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7400 Series Guide: 74HC123/74LS123 (Monostable multivibrator)

The 74×123 (ex 74LS123) is a chip that features two retriggerable monostable multivibrators with reset.

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

74HC123 / 74LS123 Package And Pinout

What does the 74HC123 / 74LS123 do?

The 74×123 is a dual retriggerable monostable multivibrator, also known as a one-shot timer. This chip can produce precise timing pulses when triggered.

Here’s how it works:

  • Each section of the chip can be triggered by a falling edge or a LOW signal.
  • Once triggered, it generates a pulse of a specific duration, which you can set using external resistors and capacitors connected to the chip.
  • If another trigger occurs before the pulse ends, the timing resets, and a new pulse starts. This is called being “retriggerable.”

The 74×123 is often used in pulse generation, signal conditioning, and timing applications in digital circuits. It’s a handy chip when you need a specific delay or timed event in your circuit!

How To Use This Chip

The 74HC123 comes in a 16-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 74HC123 supports 2V to 6V, while the 74LS123 only supports 5V.

7400 series IC connected to 5V on a breadboard
The 74HC123/74LS123 IC connected to power

Setting Pulse Length

Next, you need to connect a capacitor and a resistor to set up the width of the output pulse. This is how to connect these components:

Configuring the 74ls123

Once you’ve connected it to power and you’ve set up your resistor and capacitor, you can use the monostable multivibrator inside. The pulse length can be calculated like this:

t = Cext * Rext

For example if you use a 100kΩ resistor and a 100nF capacitor, you’ll get a pulse width of

t = 0.0000001F * 100000Ω = 0.01 seconds

If you want a negative transistion trigger, use the A input as trigger and the B input as enable signal. If you want a positive transistion trigger, use the B input as your trigger and the A input as your enable signal.

Output Current

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.

74×123 Pinout

The 74×123 has 16 pins and contains two retriggerable monostable multivibrators with overriding reset laid out as shown in the pinout diagram below:

74ls123 Pinout
Pin NamePin #TypeDescription
1A1InputNegative transistion trigger for the first monostable multivibrator.
1B2InputPositive transition trigger for the first monostable multivibrator.
1CLR3InputClear input for the first monostable multivibrator (active low).
1Q4OutputInverted output from the first monostable multivibrator.
2Q5OutputOutput from the second monostable multivibrator.
2Cext6InputExternal capacitor connection for the second monostable multivibrator timing control.
2RCext7InputExternal resistor-capacitor connection for the second monostable multivibrator timing control.
GND8PowerConnect to ground (GND).
2A9InputNegative transistion trigger for the second monostable multivibrator.
2B10InputPositive transition trigger for the second monostable multivibrator.
2CLR11InputClear input for the second monostable multivibrator (active low).
2Q12OutputInverted output from the second monostable multivibrator.
1Q13OutputOutput from the first monostable multivibrator.
1Cext14InputExternal capacitor connection for the first monostable multivibrator timing control.
1RCext15InputExternal resistor-capacitor connection for the first monostable multivibrator timing control.
VCC16PowerPositive power supply. Connect to +5V power.
Pin overview for the 74×123 IC

Alternatives and Equivalents for 74HC123 / 74LS123

There are many versions of the 74×123 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:

  • 74HC123 (High-speed CMOS)
  • 74HCT123 (High-speed CMOS, TTL compatible)
  • 74LS123 (High-speed TTL)
  • 74LVC123 (Low Voltage TTL)
  • 74AC123 (Advanced CMOS)
  • 74ALS123 (Advanced Low-Power Schottky TTL)
  • 74F123 (Very High Speed)
  • 74C123 (CMOS, similar to the 4000-series)

Some manufacturers also add a prefix, such as the SN74HC123 and SN74LS123 by Texas Instruments.

Can’t find the 74×123 anywhere? Then try one of the following IC alternatives:

  • 74×121 – Monostable multivibrator (single).
  • 74×122 – Monostable multivibrator (single, retriggerable).
  • 74×221 – Dual monostable multivibrator.
  • CD4528 – Dual monostable multivibrator.
  • CD4538 – Retriggerable precision monostable multivibrator.

If you can’t find the 74×123 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 74LS123 and 74HC123 chips

Download the PDF datasheet for your version of the 74×123 here:

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