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Product details
Overview
H21B1 from Fairchild Semiconductor is a photodarlington optical interrupter switch comprising a GaAs infrared LED coupled to a silicon photodarlington in a plastic housing with 0.035" apertures. It delivers 0.5 mA on-state collector current at 2 mA input, 1.0 V saturation voltage, and 45 µs turn-on time, used for position sensing and object detection in industrial encoders and printer paper-path monitoring.
For engineers reviewing the H21B1 datasheet, pinout, applications, or equivalent options, key selection criteria include its 30 V VCEO, -55°C to +100°C operating temperature range, opaque housing for ambient light rejection, and mechanical gap-based switching architecture requiring no external optics or alignment.
Technical Context
The H21B1 operates as a galvanically isolated digital switch: infrared emission from the GaAs diode triggers conduction in the photodarlington output stage, enabling high-current gain (IC(ON)/IF ≥ 250) without external amplification. Its 0.035" aperture defines mechanical resolution and limits stray-light coupling.
It functions exclusively in DC or low-frequency pulsed mode (PRR ≤ 100 pps), with saturation behavior defined at VCE = 1.5 V and IC up to 1.8 mA. Thermal derating applies above 25°C (1.67 mW/°C), and soldering must avoid thermal stress within 1.6 mm of the housing.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| IC(ON) @ IF=2 mA | 0.5 mA - minimum guaranteed output current for reliable logic-level switching at low drive |
| VCE(SAT) | 1.0 V max - ensures low power loss and compatibility with 3.3 V/5 V logic interfaces |
| ton | 45 µs - enables detection of objects moving at ≤ 22 cm/s across 1 mm gap |
| VCEO | 30 V - supports direct interface with 24 V industrial control lines without external clamping |
| TOPR | -55°C to +100°C - qualified for under-hood automotive and factory-floor embedded use |
| Aperture width | 0.035" (0.89 mm) - sets mechanical resolution limit and physical interrupt threshold |
Pinout & Package
Package: 4-pin DIP plastic housing with side-entry optical gap; dimensions 0.433" × 0.315" × 0.129" (11.0 × 8.0 × 3.3 mm), lead pitch 0.100", body width 0.249".
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| Pin 1 | Anode | Positive terminal of infrared emitter diode; requires current-limiting resistor when driven from 3.3 V/5 V supply |
| Pin 2 | Cathode | Negative terminal of emitter; common reference for input drive circuit |
| Pin 3 | Collector | Output high-side terminal of photodarlington; connects to load and VCC via pull-up |
| Pin 4 | Emitter | Output low-side terminal; grounded to complete output current path |
Key Features
| Feature | Design Value |
|---|---|
| Opaque housing | Blocks ambient visible/NIR light, eliminating need for external shielding in uncontrolled lighting environments |
| 0.035" aperture | Defines precise mechanical interrupt window-critical for repeatable edge detection in rotary encoders |
| Photodarlington output | Provides >100× current gain vs. phototransistor, enabling direct drive of relays or logic inputs without buffer stages |
| High VCEO (30 V) | Permits direct connection to 24 V PLC I/O modules without level-shifting or protection circuitry |
Applications
| Industrial Encoder Position Sensing | Printer Paper Path Detection |
|---|---|
Use Scenario: Detecting slotted disk rotation in CNC machine feedback systems. IC Role / Device Role / Timing Role: Optical interrupter providing digital edge pulses synchronized to mechanical motion. Use Value: 0.035" aperture ensures ±0.1 mm positional repeatability; 45 µs ton supports ≤ 22 kHz pulse rates at 3000 RPM. | Use Scenario: Monitoring paper presence and jam conditions in laser printer feed mechanisms. IC Role / Device Role / Timing Role: Gap-interrupt sensor triggering paper-out alerts and motor stop commands. Use Value: Opaque housing rejects fluorescent lamp interference; 0.5 mA IC(ON) drives logic directly without pull-up resistor redesign. |
| Automotive Throttle Position Feedback | Conveyor Belt Object Counting |
Use Scenario: Measuring throttle plate angle via slotted vane mounted on shaft. IC Role / Device Role / Timing Role: Isolated analog-adjacent digital switch converting mechanical angle to pulse train. Use Value: -55°C to +100°C rating meets AEC-Q100 Grade 2 requirements; 30 V VCEO tolerates load-dump transients. | Use Scenario: Counting boxes passing through fixed gate on packaging line. IC Role / Device Role / Timing Role: Ambient-light-immune presence detector generating count pulses per object. Use Value: High IC(ON)/IF ratio reduces required LED drive current, extending LED lifetime beyond 100k hours. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar optical interrupter switch applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| HCPL-2630 | Phototransistor output (not photodarlington); lower IC(ON) (0.4 mA @ 10 mA IF), faster ton (2 µs), higher VCEO (70 V) | Better for high-speed counting (>100 kHz); unsuitable where high output current or low drive is needed | Select HCPL-2630 only if speed >10× H21B1 is required and output current ≥0.4 mA suffices |
| PS2561-1 | Phototransistor output; 5000 VRMS isolation; IC(ON) = 0.5 mA @ 5 mA IF; TO-5 metal can package | Superior noise immunity and creepage/clearance for medical/industrial safety compliance; larger footprint and higher cost | Choose PS2561-1 when reinforced isolation or EMI robustness is mandatory, not for cost-sensitive consumer encoders |
Compared with HCPL-2630 and PS2561-1, the H21B1 provides higher output current per input mA and lower system cost due to its plastic DIP package and photodarlington gain-making it optimal for medium-speed, high-reliability mechanical sensing where isolation voltage and ultra-fast response are secondary.
Availability
H21B1 is available at Aetrix Electronics and suitable for industrial encoder position sensing, printer paper path detection, automotive throttle position feedback, and conveyor belt object counting requiring stable component supply and long-term lifecycle support.
Supply support for H21B1 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 semiconductor company specializing in power management, signal conditioning, and optoelectronics before its acquisition by ON Semiconductor in 2016.
The H21B1 belongs to Fairchild's photodarlington optical interrupter family, designed specifically for cost-sensitive, mechanically coupled position and presence detection in industrial and office equipment.
FAQ
What is the maximum continuous forward current rating for the H21B1 emitter?
The H21B1 emitter has a maximum continuous forward current (IF) rating of 50 mA at TA = 25°C. Derating is required above 25°C per the datasheet's linear derating curve (1.67 mW/°C). Exceeding this rating risks irreversible degradation of the GaAs LED. The H21B1 is typically operated at 2–10 mA for optimal lifetime and coupling efficiency.
Does the H21B1 require an external base connection for the photodarlington?
No, the H21B1 photodarlington output stage has no base terminal exposed-it is internally connected and optimized for simplicity. Only Collector (Pin 3) and Emitter (Pin 4) are accessible. This eliminates bias network design complexity and improves reliability. The H21B1 functions as a two-terminal output switch, unlike discrete phototransistors that may offer base access.
Can the H21B1 be used in AC-coupled or high-frequency switching applications?
No-the H21B1 is specified only for DC and low-frequency pulsed operation (max 100 pps per datasheet). Its 250 µs turn-off time and photodarlington structure make it unsuitable for AC line sensing or >1 kHz modulation. For such uses, consider phototransistor-based alternatives like the HCPL-2630. The H21B1 is engineered for mechanical interrupt timing, not signal transmission.
What is the meaning of "opaque housing" in the H21B1 datasheet features?
The opaque housing of the H21B1 blocks ambient visible and near-infrared light, preventing false triggering from room lighting, sunlight, or nearby LEDs. This eliminates the need for external shrouds or filters in most applications. The housing material and geometry are integral to the H21B1's specification-its performance assumes no additional optical shielding is added or removed.
Is the H21B1 RoHS compliant and halogen-free?
Yes-the H21B1 manufactured by Fairchild Semiconductor meets RoHS Directive 2011/65/EU and is halogen-free per IEC 61249-2-21. Lead-free soldering profiles (e.g., reflow peak 260°C for 10 sec) are supported, consistent with its flow-soldering absolute maximum rating. Compliance documentation is available upon request for the H21B1 production lot.
H21B1 Specifications
- Product attributes
- Attribute value
- Manufacturer:
- onsemi
- Series:
- -
- Package/Case:
- Slotted, PC Pins
- Packaging:
- Tube
- Product Status:
- Obsolete
- Sensing Distance:
- 0.118" (3mm)
- Sensing Method:
- Through-Beam
- Output Configuration:
- Photodarlington
- Current - DC Forward (If) (Max):
- 50 mA
- Current - Collector (Ic) (Max):
- 40 mA
- Voltage - Collector Emitter Breakdown (Max):
- 30 V
- Response Time:
- 7µs, 45µs
- Operating Temperature:
- -55°C ~ 100°C
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Through Hole, Flange
H21B1 FAQ
1.How can I place an order for H21B1 through Aetrix?
Please submit a Request for Quotation (RFQ) for H21B1 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 H21B1 reliable?
The price and inventory of H21B1 are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for H21B1 is usually 5 days.
3.What payment methods are accepted for H21B1?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for H21B1 transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for H21B1?
H21B1 orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your H21B1 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 H21B1?
For technical support, including H21B1 datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your H21B1 requirements.
6.How does Aetrix verify that H21B1 is sourced from the original manufacturer or authorized distributors?
All H21B1 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 H21B1 meets industry standards.
7.What is the process for return or replacement of H21B1?
All H21B1 units undergo pre-shipment inspection (PSI). If there is an issue with H21B1, 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 H21B1 part is unused and in its original packaging.
Return procedure for H21B1:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
H21B1 Tags
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