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Home > Products > Integrated Circuits (ICs) > Logic > Multivibrators

Texas Instruments SN74121DR

Name: SN74121DR
Brand: Texas Instruments
SKU: SN74121DR
Price: 2.40 USD
Availability: InStock
Rating: 5 (2 reviews)

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Part No.:: SN74121DR

Manufacturer:: Texas Instruments

Category:: Multivibrators

Package:: 14-SOIC (0.154", 3.90mm Width)

Datasheet:: SN74121DR.pdf

Description:: IC MULTIVIBRATOR 45NS 14SOIC

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Product details

Overview

SN74121DR from Texas Instruments is a dual monostable multivibrator (one-shot) IC in 14-pin SOIC package, operating from 4.75 V to 5.25 V, with typical output pulse width of 30 ns at 5 V and propagation delay of 25 ns. It supports edge-triggered inputs (A1/A2/B) and complementary Q/Q̅ outputs, used in digital timing, pulse shaping, and debouncing circuits.

For engineers reviewing the SN74121DR datasheet, SN74121DR pinout, SN74121DR application, or SN74121DR equivalent, key selection criteria include its fixed-output-width architecture, dual independent one-shot functionality, TTL-compatible input thresholds, and RoHS-compliant SOIC-14 packaging for industrial control and legacy logic interface designs.

Technical Context

The SN74121DR implements two independent retriggerable monostable multivibrators using bipolar TTL circuitry. Each section features three inputs (two low-active A1/A2 and one high-active B), enabling flexible trigger logic combinations. Output pulse width is externally set via RC network connected to pins 9–11 (R_EXT/C_EXT), with internal 2-kΩ resistor providing default timing when external R is omitted.

It operates within 0°C to 70°C ambient temperature range and meets standard TTL voltage thresholds: V_IH = 2 V min, V_IL = 0.8 V max, V_OH = 2.4 V min at I_OH = –0.4 mA, and V_OL = 0.4 V max at I_OL = 16 mA. The device is not designed for CMOS-level interfacing without level translation.

Key Specifications

Parameter	Value and Actual Design Meaning
Supply Voltage	4.75 V to 5.25 V - ensures compatibility with standard 5-V TTL logic rails and stable operation under ±5% regulation.
Propagation Delay	25 ns typical at V_CC = 5 V - enables precise timing in high-speed digital control loops and clock conditioning.
Output Pulse Width	30 ns minimum, adjustable up to seconds via external R/C - supports both short-duration glitch suppression and long-delay timing functions.
Input Thresholds	V_IL ≤ 0.8 V, V_IH ≥ 2.0 V - guarantees reliable triggering from standard TTL outputs without additional buffering.
Output Drive	I_OL = 16 mA, I_OH = –0.4 mA - sufficient to directly drive multiple 74LS loads or small LEDs without external drivers.
Operating Temperature	0°C to 70°C - suitable for commercial-grade embedded systems, instrumentation, and industrial control panels.

Pinout & Package

SN74121DR is housed in a 14-pin Small Outline Integrated Circuit (SOIC) package (Package Drawing D), 3.9 mm wide, 8.65 mm long, 1.75 mm max height, RoHS-compliant with NiPdAu lead finish and JEDEC Level-2 moisture sensitivity rating.

Pin/Terminal	Circuit Role	Design Meaning
1 (A1)	Low-active trigger input	Active-low input for first multivibrator; tied low to enable triggering on rising edge of B input.
2 (B1)	High-active trigger input	Active-high input for first multivibrator; rising edge initiates output pulse when A1 is low.
3 (Q1)	Inverted output	Complementary output of first multivibrator; low during active pulse, high otherwise.
4 (Q1̅)	Non-inverted output	True output of first multivibrator; high during active pulse, low otherwise.
9 (R_EXT)	External timing resistor connection	Connects to external resistor (or internal 2-kΩ) to set pulse width with C_EXT.
10 (C_EXT)	External timing capacitor connection	Connects to external capacitor; pulse width ≈ 0.7 × R_EXT × C_EXT for standard configuration.
14 (V_CC)	Positive supply	5-V nominal power rail; must be decoupled locally with 0.1-µF ceramic capacitor.
7 (GND)	Ground reference	Common return path for all inputs, outputs, and timing components; requires low-impedance PCB connection.

Key Features

Feature	Design Value
Dual independent monostables	Two fully separate one-shot circuits share only V_CC and GND, enabling concurrent timing tasks without crosstalk.
Retriggerable operation	Allows new trigger events to restart the output pulse before expiration - critical for variable-length pulse generation.
Flexible input logic	Each section accepts either low-active (A1/A2) or high-active (B) triggers, supporting NAND/NOR-style enable conditions.
Externally programmable width	Pulse duration set by user-selected R/C values (or internal 2-kΩ + external C), offering wide timing range from nanoseconds to seconds.
TTL-compatible I/O	Direct interface with 74LS, 74S, and other standard TTL families without level-shifting circuitry.

Applications

Keyboard Debouncing	Digital Pulse Stretching
Use Scenario: Mechanical keyboard matrix scanning where contact bounce causes false keypress detection. IC Role / Device Role / Timing Role: Monostable multivibrator generating clean, fixed-duration pulses per keypress to eliminate bounce artifacts. Use Value: Converts erratic switch transitions into single, well-defined 20-ms pulses compatible with microcontroller GPIO sampling.	Use Scenario: Extending narrow logic pulses from sensors or encoders to meet minimum width requirements of downstream logic. IC Role / Device Role / Timing Role: Pulse stretcher converting sub-100-ns edges into configurable 1–10 µs output windows for reliable latch capture. Use Value: Ensures sufficient setup/hold time for 74LS flip-flops and eliminates missed triggers in high-noise industrial environments.
Glitch Filtering	Legacy System Timing Control
Use Scenario: Removing sub-microsecond noise spikes on control lines in PLC backplanes or relay driver interfaces. IC Role / Device Role / Timing Role: Digital filter using monostable output pulse width as rejection threshold for transient interference. Use Value: Rejects glitches shorter than 50 ns while passing valid command signals - no software overhead or FPGA resources required.	Use Scenario: Synchronizing asynchronous reset signals across mixed-logic subsystems in retro-computing or test equipment. IC Role / Device Role / Timing Role: Timing coordinator generating aligned, jitter-free reset strobes from unsynchronized event sources. Use Value: Provides deterministic 100-ns resolution timing alignment between TTL, ECL, and discrete logic blocks in vintage hardware restoration.

Keyboard Debouncing

Digital Pulse Stretching

Use Scenario: Mechanical keyboard matrix scanning where contact bounce causes false keypress detection.

IC Role / Device Role / Timing Role: Monostable multivibrator generating clean, fixed-duration pulses per keypress to eliminate bounce artifacts.

Use Value: Converts erratic switch transitions into single, well-defined 20-ms pulses compatible with microcontroller GPIO sampling.

Use Scenario: Extending narrow logic pulses from sensors or encoders to meet minimum width requirements of downstream logic.

IC Role / Device Role / Timing Role: Pulse stretcher converting sub-100-ns edges into configurable 1–10 µs output windows for reliable latch capture.

Use Value: Ensures sufficient setup/hold time for 74LS flip-flops and eliminates missed triggers in high-noise industrial environments.

Glitch Filtering

Legacy System Timing Control

Use Scenario: Removing sub-microsecond noise spikes on control lines in PLC backplanes or relay driver interfaces.

IC Role / Device Role / Timing Role: Digital filter using monostable output pulse width as rejection threshold for transient interference.

Use Value: Rejects glitches shorter than 50 ns while passing valid command signals - no software overhead or FPGA resources required.

Use Scenario: Synchronizing asynchronous reset signals across mixed-logic subsystems in retro-computing or test equipment.

IC Role / Device Role / Timing Role: Timing coordinator generating aligned, jitter-free reset strobes from unsynchronized event sources.

Use Value: Provides deterministic 100-ns resolution timing alignment between TTL, ECL, and discrete logic blocks in vintage hardware restoration.

Equivalent & Alternatives

The following parts are listed as comparable options for similar monostable multivibrator applications.

Alternative Part	Technical Difference	Application Difference	Selection Advice
SN74LS122N	Retriggerable dual one-shot with wider pulse width range (10 ns–30 s), higher speed (t_PLH/t_PHL = 15 ns), but requires external R/C for all timing; no internal resistor.	Preferred for ultra-wide timing range and faster response; lacks internal timing resistor option.	Select SN74LS122N when precise sub-10-ns timing or multi-second delays are required, and board space allows full external R/C networks.
CD74HC221M	CMOS dual monostable with lower power (I_CC = 8 µA typ), wider V_CC range (2–6 V), but incompatible TTL input thresholds and slower min pulse width (30 ns).	Suitable for battery-powered or mixed-voltage systems; requires level-shifting for direct TTL interface.	Choose CD74HC221M for low-power or 3.3-V designs; avoid in pure 5-V TTL-only systems without input conditioning.

Compared with SN74LS122N and CD74HC221M, the SN74121DR offers plug-and-play TTL compatibility, built-in timing resistor, and guaranteed 25-ns propagation - making it optimal for drop-in replacement in legacy 5-V logic systems where design cycle time and signal integrity are prioritized over ultra-low power or extreme timing range.

Availability

SN74121DR is available at Aetrix Electronics and suitable for industrial control panels, test equipment interfaces, and legacy system repairs requiring stable component supply and long-term obsolescence mitigation.

Supply support for SN74121DR includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Texas Instruments is a U.S.-based semiconductor company founded in 1930, specializing in analog, embedded processing, and logic solutions with broad industrial, automotive, and consumer reach.

The SN74121DR belongs to TI's classic 74xx TTL logic family, designed specifically for robust, deterministic timing in noise-prone industrial and instrumentation environments where predictable propagation delay and TTL-level interoperability are essential.

FAQ

What is the minimum output pulse width achievable with SN74121DR?

The SN74121DR achieves a minimum output pulse width of 30 ns at V_CC = 5 V with no external timing components. This value is specified in the TI SDLS042A datasheet and reflects internal propagation and switching limits. Using external R/C can extend the pulse but cannot reduce it below this intrinsic minimum.

Can SN74121DR operate from a 3.3-V supply?

No, SN74121DR is a bipolar TTL device requiring 4.75 V to 5.25 V for guaranteed operation. At 3.3 V, input thresholds (V_IH ≥ 2.0 V, V_IL ≤ 0.8 V) and output drive (V_OH ≥ 2.4 V) are not met, resulting in unreliable triggering and weak logic levels. Use CD74HC221M or SN74LVC1G123 for 3.3-V compatibility.

Does SN74121DR support non-retriggerable mode?

No, SN74121DR is inherently retriggerable - a new trigger input during an active output pulse will restart the timing interval. Non-retriggerable behavior requires external gating logic or alternative devices like SN74LS221. The internal architecture does not provide a mode-select pin or internal latch to disable retriggering.

What is the function of pin 9 (R_EXT) on SN74121DR?

Pin 9 (R_EXT) on SN74121DR connects to the external timing resistor that sets output pulse width in conjunction with C_EXT on pin 10. When left unconnected, the internal 2-kΩ resistor is enabled, allowing pulse width determination using only C_EXT. This simplifies design for fixed-delay applications without discrete resistor placement.

Is SN74121DR pin-compatible with SN74123?

No, SN74121DR and SN74123 are not pin-compatible. While both are dual monostables in 14-pin packages, SN74123 uses different pin assignments: its Q/Q̅ outputs are on pins 12/13 (vs. SN74121DR's pins 3/4), and timing pins are located at 6/7 instead of 9/10. Direct substitution requires PCB redesign and signal routing changes.

SN74121DR Specifications

Product attributes: Attribute value

Manufacturer:: Texas Instruments

Series:: 7400

Package/Case:: 14-SOIC (0.154", 3.90mm Width)

Packaging:: Tape & Reel (TR)

Product Status:: Obsolete

Logic Type:: Monostable

Independent Circuits:: 2

Schmitt Trigger Input:: No

Propagation Delay:: 45 ns

Current - Output High, Low:: 400µA, 16mA

Voltage - Supply:: 4.75 V ~ 5.25 V

Operating Temperature:: 0°C ~ 70°C

Grade:: -

Qualification:: -

Mounting Type:: Surface Mount

Supplier Device Package:: 14-SOIC

SN74121DR FAQ

1.How can I place an order for SN74121DR through Aetrix?

Please submit a Request for Quotation (RFQ) for SN74121DR on Aetrix. Our sales agent will provide a competitive quotation and guide you through the order confirmation once you accept the terms.

2.Are the price and stock information for SN74121DR reliable?

The price and inventory of SN74121DR are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74121DR is usually 5 days.

3.What payment methods are accepted for SN74121DR?

We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74121DR transactions.

Note: Certain payment methods may incur a processing fee.

4.How is shipping managed for SN74121DR?

SN74121DR orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.

Once your SN74121DR order is processed, you will receive an email with the shipment details and tracking number.

Note: Tracking information may take up to 24 hours to appear. Express delivery typically takes 3–5 business days.

5.How can I obtain technical support or documentation for SN74121DR?

For technical support, including SN74121DR datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74121DR requirements.

6.How does Aetrix verify that SN74121DR is sourced from the original manufacturer or authorized distributors?

All SN74121DR products on Aetrix are procured from qualified distributors and authorized channels. Our dedicated quality assurance team conducts strict verification, including traceability checks and, if necessary, third-party testing. This ensures that SN74121DR meets industry standards.

7.What is the process for return or replacement of SN74121DR?

All SN74121DR units undergo pre-shipment inspection (PSI). If there is an issue with SN74121DR, returns or replacements are accepted under the following conditions:

1.Quantity discrepancies, incorrect items, or visible external defects (such as breakage or corrosion), acknowledged by Aetrix.

2.The issue is reported within 90 days of delivery.

3.The SN74121DR part is unused and in its original packaging.

Return procedure for SN74121DR:

1.Submit a request within 90 days.

2.Obtain a Return Material Authorization (RMA) from Aetrix.

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