7400 Series Guide: 74HC221/74LS221 (Monostable multivibrators)

The 74×221 (ex 74LS221) is a with two monostable multivibrators with Schmitt-trigger inputs. It is designed to produce precise timing intervals and is often used in circuits that need a stable pulse length.

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.

What does the 74HC221 / 74LS221 do?

The 74×221 is a dual monostable multivibrator, which means it has two separate circuits that can generate a single output pulse of a specific width when triggered. Also called a one-shot circuit.

You can use it to create timed delays or pulse-widths in circuits.

Here’s how it works:

• Each monostable circuit has two trigger input signals (TR). One normal and one active LOW. Use one of them as your Enable signal and the other one as your trigger signal.
• When a trigger signal is received, the output of the circuit goes from LOW to HIGH for a specific amount of time, and then it goes back to LOW.
• You can adjust the length of the output pulse using external resistors and capacitors connected to the chip.

This IC is commonly used for timing applications, like generating precise time delays, pulse stretching, and debouncing switches in digital circuits.

How To Use This Chip

The 74HC221 comes in a 16-pin package, and you need to connect it to power and connect the timing components before you can use it.

Voltage and Current

Most 7400 ICs support a VCC voltage of 5V. One difference between the HC and LS version of the chip is that the 74HC221 supports 2V to 6V, while the 74LS221 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.

Setting The Pulse Width

To use the Monostable Multivibrators, you need to set up the timing resistor and capacitor like this:

As long as C_ext is larger than 1000 pF, you can use this formula to calculate the pulse width (t_W):

t_W = 0.7 * R_ext * C_ext

If you are using an C_ext that is smaller than 1000 pF, check the datasheet for a graph that gives you the resulting pulse width.

74×221 Pinout

The 74×221 has 16 pins and contains two monostable multivibrators with Schmitt-trigger inputs laid out as shown in the pinout diagram below:

Pin Name	Pin #	Type	Description
1TR	1	Input	Trigger input for the first monostable multivibrator (active low).
1TR	2	Input	Non-inverting trigger input for the first monostable multivibrator.
1RST	3	Input	Reset input for the first monostable multivibrator (active low).
1Q	4	Output	Inverted output from the first monostable multivibrator.
2Q	5	Output	Non-inverted output from the second monostable multivibrator.
2Cext	6	Input	External capacitance pin for the second monostable multivibrator.
2RCext	7	Input	External resistor-capacitor pin for the second monostable multivibrator.
GND	8	Power	Connect to ground (GND).
2TR	9	Input	Trigger input for the second monostable multivibrator (active low).
2TR	10	Input	Non-inverting trigger input for the second monostable multivibrator.
2RST	11	Input	Reset input for the second monostable multivibrator (active low).
2Q	12	Output	Inverted output from the second monostable multivibrator.
1Q	13	Output	Non-inverted output from the first monostable multivibrator.
1Cext	14	Input	External capacitance pin for the first monostable multivibrator.
1RCext	15	Input	External resistor-capacitor pin for the first monostable multivibrator.
VCC	16	Power	Positive power supply. Connect to +5V power.

Alternatives and Equivalents for 74HC221 / 74LS221

There are many versions of the 74×221 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:

• 74HC221 (High-speed CMOS)
• 74HCT221 (High-speed CMOS, TTL compatible)
• 74LS221 (High-speed TTL)
• 74LVC221 (Low Voltage TTL)
• 74AC221 (Advanced CMOS)
• 74ALS221 (Advanced Low-Power Schottky TTL)
• 74F221 (Very High Speed)
• 74C221 (CMOS, similar to the 4000-series)

Some manufacturers also add a prefix, such as the SN74HC221 and SN74LS221 by Texas Instruments.

Can’t find the 74×221 anywhere? Then try one of the following IC alternatives:

• 74×121 – Monostable multivibrator.
• 74×123 – Dual monostable multivibrator.
• CD4098 – Dual monostable multivibrator.

If you can’t find the 74×221 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 74LS221 and 74HC221 chips

Download the PDF datasheet for your version of the 74×221 here:

• SN74LS221 (Texas Instruments)
• 74HC221 (Futurlec)
• 74LS221 (Futurlec)