Texas Instruments SN74LS221NSR

Overview

SN74LS221NSR from Texas Instruments is a dual monostable multivibrator IC with TTL-compatible inputs and outputs, designed for precise pulse generation in digital timing circuits. It features two independent one-shot circuits, each with negative- and positive-transition-triggered inputs (A and B), overriding clear (CLR), and external R_ext/C_ext-controlled pulse width up to 70 ms. Used in clock stretching, debouncing, and delay generation within industrial control logic and legacy TTL systems.

For engineers reviewing the SN74LS221NSR datasheet, SN74LS221NSR pinout, SN74LS221NSR application, or SN74LS221NSR equivalent, key selection considerations include its 16-pin SOP-NS package, 0°C to 70°C operating range, 70 ms maximum output pulse length, Schmitt-trigger B-input with 1.2 V noise immunity, and compatibility with SN74LS123-based designs via R/C value adjustment.

Technical Context

The SN74LS221NSR implements two independent TTL-level monostable multivibrators using internal NAND-gate latches and timing networks. Each section accepts triggering via either A (positive-edge) or B (negative-edge) inputs, with B featuring Schmitt-trigger hysteresis for jitter-free operation down to 1 V/s input slew rate.

Pulse width is defined by t_w ≈ 0.7 × R_ext × C_ext, supported over R_ext = 1.4 kΩ–100 kΩ and C_ext = 0–1000 µF. An overriding clear (CLR) forces Q low independently of timing components, enabling synchronous reset across both sections.

Key Specifications

Pinout & Package

The SN74LS221NSR is housed in a 16-pin SOP (Small Outline Package) with NS suffix, measuring 10.4 mm × 5.3 mm × 2.0 mm max height, RoHS-compliant, tape-and-reel packaged (2000 units/reel), MSL Level-1.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS123DR and SN74HC221DR, the SN74LS221NSR uniquely combines non-retriggerable operation, overriding clear, Schmitt-trigger noise immunity, and pin-for-pin compatibility with legacy LS123 layouts-making it the preferred choice for maintenance of existing TTL timing subsystems.

Availability

SN74LS221NSR is available at Aetrix Electronics and suitable for industrial control panels, legacy instrumentation upgrades, and military-grade repair programs requiring stable component supply and long-term obsolescence management.

Supply support for SN74LS221NSR 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 and embedded processing technologies with broad industrial, automotive, and aerospace design expertise.

The SN74LS221NSR belongs to TI's legacy TTL logic product line, engineered specifically for high-reliability, noise-immune digital timing in mission-critical and long-lifecycle systems where compatibility and predictability outweigh power efficiency.

FAQ

What is the maximum output pulse width achievable with SN74LS221NSR?

The SN74LS221NSR supports a maximum output pulse width of 70 ms when configured with R_ext = 100 kΩ and C_ext = 1 µF (t_w ≈ 0.7 × R_ext × C_ext). With C_ext extended to 1000 µF and R_ext at 1.4 kΩ, pulse widths up to several seconds are possible-though jitter increases beyond recommended ranges. The SN74LS221NSR datasheet specifies 70 ms as the guaranteed maximum under full-spec conditions.

Does SN74LS221NSR support retriggering during an active output pulse?

No, the SN74LS221NSR is a non-retriggerable monostable multivibrator. Once triggered, the output pulse duration is determined solely by R_ext and C_ext, and subsequent input transitions on A or B are ignored until the pulse completes or is terminated by CLR. This behavior distinguishes it from retriggerable alternatives like SN74LS123 and ensures deterministic timing in critical control sequences.

Can SN74LS221NSR replace SN74LS123 in an existing PCB layout?

Yes, the SN74LS221NSR has identical pinout to SN74LS123 and SN74123, allowing direct physical replacement. However, timing component values must be adjusted: SN74LS221NSR uses t_w ≈ 0.7 × R_ext × C_ext, whereas SN74LS123 uses t_w ≈ 0.45 × R × C. Also, the capacitor polarity must be reversed due to internal circuit differences. The SN74LS221NSR provides overriding clear functionality not present in SN74LS123.

What is the input hysteresis voltage of the B input on SN74LS221NSR?

The B input of the SN74LS221NSR features Schmitt-trigger hysteresis with a typical difference of 1.2 V between positive-going (V_T+ ≈ 2 V) and negative-going (V_T− ≈ 0.8 V) thresholds at V_CC = 4.75 V. This hysteresis enables reliable triggering from slowly varying or noisy signals-such as mechanical switch closures-without external filtering. The SN74LS221NSR maintains this performance across its full 0°C to 70°C operating range.

Is SN74LS221NSR compatible with 3.3 V logic systems?

No, the SN74LS221NSR is a bipolar TTL device requiring a nominal 5 V supply (4.75–5.25 V). Its input thresholds and output voltage levels are specified for TTL logic families and are not compatible with 3.3 V CMOS or LVTTL interfaces without level translation. Attempting to operate the SN74LS221NSR below 4.75 V may result in undefined output states or increased propagation delay. For 3.3 V systems, consider CMOS alternatives like SN74HC221.