Photointerrupter LTH-872-N55H Datasheet - Slot Type - Dimensions 4.0mm - Voltage 5V - English Technical Document

1. Product Overview

The LTH-872-N55H is a slot-type photointerrupter, a fundamental optoelectronic component used for non-contact sensing and switching applications. It integrates an infrared light-emitting diode (LED) and a phototransistor within a single housing, separated by a physical gap or slot. The core principle of operation is straightforward: when an object passes through this slot, it interrupts the infrared light beam traveling from the emitter to the detector, causing a corresponding change in the phototransistor's output state. This simple yet reliable mechanism makes it an ideal solution for detecting the presence, absence, position, or speed of objects without physical contact.

The device is designed for direct PCB (Printed Circuit Board) mounting or insertion into a standard dual-in-line socket, offering flexibility in assembly and prototyping. Its primary advantages include high reliability, fast switching speed, and long operational life due to the absence of mechanical contacts that can wear out. Typical applications span a wide range of office automation and industrial equipment, including but not limited to printers, photocopiers, scanners, facsimile machines, and various automated systems where precise object detection is required.

1.1 Core Features

• Non-contact Switching: Eliminates mechanical wear and tear, ensuring high reliability and a long operational lifespan.
• Versatile Mounting: Compatible with direct PCB soldering or standard dual-in-line sockets, facilitating easy integration into various circuit designs.
• Fast Response Time: Enables detection of high-speed events, suitable for applications requiring quick sensing, such as paper feed detection in printers or rotary encoder systems.

2. Technical Parameters: In-Depth Objective Interpretation

The datasheet provides critical parameters that define the device's operational limits and performance under standard conditions. Understanding these parameters is essential for proper circuit design and ensuring long-term reliability.

2.1 Absolute Maximum Ratings

These ratings specify the stress limits that, if exceeded, may cause permanent damage to the device. They are not conditions for normal operation.

• Input LED:
  • Power Dissipation (P_D): 75 mW maximum. This is the total power the LED can safely dissipate as heat.
  • Continuous Forward Current (I_F): 50 mA maximum. The LED should not be driven with a continuous current exceeding this value.
  • Reverse Voltage (V_R): 5 V maximum. Applying a reverse voltage higher than this can break down the LED junction.
• Output Phototransistor:
  • Power Dissipation (P_D): 100 mW maximum for the phototransistor.
  • Collector-Emitter Voltage (V_CEO): 30 V maximum. This is the maximum voltage that can be applied between the collector and emitter when the base (light input) is open.
  • Emitter-Collector Voltage (V_ECO): 5 V maximum, which is the reverse voltage rating for the collector-emitter junction.
  • Collector Current (I_C): 20 mA maximum. The load current through the phototransistor must stay below this limit.
• Environmental:
  • Operating Temperature Range (T_A): -25°C to +85°C. The device is guaranteed to operate within specifications across this ambient temperature range.
  • Storage Temperature Range (T_stg): -40°C to +100°C.
  • Lead Soldering Temperature: 260°C for 5 seconds maximum, specified for a lead form case of 1.6mm (0.063 inches). This is critical for reflow or wave soldering processes.

2.2 Electrical and Optical Characteristics

These parameters define the device's performance under typical operating conditions at an ambient temperature (T_A) of 25°C.

• Input LED Characteristics:
  • Forward Voltage (V_F): Typically 1.2V, with a maximum of 1.6V at a forward current (I_F) of 20 mA. This parameter is used to calculate the current-limiting resistor value for the LED driver circuit: R = (V_CC - V_F) / I_F.
  • Reverse Current (I_R): Maximum 100 µA at a reverse voltage (V_R) of 5V, indicating the LED's leakage current when reverse-biased.
• Output Phototransistor Characteristics:
  • Collector-Emitter Dark Current (I_CEO): Maximum 100 nA at V_CE = 10V. This is the leakage current when the LED is off (no light incident on the phototransistor). A low dark current is desirable for good signal-to-noise ratio, especially in low-light or high-gain applications.
• Coupler (System) Characteristics:
  • Collector-Emitter Saturation Voltage (V_CE(SAT)): Maximum 0.4V when the phototransistor is fully on (I_C = 0.25 mA, I_F = 20 mA). A low saturation voltage is crucial when the output is used to drive logic inputs or other low-voltage circuits, as it defines the logical \"LOW\" level.
  • On-State Collector Current (I_C(ON)): Minimum 2.0 mA at V_CE = 5V and I_F = 20 mA. This is the guaranteed minimum output current when the LED is driven at its typical current and the beam is unobstructed. This parameter, often called the \"current transfer ratio\" (CTR) when expressed as a ratio I_C/I_F, defines the sensitivity of the coupler. Here, the minimum CTR is (2.0 mA / 20 mA) = 0.1 or 10%.
  • Response Time:
    • Rise Time (T_r): Typical 3 µs, maximum 15 µs. This is the time for the output to transition from 10% to 90% of its final value when the input LED is turned on.
    • Fall Time (T_f): Typical 4 µs, maximum 20 µs. This is the time for the output to transition from 90% to 10% of its final value when the input LED is turned off. These fast switching speeds enable the detection of rapidly moving objects.

3. Mechanical and Package Information

The LTH-872-N55H features a standard through-hole package designed for easy PCB integration.

3.1 Outline Dimensions

The datasheet provides a detailed mechanical drawing. Key dimensions include the overall slot width, which defines the size of the object that can be detected, and the pin spacing for PCB layout. All dimensions are specified in millimeters (mm) with a standard tolerance of ±0.25 mm unless otherwise noted. The drawing typically shows the top view, side view, and pin identification (emitter anode, emitter cathode, collector, emitter).

3.2 Polarity Identification and Pinout

Correct polarity is essential for device operation. The package has a marking or a specific pin shape (often a flat side or a notch) to identify Pin 1. The standard pinout for a 4-pin photointerrupter is: Pin 1 - Anode of IR LED, Pin 2 - Cathode of IR LED, Pin 3 - Emitter of Phototransistor, Pin 4 - Collector of Phototransistor. Always refer to the datasheet diagram to confirm the exact pin assignment for the LTH-872-N55H before designing the PCB footprint.

4. Soldering and Assembly Guidelines

4.1 Soldering Process

The device is rated for a maximum lead soldering temperature of 260°C for 5 seconds. This specification is critical for wave soldering or reflow processes. Exceeding this temperature or time can damage the internal semiconductor junctions or the plastic housing. It is recommended to follow standard IPC guidelines for through-hole component soldering.

4.2 Handling and Storage

While not explicitly detailed in the provided excerpt, general best practices apply: store components in a dry, anti-static environment within the specified storage temperature range (-40°C to +100°C). Avoid exposing the device to excessive moisture before soldering to prevent \"popcorning\" during reflow, although this is more critical for surface-mount devices.

5. Application Suggestions and Design Considerations

5.1 Typical Application Circuits

The most common configuration is to use the photointerrupter as a digital switch. A simple circuit involves:
1. LED Driver: Connect a current-limiting resistor in series with the infrared LED to a voltage source (e.g., 5V). Set the resistor value to achieve the desired I_F (e.g., 20 mA). Example: R_limit = (5V - 1.2V) / 0.02A = 190Ω (use a standard 200Ω resistor).
2. Phototransistor Output: Connect a pull-up resistor (R_L) from the phototransistor's collector to a voltage source (e.g., 5V). The emitter is connected to ground. When the light path is clear, the phototransistor conducts, pulling the collector voltage (output) low. When the beam is blocked, the phototransistor turns off, and the pull-up resistor pulls the output high. The value of R_L affects switching speed and current consumption; a lower value gives faster speed but higher power dissipation. The test condition in the datasheet uses R_L = 100Ω.

5.2 Design Considerations

• Ambient Light Immunity: Since the device uses infrared light, it is somewhat immune to visible ambient light. However, strong infrared sources (sunlight, some lamps) can cause interference. Using a modulated LED signal and a corresponding demodulating circuit can greatly enhance noise immunity.
• Alignment: The emitter and detector must be precisely aligned across the slot. The mechanical housing ensures this alignment, but the PCB design must place the component correctly.
• Object Characteristics: The object interrupting the beam should be opaque to infrared light. Transparent or highly reflective materials may not be reliably detected.
• Debouncing: In mechanical systems (e.g., detecting a chopper wheel), the output signal may chatter as an object enters or leaves the slot. Software or hardware debouncing techniques should be employed for clean digital signals.

6. Performance Curve Analysis

The datasheet mentions \"Typical Electrical / Optical Characteristics Curves.\" While the specific curves are not provided in the excerpt, typical plots for such devices include:

• Forward Current vs. Forward Voltage (I_F-V_F): Shows the non-linear relationship for the IR LED, important for driver design.
• Collector Current vs. Collector-Emitter Voltage (I_C-V_CE): Family of curves with incident light intensity (or I_F) as a parameter, similar to a transistor's output characteristics.
• Current Transfer Ratio (CTR) vs. Forward Current (I_F): Shows how sensitivity changes with LED drive current.
• Current Transfer Ratio (CTR) vs. Ambient Temperature: A crucial curve showing that CTR typically decreases as temperature increases. Designers must ensure sufficient margin at the highest operating temperature to guarantee the minimum required I_C(ON).
• Response Time vs. Load Resistance (R_L): Illustrates the trade-off between switching speed and power consumption.

7. Technical Comparison and Differentiation

Compared to mechanical micro-switches, the LTH-872-N55H offers superior life expectancy and reliability due to non-contact operation. It is immune to contact bounce. Compared to reflective sensors, slot-type photointerrupters provide more precise and consistent detection as they are less sensitive to the color, texture, or reflectivity of the target object; they simply detect the physical interruption of a beam. The key differentiator among photointerrupters themselves is often the slot dimensions, sensitivity (CTR), response speed, and package type (through-hole vs. surface-mount).

8. Frequently Asked Questions (Based on Technical Parameters)

Q: What happens if I drive the LED with more than 50 mA?
A: Exceeding the Absolute Maximum Rating for continuous forward current can cause excessive heating, leading to accelerated degradation of the LED's light output or catastrophic failure. Always use a current-limiting resistor.

Q: My output signal is noisy. What could be the cause?
A: Potential causes include electrical noise on the power supply lines, interference from ambient light (especially fluorescent lights operating at 50/60 Hz), or a load resistor value that is too high, making the node high-impedance and susceptible to noise. Ensure stable power, consider shielding, use a lower pull-up resistor, or implement signal modulation/demodulation.

Q: The device works at room temperature but fails when my system heats up. Why?
A: The phototransistor's sensitivity (CTR) decreases with increasing temperature. You may be operating with minimal margin at 25°C. Re-evaluate your design using the minimum I_C(ON) specification and consider the typical CTR vs. Temperature curve. You may need to increase the LED drive current (within limits) or use a phototransistor with higher guaranteed CTR at elevated temperatures.

Q: Can I use this to detect a transparent object?
A: Generally, no. Standard infrared photointerrupters require the object to be opaque to the infrared wavelength emitted (typically around 940 nm). Transparent plastics or glass may allow enough IR light to pass, preventing reliable detection. Special sensors with different wavelengths or detection principles are needed for transparent materials.

9. Practical Use Case Example

Application: Paper Jam Detection in a Desktop Printer.
Implementation: The LTH-872-N55H is mounted along the paper path with the paper feeding through its slot. A microcontroller GPIO pin drives the LED via a current-limiting resistor. Another GPIO pin, configured with an internal pull-up resistor, reads the state of the phototransistor's collector. During normal operation, the paper interrupts the beam, and the output is in one logic state (e.g., HIGH). If a paper jam occurs, the paper either remains stuck (keeping the beam interrupted) or fails to reach the sensor (leaving the beam unbroken), causing the output to be in an unexpected state for too long. The microcontroller firmware monitors this signal and triggers a \"Paper Jam\" error message if the expected timing sequence is violated. The fast response time of the sensor ensures even small gaps between sheets of paper can be detected for precise paper feed monitoring.

10. Operating Principle Introduction

A photointerrupter operates on the principle of optoelectronic conversion and interruption. Internally, it houses two discrete components in alignment: an infrared light-emitting diode (IR LED) and a silicon phototransistor. The IR LED acts as the light source. When forward-biased by an external current, it emits invisible infrared photons. The phototransistor acts as the light detector. Its base region is sensitive to light. When photons from the LED strike the base, they generate electron-hole pairs, which act as base current, turning the transistor on and allowing a much larger collector current to flow. This collector current is proportional to the intensity of the incident light. The slot physically separates these two elements. An object placed in the slot blocks the light path, drastically reducing the light incident on the phototransistor, which in turn switches it off (or reduces its current). This change in output current/voltage is detected by external circuitry to register an \"interruption.\"

11. Industry Trends and Developments

The trend in optoelectronic sensors, including photointerrupters, is towards miniaturization, higher integration, and surface-mount technology (SMT) packages to accommodate smaller and denser PCB designs. There is also a move towards digital output sensors with built-in signal conditioning, which provide a clean, buffered logic-level output, simplifying interface with microcontrollers. Some advanced versions incorporate Schmitt triggers for hysteresis to improve noise immunity. Furthermore, the demand for higher precision and speed in automation drives the development of devices with narrower slots, faster response times, and improved temperature stability. The fundamental slot-type photointerrupter, as represented by the LTH-872-N55H, remains a cost-effective and highly reliable solution for a vast array of standard detection applications where its simplicity and robustness are key advantages.