SN74121N Monostable Multivibrator

SN74121N IC is a Monostable multivibrator with Schmitt-trigger inputs. Monostable multivibrators are also known as one-shot multivibrator. The SN74121N have a feature for dual negative/positive transition-triggered inputs. It has programmable output pulse width from 40 nano seconds to 28 seconds.

 

Features and Specifications 

This section mentions some of the important features and specifications of the multivibrator IC.

1. Supply voltage: 4.25-5.25V DC
2. Input Type: Bipolar, Output Type: Push-pull
3. Low level Output Current (Max): 16mA
4. High level Output Current (Max): 0.8mA
5. Operating Temperature Range: 0°C to +70°C
6. Programmable Output Pulse Width: Rint - 35 ns, Rext/Cext - 40 ns to 28 Sec
7. Output pulse timings: 40 nanoseconds to 28 seconds
8. Jitter-Free Operation up to 90% Duty Cycle

 

Note: More technical information can be found in the SN74121N Datasheet, linked at the bottom of this page.

 

Pin configuration

The Monostable multivibrator IC consists of 14 pins in total, the table below can be referred to understand the pin configuration of the IC.

Pin Number	Pin Type	Pin Description
1	Q’	Inverse of the output
2	NC	Not Connected
3	A1	Negative Edge Trigger pin 1
4	A2	Negative Edge Trigger pin 2
5	B	Positive Edge Trigger
6	Q	Output
7	GND	Ground
8	NC	Not Connected
9	Rint	Internal Resistor
10	Cext	External Capacitor
11	Rint/Cext	External Resistor/Capacitor
12	NC	Not Connected
13	NC	Not Connected
14	VCC	Positive supply voltage

 

Working of the Monostable Multivibrator

Monostable multivibrator, mono means single and monostable means that it is a single state stable multivibrator. This type of multivibrator changes a state depending upon the trigger input.

The output always stays in a stable state which could be either LOW or HIGH, but by applying a trigger signal, the state changes. The time required for a signal to come back to its stable state can be varied by changing the resistance and capacitance values.

 

Logic Diagram

The image shows the internal schematic of the monostable multivibrator IC.

There are three trigger inputs to the device, two of which are for negative edge triggering and one which is for positive edge triggering. The output pulse width can be determined by varying or choosing the value of internal resistor(Rint) or external resistors(Rext,Cext). By doing this we can vary the pulse width from 40 nano seconds to 28 seconds.

 

Note: If no external timing components are used, for example Rint connected to VCC and Rext and Cext pin left open; an output pulse of typically 30 or 35 nanoseconds is achieved.

 

Alternatives 

CD4047, 74123, 74HC221

 

Available packages

SOIC, PDIP, SO

 

Applications 

Here are some of the applications of the monostable multivibrator.

1. Timing circuits
2. Delay applications
3. Frequency multiplication/division

 

2D Model 

Below is the 2D model of the monostable multivibrator IC along with its dimensions in millimeters and inches. The following information can be used to create custom footprints and be used while PCB designing and CAD modelling.