-speed CMOS, TTL compatible)
  • 74LS122 (High-speed TTL)
  • 74LVC122 (Low Voltage TTL)
  • 74AC122 (Advanced CMOS)
  • 74ALS122 (Advanced Low-Power Schottky TTL)
  • 74F122 (Very High Speed)
  • 74C122 (CMOS, similar to the 4000-series)

Some manufacturers also add a prefix, such as the SN74HC122 and SN74LS122 by Texas Instruments.

Can’t find the 74×122 anywhere? Then try one of the following IC alternatives:

  • 74×121 – Monostable multivibrator.
  • 74×123 – Dual retriggerable monostable multivibrator.
  • 74×221 – Dual monostable multivibrator.
  • 74×223 – Dual monostable multivibrator with clear.
  • CD4098 – Dual monostable multivibrator.
  • CD4538 – Dual precision monostable multivibrator.

If you can’t find the 74×122 IC in your local electronics store, don’t worry, you’ll most likely find it in one of the stores listed on this page of online stores where you’ll find components and tools for all your electronics projects.

Datasheet for the 74LS122 and 74HC122 chips

Download the PDF datasheet for your version of the 74×122 here:

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