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7400 Series Guide: 74HC121/74LS121 (Monostable Multivibrator)

The 74×121 (ex 74LS121) is a chip that features a monostable multivibrator, also known as a one-shot pulse generator. This chip is designed to generate a single output pulse of a specified duration in response to a triggering 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.

74x121 (74121) Package And Pinout

What does the 74121 do?

The 74×121 is a type of pulse generator called a “monostable multivibrator.” When it gets triggered, it creates a single output pulse of a specific length, and then goes back to a resting state.

The pulse width can range from 40 ns to 20 seconds. And you can control the length of this pulse using external resistors and capacitors (connected to pin 10 and 11).

You also have the option of using an internal resistor to set the timing instead. To do this, connect Rint (pin 9) to VCC. This will give an output pulse of about 30 to 35 ns.

This chip is useful in applications like creating timed signals, debouncing switches, or generating clock pulses.

How To Use This Chip

The 74×121 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 74HC121 supports 2V to 6V, while the 74LS121 only supports 5V.

74HC chips can normally supply a maximum of 4 mA from an 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.

Once you’ve connected it to power, you can use the Monostable multivibrator inside.

7400 series IC connected to 5V on a breadboard
The 74HC121/74LS121 IC connected to power

74×121 Pinout

The 74×121 has 14 pins and contains Programmable output pulse width from 40 ns to 20 seconds laid out as shown in the pinout diagram below:

Pin NamePin #TypeDescription
Q1OutputInverted output from the Monostable multivibrator.
2NCNot connected.
A13InputTrigger input 1 for the Monostable multivibrator (active low).
A24InputTrigger input 2 for the Monostable multivibrator (active low).
B5InputControl input for triggering the Monostable multivibrator.
Q6OutputOutput from the Monostable multivibrator.
GND7PowerConnect to ground (GND).
8NCNot connected.
Rint9InputConnect to VCC to use without external components.
Cext10InputExternal capacitor connection for timing.
RCext11InputExternal resistor-capacitor connection for timing.
12NCNot connected.
13NCNot connected.
VCC14PowerPositive power supply. Connect to +5V power.
Pin overview for the 74×121 IC

Alternatives and Equivalents for 74×121

There are many versions of the 74×121 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:

  • 74HC121 (High-speed CMOS)
  • 74HCT121 (High-speed CMOS, TTL compatible)
  • 74LS121 (High-speed TTL)
  • 74LVC121 (Low Voltage TTL)
  • 74AC121 (Advanced CMOS)
  • 74ALS121 (Advanced Low-Power Schottky TTL)
  • 74F121 (Very High Speed)
  • 74C121 (CMOS, similar to the 4000-series)

Some manufacturers also add a prefix, such as the SN74HC121 and SN74LS121 by Texas Instruments.

Can’t find the 74×121 anywhere? Then try one of the following IC alternatives:

  • 74×122 – Retriggerable monostable multivibrator.
  • 74×123 – Dual retriggerable monostable multivibrator.
  • 74×221 – Dual monostable multivibrator with choice of 1-shot or 9× Resistor-Capacitor.
  • 74×223 – Dual monostable multivibrator with clear.
  • CD4098 – Dual monostable multivibrator.
  • CD4538 – Dual precision monostable multivibrator.

If you can’t find the 74×121 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 74×121 chip

Download the PDF datasheet for your version of the 74×121 here:

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