LTH-301A Photointerrupter Datasheet - Non-Contact Switching - English Technical Document

1. Product Overview

The LTH-301A is a compact, through-hole mounted optoelectronic component designed for non-contact switching applications. Its core function is to detect the presence or absence of an object by interrupting an infrared light beam between an integrated emitter and detector. This device is engineered for direct PCB mounting or use with dual-in-line sockets, offering a reliable and fast solution for position sensing, object detection, and limit switching in various electronic systems.

The primary advantage of this component lies in its non-contact operation, which eliminates mechanical wear and tear associated with physical switches, leading to enhanced reliability and longevity. Its fast switching speed makes it suitable for applications requiring quick response times, such as in encoders, printers, and automated equipment. The target market includes industrial automation, consumer electronics, office equipment, and any application requiring precise, wear-free object detection.

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 these conditions is not guaranteed.

• Input LED:
  • Power Dissipation: 75 mW
  • Peak Forward Current (300 pps, 10 μs pulse): 1 A
  • Continuous Forward Current: 50 mA
  • Reverse Voltage: 5 V
• Output Phototransistor:
  • Power Dissipation: 100 mW
  • Collector-Emitter Voltage (V_CEO): 30 V
  • Emitter-Collector Voltage (V_ECO): 5 V
  • Collector Current: 20 mA
• Environmental:
  • Operating Temperature Range: -25°C to +85°C
  • Storage Temperature Range: -40°C to +100°C
  • Lead Soldering Temperature (1.6mm from case): 260°C for 5 seconds

These parameters are critical for circuit design. For instance, the LED driver circuit must limit the continuous current to 50mA and include protection against reverse voltage spikes exceeding 5V. The phototransistor's collector load must be chosen to keep the collector-emitter voltage below 30V and the collector current below 20mA under all operating conditions.

2.2 Electrical & Optical Characteristics

These specifications 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 to 1.6V at a forward current (I_F) of 20mA. This parameter is essential for calculating the current-limiting resistor value in the driver circuit.
  • Reverse Current (I_R): Maximum 100 μA at a reverse voltage (V_R) of 5V, indicating the LED's leakage in the off-state.
• Output Phototransistor Characteristics:
  • Collector-Emitter Breakdown Voltage (V_(BR)CEO): Minimum 30V at I_C=1mA.
  • Emitter-Collector Breakdown Voltage (V_(BR)ECO): Minimum 5V at I_E=100μA.
  • Collector-Emitter Dark Current (I_CEO): Maximum 100 nA at V_CE=10V. This is the leakage current when the LED is off, a key parameter for noise and off-state signal integrity.
• Coupler (Combined) Characteristics:
  • Collector-Emitter Saturation Voltage (V_CE(SAT)): Maximum 0.4V at I_C=0.25mA and I_F=20mA. This low voltage is desirable when the phototransistor is used as a switch in the saturation region.
  • On-State Collector Current (I_C(ON)): Minimum 0.5mA at V_CE=5V and I_F=20mA. This is the phototransistor's output current when the LED is energized, defining the device's current transfer ratio (CTR), a measure of sensitivity.

The relationship between I_F and I_C(ON) is crucial. A higher I_F generally increases I_C(ON), improving signal strength but also increasing power consumption and LED aging. Designers must balance these factors based on required sensitivity, speed, and lifetime.

3. Performance Curve Analysis

The datasheet references typical electrical/optical characteristic curves. While the specific graphs are not provided in the text, standard curves for such devices typically include:

• Forward Current vs. Forward Voltage (I_F-V_F): Shows the exponential relationship, crucial for thermal management and driver design.
• Collector Current vs. Collector-Emitter Voltage (I_C-V_CE): Family of curves with I_F as a parameter, illustrating the phototransistor's output characteristics and saturation region.
• Current Transfer Ratio (CTR) vs. Forward Current (I_F): Demonstrates how sensitivity changes with LED drive current, often showing an optimal range.
• On-State Collector Current vs. Ambient Temperature (I_C(ON)-T_A): Indicates how the output signal degrades with increasing temperature, which is vital for designing systems operating over the specified temperature range.
• Switching Time vs. Load Resistance: Illustrates the trade-off between switching speed and the value of the pull-up resistor at the collector.

These curves allow designers to predict performance under non-standard conditions and optimize their circuits for specific requirements like speed, power, or temperature stability.

4. Mechanical & Package Information

4.1 Package Dimensions

The LTH-301A is housed in a standard, compact through-hole package. Key dimensional notes from the datasheet:

• All dimensions are provided in millimeters, with inches in parentheses.
• The standard tolerance is ±0.25mm (±0.010") unless a specific feature note states otherwise.

The package features a molded body with a slot that allows an external object to pass between the internal LED and phototransistor. The leads are designed for standard 0.1" (2.54mm) grid spacing, compatible with common PCB layouts and DIP sockets. Precise mechanical drawings are essential for designing the PCB cutout and ensuring proper alignment of the interrupting object.

4.2 Polarity Identification & Pinout

Correct orientation is critical. The device pinout is typically indicated by a marking on the package body, such as a dot or a notch near pin 1. The standard pin configuration for a 4-pin photointerrupter is: Pin 1: LED Anode, Pin 2: LED Cathode, Pin 3: Phototransistor Emitter, Pin 4: Phototransistor Collector. Designers must always verify this against the specific datasheet diagram to avoid incorrect connections that could damage the device.

5. Soldering & Assembly Guidelines

The datasheet specifies a lead soldering temperature of 260°C for a maximum of 5 seconds, measured 1.6mm (0.063") from the plastic case. This is a critical parameter for wave soldering or hand-soldering processes.

• Reflow Soldering: While not explicitly mentioned for this through-hole part, if used in a mixed-technology board, the thermal profile must ensure the body temperature does not exceed the maximum storage temperature (100°C) or the soldering temperature limit at the leads.
• Cleaning: Use cleaning agents compatible with the device's plastic material. Avoid ultrasonic cleaning unless verified to be safe for the internal wire bonds.
• Handling: Avoid mechanical stress on the leads, especially bending them right at the package body. Use proper ESD precautions during handling and assembly.
• Storage Conditions: Store in a dry, anti-static environment within the specified temperature range of -40°C to +100°C to prevent moisture absorption and degradation.

6. Application Suggestions

6.1 Typical Application Circuits

The LTH-301A can be used in two primary configurations:

1. Digital Switch/Interrupter: The phototransistor is used in saturation mode. A pull-up resistor is connected from the collector to a logic supply voltage (e.g., 5V). The emitter is grounded. When the beam is unblocked, the phototransistor turns on, pulling the collector voltage low (to V_CE(SAT)). When blocked, it turns off, and the pull-up resistor pulls the collector voltage high. This provides a clean digital signal to a microcontroller or logic gate.
2. Analog Sensor: The phototransistor is used in its linear region. The collector current is proportional to the received light intensity. This current can be converted to a voltage using a transimpedance amplifier for applications requiring detection of partial obstruction or varying opacity.

6.2 Design Considerations

• LED Current Setting: Choose I_F based on required sensitivity, speed, and desired lifetime. A typical value is 10-20mA. Always use a series current-limiting resistor: R_limit = (V_supply - V_F) / I_F.
• Output Load Resistor: For digital switching, the value of the pull-up resistor (R_pull-up) affects switching speed and power consumption. A smaller resistor gives faster rise times but draws more current when the transistor is on. A value between 1kΩ and 10kΩ is common for 5V systems.
• Noise Immunity: For long wires or noisy environments, consider adding a small capacitor (e.g., 10nF to 100nF) between the phototransistor's collector and ground to filter high-frequency noise.
• Object Characteristics: The interrupting object must be opaque to the infrared wavelength emitted by the LED. The object's thickness and speed will affect the reliability and timing of detection.
• Ambient Light: While the device is modulated (the slot helps), strong ambient infrared light (e.g., from sunlight or incandescent bulbs) can affect performance. Using a modulated LED drive signal and synchronous detection in the receiver circuit can greatly improve immunity.

7. Technical Comparison & Differentiation

Compared to mechanical micro-switches, the LTH-301A offers superior life expectancy (millions of operations vs. hundreds of thousands), faster response, and no contact bounce. Compared to reflective optical sensors, transmissive photointerrupters like the LTH-301A are generally more immune to variations in the reflectivity and color of the target object, providing more consistent performance when detecting the presence of an object in a predefined gap.

Within the photointerrupter category, key differentiators for a part like the LTH-301A include its current transfer ratio (sensitivity), switching speed, package size, and operating temperature range. Its through-hole design makes it suitable for prototyping, legacy designs, or applications where mechanical robustness of the connection is preferred over the space savings of surface-mount devices.

8. Frequently Asked Questions (FAQs)

Q: What is the typical response time of the LTH-301A?
A: While not explicitly stated in the provided text, photointerrupters like this typically have rise and fall times in the range of a few microseconds, enabling kHz-range switching frequencies. The actual speed depends on the chosen load resistor and LED drive current.

Q: Can I use this sensor outdoors?
A: The operating temperature range (-25°C to +85°C) allows for many outdoor applications. However, direct exposure to sunlight, rain, or dust can interfere with operation or damage the device. It should be housed in an appropriate enclosure that protects it from the elements while allowing the target object to pass through the slot.

Q: How do I calculate the sensitivity or detection gap?
A: The "gap" is fixed by the mechanical package. The LTH-301A detects any opaque object that fully enters the slot between the emitter and detector. The minimum detectable object size is determined by the width of the slot opening. For reliable operation, the object should be wider than the infrared beam width inside the slot.

Q: Why is my output signal noisy or unstable?
A> Common causes include: 1) Insufficient LED drive current, leading to a weak output signal. 2) Electrical noise pickup on long, unshielded wires to the phototransistor. 3) Interference from ambient light sources. 4) The interrupting object may be translucent or reflective to IR. Solutions include increasing I_F, adding a filter capacitor at the output, shielding cables, and ensuring the target object is opaque.

9. Practical Application Examples

Example 1: Paper Detection in a Printer. The LTH-301A can be placed along the paper path. When paper is present, it blocks the IR beam, changing the output state. This signal can be used to detect paper jams, the leading/trailing edge of paper, or to count pages.

Example 2: Rotary Encoder for Motor Speed. A slotted disk attached to a motor shaft rotates through the slot of the photointerrupter. As each slot passes, the beam is interrupted, generating a pulse train. The frequency of this pulse train is directly proportional to the motor's rotational speed.

Example 3: Door/ Lid Interlock Safety Switch. Mounted on a cabinet or machine, the photointerrupter can detect if a door or protective cover is closed (beam unblocked) or open (beam blocked). This digital signal can be used to enable or disable machine operation for safety purposes.

10. Operating Principle

The LTH-301A is a transmissive optical sensor. It integrates an infrared light-emitting diode (IR LED) and a silicon phototransistor facing each other across a small air gap. In operation, a current is passed through the LED, causing it to emit infrared light. This light travels across the gap and strikes the base region of the phototransistor. The photons generate electron-hole pairs in the base, which act as base current, turning the transistor on and allowing a much larger collector current to flow. When an opaque object enters the gap, it blocks the light path. The phototransistor receives no light, its effective base current drops to zero, and it turns off, stopping the collector current. This on/off change in collector current provides a clear electrical signal corresponding to the object's presence or absence.

11. Technology Trends

The fundamental principle of photointerruption remains stable. However, trends in the industry include a shift towards surface-mount device (SMD) packages for automated assembly and reduced board space. There is also a move towards integrating more functionality, such as built-in amplifiers, Schmitt triggers for hysteresis, and even digital interfaces (I2C) within the sensor package to provide a cleaner, more robust output signal directly to microcontrollers. Furthermore, advancements in LED and photodetector materials continue to improve sensitivity, speed, and temperature stability while reducing power consumption. Despite these trends, through-hole components like the LTH-301A remain relevant for applications requiring high mechanical bond strength, easier manual prototyping, or servicing in harsh environments.