onsemi H21B2

Overview

H21B2 from Fairchild Semiconductor is a gallium arsenide infrared emitting diode coupled with a silicon photodarlington in a plastic optical interrupter housing, designed for mechanical position sensing and object detection. It delivers 1.0 mA on-state collector current at 2 mA input, features 30 V V_CEO, 1.0 V V_CE(SAT), and operates across –55°C to +100°C ambient range - used in industrial encoder wheels and printer paper-path detection.

For engineers reviewing the H21B2 datasheet, pinout, applications, or equivalent options, key selection criteria include its 1.0 mA I_C(ON) at 2 mA I_F, 45 µs turn-on time, 250 µs turn-off time, opaque housing with 0.035" aperture, and compatibility with standard through-hole PCB assembly.

Technical Context

The H21B2 functions as a reflective or transmissive optical interrupter switch: IR LED emission is optically coupled to a photodarlington detector, enabling high-current gain (β ≈ 1000) and low-output saturation voltage. Its output stage provides open-collector switching with 30 V V_CEO and 40 mA I_C rating.

Designed for ambient light rejection via opaque housing and precise 0.035" aperture geometry, it supports reliable edge detection in dusty or variable-light environments. Thermal derating applies above 25°C (1.67 mW/°C), and soldering must follow specified iron (240°C/5 s) and flow (260°C/10 s) limits.

Key Specifications

Pinout & Package

Package: 4-pin DIP (dual in-line package) with 0.3" row spacing, plastic opaque housing, 0.035" optical aperture gap between emitter and detector.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar optical interrupter switch applications.

Compared with H21A2 and PC817XNIP0X, the H21B2 provides optimal balance of sensitivity (1.0 mA I_C(ON)), speed (45 µs t_on), and ease of integration - requiring no external amplification while maintaining robust noise immunity in industrial settings.

Availability

H21B2 is available at Aetrix Electronics and suitable for industrial encoder wheel detection, printer paper-path sensing, and automotive throttle position feedback requiring stable component supply and long-term lifecycle support.

Supply support for H21B2 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

Fairchild Semiconductor was a U.S.-based analog and power IC manufacturer acquired by ON Semiconductor in 2016; known for high-reliability optocouplers and discrete power solutions.

The H21B2 belongs to Fairchild's photodarlington optical interrupter family, engineered specifically for robust position and presence sensing in noisy, thermally demanding industrial and automotive environments.

FAQ

What is the maximum continuous forward current rating for the H21B2 emitter?

The H21B2 emitter has a maximum continuous forward current (I_F) rating of 50 mA at T_A = 25°C. Derating is required above 25°C per the datasheet's linear derating curve (1.67 mW/°C). This limit ensures long-term reliability of the GaAs infrared LED and prevents thermal runaway in the photodarlington pair. The H21B2 is typically operated at 2–10 mA for optimal lifetime and coupling efficiency.

Does the H21B2 require an external pull-up resistor on its output?

Yes, the H21B2 output is an open-collector photodarlington configuration (PIN 3 = Collector, PIN 4 = Emitter), so it requires an external pull-up resistor to V_CC (e.g., 4.7 kΩ for 5 V logic). Without this resistor, the output cannot transition to a valid HIGH state. The H21B2 itself does not contain internal pull-up circuitry, and proper resistor selection ensures fast turn-off (t_off ≤ 250 µs) and adequate noise margin in the receiving logic stage.

Can the H21B2 be used in transmissive or reflective mode?

The H21B2 is designed exclusively for transmissive optical interrupter operation - its physical housing contains a fixed 0.035" gap separating the integrated GaAs emitter and silicon photodarlington, intended for interruption by an opaque object. It is not configured for reflective sensing (no dedicated detector orientation or lens for bounce-back light), and attempting reflective use results in insufficient signal margin due to low reflected IR intensity and poor optical coupling geometry.

What is the saturation voltage V_CE(SAT) specification for the H21B2, and under what conditions is it measured?

The H21B2 has a maximum V_CE(SAT) of 1.0 V, measured under I_F = 10 mA and I_C = 1.8 mA. A second condition specifies ≤1.5 V at I_F = 60 mA and I_C = 50 mA. This low saturation voltage ensures minimal power dissipation in the output stage and allows direct interfacing with 3.3 V or 5 V logic families without level shifting. The value reflects the photodarlington's inherent gain and junction characteristics, not external circuit dependencies.

Is the H21B2 RoHS compliant and halogen-free?

The original H21B2 datasheet (2002) predates RoHS Directive 2002/95/EC and does not declare RoHS or halogen-free status. As a legacy Fairchild part, it was manufactured prior to mandatory compliance timelines. For new designs requiring RoHS/halogen-free assurance, engineers should consult ON Semiconductor's current cross-reference documentation or consider modern alternatives like the FODM8071, as the H21B2 may contain leaded solder and brominated flame retardants per 2002 process standards.