LTH-872-T55T1 Photointerrupter Datasheet - Slotted Optical Switch - English Technical Document

1. Product Overview

The LTH-872-T55T1 is a slotted-type photointerrupter, a fundamental optoelectronic component designed for non-contact sensing 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 operational principle involves the interruption of the infrared light beam traveling from the emitter to the detector. When an opaque object passes through this slot, it blocks the light, causing a significant change in the phototransistor's output current. This change is electronically detected, providing a reliable digital switching signal. Photointerrupters are favored for their high reliability, accuracy, and immunity to environmental factors like dust or surface contamination compared to mechanical switches.

Core Advantages: The primary advantages of this device include true non-contact switching, which eliminates mechanical wear and ensures a long operational lifespan. It offers fast response times, enabling detection of high-speed events. The design is suitable for direct PCB mounting or use with a dual-in-line socket, providing flexibility in assembly. Its construction provides inherent protection against ambient light interference.

Target Market & Applications: This component is widely utilized across various office automation and consumer electronics equipment. Typical application scenarios include paper detection in facsimile machines, printers, and photocopiers, where it senses the presence or absence of paper, paper jams, or the position of print heads and carriages. It is also found in scanners, vending machines, industrial automation for position sensing, and any device requiring precise, reliable object detection without physical contact.

2. In-Depth Technical Parameter Analysis

2.1 Absolute Maximum Ratings

These ratings define the stress limits beyond which permanent damage to the device may occur. Operation under or at these limits is not guaranteed.

• Input LED:
  • Power Dissipation (P_D): 75 mW. This is the maximum power the LED chip can dissipate as heat at an ambient temperature (T_A) of 25°C. Exceeding this can lead to thermal runaway and failure.
  • Continuous Forward Current (I_F): 50 mA. The maximum DC current that can be continuously passed through the LED.
  • Peak Forward Current: 1 A (pulse width = 10 µs, 300 pps). This rating allows for brief, high-current pulses, useful for driving the LED with higher instantaneous optical output without exceeding the average power rating.
  • Reverse Voltage (V_R): 5 V. The maximum reverse-bias voltage that can be applied across the LED. Exceeding this can cause junction breakdown.
• Output Phototransistor:
  • Power Dissipation (P_D): 100 mW.
  • Collector-Emitter Voltage (V_CEO): 30 V. The maximum voltage that can be applied between the collector and emitter when the base (light input) is open.
  • Collector Current (I_C): 20 mA. The maximum current that can flow through the collector-emitter path.
• Thermal Limits:
  • Operating Temperature Range: -25°C to +85°C. The ambient temperature range over which the device is specified to operate correctly.
  • Storage Temperature Range: -55°C to +100°C.
  • Lead Soldering Temperature: 260°C for 5 seconds (at 1.6mm from the case body). This defines the reflow soldering profile constraint to prevent damage to the plastic package and internal wire bonds.

2.2 Electrical & Optical Characteristics

These parameters are measured under standard test conditions (T_A=25°C) and define the typical performance of the device.

• Input LED Characteristics:
  • Forward Voltage (V_F): Typically 1.2V, with a maximum of 1.6V at I_F = 20 mA. This parameter is crucial for designing the current-limiting resistor for the LED driver circuit. A typical design would aim for I_F=20mA, using V_F~1.2V for calculation.
  • Reverse Current (I_R): Maximum 100 µA at V_R = 5V. This indicates the quality of the LED's PN junction under reverse bias.
• Output Phototransistor Characteristics:
  • Collector-Emitter Breakdown Voltage (V_(BR)CEO): Minimum 30V at I_C=1mA. This ensures a good safety margin for typical 5V or 12V logic circuits.
  • 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). A low value is essential for a well-defined "OFF" state, especially in high-gain circuits.
• Coupler (System) Characteristics:
  • On-State Collector Current (I_C(ON)): Minimum 0.5 mA when V_CE = 5V and I_F = 20 mA. This is the key sensitivity parameter. It defines the minimum output current when the slot is unobstructed. Designers must ensure the load resistor (R_L) is chosen so that this current produces a usable voltage swing.
  • Collector-Emitter Saturation Voltage (V_CE(SAT)): Maximum 0.4V at I_C = 0.25mA and I_F = 20mA. This low saturation voltage indicates good performance when the phototransistor is driven into saturation (fully ON), allowing it to pull a line very close to ground.
  • Response Time:
    • Rise Time (T_r): Typically 3 µs, maximum 15 µs.
    • Fall Time (T_f): Typically 4 µs, maximum 20 µs.
    Measured under V_CC=5V, I_C=2mA, R_L=100Ω. These times limit the maximum switching frequency. The asymmetric times are common in phototransistors. For a conservative design, assume a maximum frequency of 1/(T_r+T_f) ~ 1/40µs = 25 kHz.

3. Performance Curve Analysis

The datasheet references typical performance curves. While the specific graphs are not provided in the text, their standard interpretations are as follows:

• Forward Current vs. Forward Voltage (I_F-V_F): This curve shows the exponential relationship typical of a diode. It helps in understanding the V_F variation with temperature and current.
• Collector Current vs. Collector-Emitter Voltage (I_C-V_CE): For a given LED current (I_F), this graph shows the phototransistor's output characteristics, similar to a bipolar transistor's output curves. It illustrates the transition from the active region to saturation.
• Current Transfer Ratio (CTR) vs. Forward Current: CTR is the ratio I_C / I_F (often expressed as a percentage). This is a critical efficiency parameter for the coupler. The curve typically shows CTR peaking at a specific I_F and decreasing at higher currents due to heating or other effects.
• Temperature Characteristics: Curves showing how parameters like I_C(ON), V_F, and CTR vary over the operating temperature range (-25°C to +85°C). Phototransistor gain generally decreases with increasing temperature, which must be accounted for in designs requiring stable performance across temperature.

4. Mechanical & Packaging Information

4.1 Outline Dimensions

The device features a standard through-hole package with a molded plastic body containing the slot. Key dimensional notes from the datasheet:

• All dimensions are provided in millimeters (mm).
• The default tolerance for unspecified dimensions is ±0.25 mm.
• The specific slot width, body height, and lead spacing are defined in the dimensional drawing (not fully detailed in text). This information is critical for mechanical integration, ensuring the object to be detected fits through the slot and for PCB footprint design.

4.2 Polarity Identification & Pinout

For proper operation, correct pin identification is essential. The package uses a standard pin arrangement for slotted photointerrupters: one pair of pins for the infrared LED (anode and cathode) and another pair for the phototransistor (collector and emitter). The datasheet drawing specifies the pin numbers. Typically, when viewing the device from the top (slot side), pins are numbered counter-clockwise. The designer must consult the drawing to correctly connect the anode, cathode, collector, and emitter.

5. Soldering & Assembly Guidelines

Adherence to these guidelines is necessary to prevent damage during the manufacturing process.

• Reflow Soldering: The absolute maximum rating specifies lead soldering at 260°C for 5 seconds, measured 1.6mm from the case body. This translates to a standard leaded reflow profile. The plastic package has a limited thermal mass, so prolonged exposure to high temperatures must be avoided to prevent cracking or internal damage.
• Hand Soldering: If hand soldering is necessary, use a temperature-controlled iron. Apply heat to the lead/pin, not the plastic body, and complete the joint within 3-5 seconds per lead.
• Cleaning: Use cleaning solvents that are compatible with the device's plastic material to avoid stress cracking or degradation.
• Storage Conditions: Store in an environment within the specified storage temperature range (-55°C to +100°C) and at low humidity. Moisture-sensitive devices should be kept in sealed, dry packaging until use.

6. Application Design Considerations

6.1 Typical Application Circuit

A standard interface circuit involves two main parts:

1. LED Driver: A current-limiting resistor (R_LIMIT) is connected in series with the LED. Its value is calculated as R_LIMIT = (V_CC - V_F) / I_F. For a 5V supply, V_F=1.2V, and I_F=20mA, R_LIMIT = (5 - 1.2) / 0.02 = 190Ω. A 180Ω or 200Ω resistor would be suitable.
2. Phototransistor Output: The phototransistor is typically connected as a common-emitter switch. A pull-up resistor (R_L) is connected between the collector and the positive supply (V_CC). The emitter is connected to ground. When light falls on the transistor (unobstructed slot), it turns ON, pulling the collector voltage low (near V_CE(SAT)). When the light is blocked, the transistor turns OFF, and the collector voltage is pulled high by R_L. The value of R_L determines the output voltage swing and speed. A smaller R_L provides faster response but draws more current. Using the test condition of R_L=100Ω as a starting point is common.

6.2 Design Challenges & Solutions

• Ambient Light Immunity: While the slotted design offers some protection, strong ambient light (especially infrared) can affect the phototransistor. Using a modulated LED drive signal and synchronous detection in the receiver circuit can greatly enhance immunity. Alternatively, ensuring the slot is shrouded can help.
• Temperature Compensation: As phototransistor gain decreases with temperature, the I_C(ON) will drop. For critical applications, design the circuit to have sufficient margin at the highest operating temperature, or use a comparator with a adjustable threshold instead of a simple pull-up resistor interface.
• Object Characteristics: The object interrupting the beam must be opaque to the infrared wavelength emitted (~940nm). Thin or translucent materials may not be reliably detected. The object's size must be sufficient to fully block the beam within the slot.

7. Technical Comparison & Differentiation

Compared to other sensing technologies:

• vs. Mechanical Micro-Switches: Photointerrupters offer superior reliability (no moving parts to wear out), faster response, and silent operation. They are immune to contact bounce.
• vs. Reflective Optical Sensors: Slotted types are generally more reliable for edge detection or precise position sensing because they are less susceptible to variations in the reflectivity or color of the target object. The beam is either fully blocked or unblocked.
• vs. Hall Effect Sensors: Hall sensors detect magnetic fields, not light interruption. They are used for different physical phenomena (e.g., detecting a magnet). Photointerrupters are for detecting any opaque object.
• Within Photointerrupters: The LTH-872-T55T1's specific differentiation lies in its combination of electrical ratings (e.g., V_CEO=30V, I_C(ON) min=0.5mA), package dimensions, and cost-effectiveness for high-volume office automation applications.

8. Frequently Asked Questions (FAQ)

• Q: What is the typical operating current for the LED? A: The standard test condition and a common operating point is I_F = 20 mA. This provides a good balance between optical output, power consumption, and longevity.
• Q: Can I drive the LED directly from a microcontroller pin? A: Most microcontroller GPIO pins cannot source or sink 20mA continuously. It is recommended to use a simple transistor or MOSFET driver circuit, or a dedicated LED driver IC, to provide the necessary current.
• Q: How do I connect the output to a digital input? A: The phototransistor collector (with pull-up resistor) can be connected directly to a standard CMOS or TTL logic input. When the slot is clear, the input will read LOW. When blocked, it will read HIGH. Ensure the pull-up voltage is compatible with the logic family (e.g., 5V for 5V logic, 3.3V for 3.3V logic).
• Q: Why is my output not switching fully to the supply rail when blocked? A: This is likely due to the dark current (I_CEO) flowing through the pull-up resistor. With a very large pull-up resistor (e.g., 100kΩ), even 100nA of leakage can create a significant voltage drop. Use a smaller pull-up resistor (e.g., 1kΩ to 10kΩ) to ensure a solid HIGH level, balancing current draw and speed.
• Q: What is the recommended PCB layout practice? A: Keep the LED driver traces and the phototransistor output traces separate to minimize noise coupling. Place the current-limiting and pull-up resistors close to the device. Ensure the slot area on the PCB is clear of solder mask or components that could obstruct the infrared beam path.

9. Operational Principle

The photointerrupter operates on the principle of direct optical coupling interrupted by a physical object. An infrared LED emits light at a wavelength typically around 940 nm, which is invisible to the human eye. Directly opposite, a silicon phototransistor is sensitive to this wavelength. In the unobstructed state, the infrared light strikes the base region of the phototransistor, generating electron-hole pairs. This photocurrent acts as base current, causing the transistor to turn on and conduct a much larger collector current (I_C(ON)). When an opaque object enters the slot, it completely blocks the light path. The photocurrent ceases, the effective base current drops to zero, and the phototransistor turns off, allowing only a tiny leakage current (I_CEO) to flow. This stark contrast between the ON and OFF states provides a clean, reliable digital signal indicative of the object's presence or absence.

10. Industry Trends

The photointerrupter remains a mature and widely used technology due to its simplicity, robustness, and low cost. Current trends in the industry focus on several areas:

• Miniaturization: Development of smaller package sizes (e.g., surface-mount devices with very narrow slots) to fit into increasingly compact consumer electronics and mobile devices.
• Enhanced Performance: Improving parameters such as higher speed for faster machinery, lower power consumption for battery-operated devices, and better temperature stability.
• Integration: Incorporating additional circuitry within the package, such as Schmitt triggers for hysteresis, amplifiers for weaker signals, or even digital interfaces (I2C), creating "smart sensors" that simplify system design.
• Material Advancements: Using advanced plastics and lens designs to improve light collimation, increase coupling efficiency, and enhance resistance to environmental factors like high temperature and humidity.

Despite the advent of newer technologies like time-of-flight (ToF) sensors or vision systems, the basic slotted photointerrupter continues to be the optimal solution for countless simple, reliable, and cost-sensitive presence detection applications.