LTH-209-01 Photointerrupter Datasheet - Reflective Object Sensor - English Technical Document

1. Product Overview

The LTH-209-01 is a reflective-type photointerrupter module designed for non-contact switching applications. This optoelectronic device integrates an infrared (IR) emitting diode and a phototransistor within a single, compact package. Its primary function is to detect the presence or absence of a reflective object placed within its sensing gap. The module is engineered for direct mounting onto printed circuit boards (PCBs) or for use with dual-in-line sockets, offering flexibility in system integration. Its core advantages include non-contact operation, which eliminates mechanical wear and ensures long-term reliability, and fast switching speeds suitable for various sensing and counting tasks. The target market includes automation equipment, consumer electronics, security systems, and industrial controls where precise, reliable object detection is required.

2. In-Depth Technical Parameter Analysis

2.1 Absolute Maximum Ratings

Operating the device beyond these limits may cause permanent damage. Key parameters include:

• IR Diode Continuous Forward Current (I_F): 50 mA maximum. This defines the upper limit for the DC current that can be continuously passed through the IR LED.
• IR Diode Reverse Voltage (V_R): 5 V maximum. Exceeding this reverse bias voltage can damage the LED junction.
• Phototransistor Collector Current (I_C): 20 mA maximum. This is the maximum continuous current the output transistor can sink.
• Phototransistor Collector-Emitter Voltage (V_CEO): 30 V maximum. This is the maximum voltage that can be applied across the phototransistor's collector and emitter pins.
• Operating Temperature Range: -35°C to +65°C. The device is guaranteed to operate within specifications across this ambient temperature range.
• Lead Soldering Temperature: 260°C for 5 seconds at a distance of 1.6mm from the case. This is critical for wave or reflow soldering processes.

Note on Power Derating: The maximum power dissipation for both the IR diode (75 mW) and the phototransistor (100 mW) must be derated linearly at a rate of 1.33 mW/°C for ambient temperatures above 25°C. This is essential for thermal management and long-term reliability.

2.2 Electrical & Optical Characteristics

These parameters are specified at an ambient temperature (T_A) of 25°C and define the device's typical performance.

2.2.1 Input IR Diode Characteristics

• Forward Voltage (V_F): Typically 1.2V to 1.6V at a forward current (I_F) of 20 mA. This parameter is crucial for designing the current-limiting driver circuit for the LED.
• Reverse Current (I_R): Maximum 100 µA at a reverse voltage (V_R) of 5V. A low reverse current indicates good junction quality.

2.2.2 Output Phototransistor Characteristics

• Collector-Emitter Breakdown Voltage (V_(BR)CEO): Minimum 30V at I_C=1mA. This high breakdown voltage allows the use of higher pull-up voltages in the output circuit.
• Collector-Emitter Dark Current (I_CEO): Maximum 100 nA at V_CE=10V. This is the leakage current when the IR diode is off (no illumination). A low dark current is essential for good signal-to-noise ratio, especially in low-light or high-gain applications.

2.2.3 Coupler (System) Characteristics

These parameters describe the performance of the complete sensor system (IR LED + phototransistor).

• Collector-Emitter Saturation Voltage (V_CE(SAT)): Maximum 0.4V at I_C=0.08mA and I_F=20mA. This low saturation voltage indicates the phototransistor can act as an efficient switch, pulling the output close to ground when activated.
• On-State Collector Current (I_C(ON)): Minimum 0.16 mA at V_CE=5V and I_F=20mA. Test Condition: This critical parameter is measured with a standard reflective surface (90% diffused reflectance white paper) placed 3.81 mm (0.15 inches) from the sensor face. This standardized distance and surface define the "sensing gap" and "minimum detectable reflectivity" for the device's specified performance.

3. Mechanical & Package Information

3.1 Package Dimensions

The LTH-209-01 comes in a standard 4-pin DIP (Dual In-line Package) style housing. All dimensions are provided in millimeters with a default tolerance of ±0.25mm unless otherwise specified on the dimensional drawing. The package is designed for through-hole PCB mounting. The exact dimensional drawing, including body length, width, height, pin spacing, and pin diameter, is essential for PCB footprint design and mechanical integration into the final product enclosure.

3.2 Pinout and Polarity Identification

The device has four pins. Typically, two pins are for the anode and cathode of the IR emitting diode, and the other two are for the collector and emitter of the NPN phototransistor. Correct identification is vital to prevent damage. The datasheet's pinout diagram must be consulted. The package often includes a notch, dot, or beveled edge to indicate pin 1. The IR diode is polarity-sensitive, and the phototransistor collector and emitter must be connected correctly for proper switching operation.

4. Soldering & Assembly Guidelines

Hand Soldering: Use a temperature-controlled soldering iron. The absolute maximum rating specifies that leads can be subjected to 260°C for 5 seconds when measured 1.6mm from the plastic case. It is recommended to use the lowest temperature and shortest time possible to make a reliable solder joint to minimize thermal stress on the internal components and plastic housing.

Wave Soldering: Possible, but the same temperature/time profile (260°C for 5 sec at 1.6mm from case) must be strictly adhered to. Preheating is recommended to reduce thermal shock.

Cleaning: If cleaning is necessary after soldering, use methods and solvents compatible with the device's plastic material to avoid cracking or clouding of the optical window.

Storage Conditions: Store in an environment within the specified storage temperature range of -40°C to +100°C. It is advisable to keep the devices in their original moisture-barrier bags until use to prevent contamination of the optical surfaces.

5. Application Suggestions

5.1 Typical Application Circuits

The most common circuit configuration uses the LTH-209-01 as a digital switch. The IR diode is driven with a constant current source or a current-limiting resistor from a voltage supply (e.g., 5V). A typical I_F of 20mA is used as per the test conditions. The phototransistor is connected in a common-emitter configuration: the collector is connected to the supply voltage (V_CC, up to 30V) through a pull-up resistor (R_L), and the emitter is connected to ground. The output signal is taken from the collector node. When no reflective object is present, the phototransistor is off (high output). When a reflective object enters the sensing gap, IR light reflects onto the phototransistor, turning it on and pulling the output low.

5.2 Design Considerations & Best Practices

• Selecting the Pull-up Resistor (R_L): The value of R_L determines the output current and voltage swing. It must be chosen based on the required I_C(ON) and the load's input characteristics (e.g., a microcontroller GPIO). A smaller R_L provides faster switching and better noise immunity but consumes more power. Ensure I_C does not exceed 20mA: R_L > (V_CC - V_CE(SAT)) / 20mA.
• Minimizing Electrical Noise: Place a bypass capacitor (e.g., 0.1µF) close to the device's power pins. Keep signal traces short, especially the phototransistor output line, to reduce susceptibility to electromagnetic interference (EMI).
• Optical Considerations: The sensing performance depends on the reflectivity, color, and distance of the target object. The specified I_C(ON) is for a 90% reflective white surface at 3.81mm. Darker or more distant objects will produce a smaller output signal. For consistent operation, design the system's detection threshold (e.g., comparator reference voltage) accordingly. Avoid ambient light sources (especially sunlight or incandescent bulbs rich in IR) from directly shining into the sensor's aperture, as this can cause false triggering. A modulated IR signal and synchronous detection can be used in high-ambient-light environments.
• Mechanical Alignment: Ensure the target object's path is consistent and passes within the optimal sensing gap (around the specified 3.81mm) for reliable detection.

6. Technical Comparison & Differentiation

The LTH-209-01, as a reflective photointerrupter, differs from other optosensor types:

• vs. Transmissive Photointerrupters (Slotted Optocouplers): Transmissive types have a physical gap between the emitter and detector; an object is detected when it blocks the light path. Reflective types like the LTH-209-01 detect an object when it reflects light back. Reflective sensors are often simpler to mount as they require access from only one side, but their performance is more dependent on the object's surface properties.
• vs. Photologic Sensors: Some photointerrupters include built-in logic circuitry (schmitt trigger, amplifier) to provide a clean digital output. The LTH-209-01 provides a simple analog phototransistor output, offering more flexibility but requiring external circuitry (like a comparator) to create a robust digital signal in noisy environments.
• Key Advantages of this Model: The combination of a relatively high collector-emitter breakdown voltage (30V), low saturation voltage, and a standardized test condition for sensitivity provides a good balance for general-purpose reflective sensing applications.

7. Frequently Asked Questions (FAQs)

Q1: What is the optimal distance for sensing an object?
A1: The datasheet specifies the On-State Current (I_C(ON)) with the target at 3.81mm (0.15"). This is the standardized test distance. The actual optimal distance depends on the target's reflectivity. For a highly reflective target, detection may work at slightly greater distances. For reliable design, use 3.81mm as the nominal operating point.

Q2: Can I drive the IR LED with a voltage source directly?
A2: No. An IR LED, like all diodes, must be current-driven. Connecting it directly to a voltage source will cause excessive current flow, potentially destroying the device. Always use a series current-limiting resistor. Calculate the resistor value as R = (V_supply - V_F) / I_F. For a 5V supply, V_F=1.4V, and I_F=20mA: R = (5 - 1.4) / 0.02 = 180 Ohms.

Q3: Why is my output signal unstable or noisy?
A3: Common causes include: 1) Insufficient pull-up resistor value leading to a slow rise time, 2) Electrical noise pickup on long output traces (use a bypass capacitor and shorter routing), 3) Ambient IR light interference (shield the sensor or use modulation), 4) The target object has variable reflectivity or is at an inconsistent distance.

Q4: What does the "Derate Linearly 1.33 mW/°C" note mean?
A4: This is a thermal derating rule. The maximum allowed power dissipation (75 mW for the diode, 100 mW for the transistor) is specified at 25°C. For every degree Celsius the ambient temperature increases above 25°C, you must reduce the maximum allowed power by 1.33 mW. For example, at 65°C (40°C above 25°C), the derated max power for the transistor is 100 mW - (40 * 1.33 mW) = 100 - 53.2 = 46.8 mW.

8. Practical Application Case Study

Scenario: Paper Detection in a Printer.
The LTH-209-01 can be used to detect the leading edge of paper as it feeds through a printer mechanism. The sensor is mounted on the main board with its sensing face oriented towards the paper path. A reflective strip or the paper itself (if sufficiently reflective) acts as the target. When no paper is present, the output is high. When the paper edge passes under the sensor, the reflected IR light activates the phototransistor, pulling the output low. This digital signal informs the printer's microcontroller of the paper's position, allowing it to control print timing accurately. Key design points here include choosing a pull-up resistor to interface cleanly with the MCU's 3.3V or 5V logic, ensuring the paper path is mechanically stable to maintain the correct sensing gap, and possibly adding a simple RC filter on the output to debounce the signal caused by paper texture.

9. Operating Principle

The LTH-209-01 operates on the principle of modulated light reflection and photoelectric conversion. Internally, an infrared light-emitting diode (IRED) emits light at a wavelength typically around 940nm, which is invisible to the human eye. This light projects out of the front of the device. When a suitably reflective object enters the field of view and is within the effective range, a portion of the emitted IR radiation reflects off the object's surface and back towards the device. A silicon NPN phototransistor, positioned adjacent to the IRED inside the same package, receives this reflected light. The photons incident on the phototransistor's base region generate electron-hole pairs, effectively creating a base current. This photogenerated base current is amplified by the transistor's gain, resulting in a much larger collector current that can be measured externally. This change in collector current (from a very low dark current to the specified I_C(ON)) is the fundamental detection mechanism. The device thus converts an optical event (the presence of a reflective object) into an electrical signal.

10. Industry Trends & Context

Reflective photointerrupters like the LTH-209-01 represent a mature and reliable technology within the broader optoelectronics sensor market. The general trend in this field is towards miniaturization, increased integration, and enhanced functionality. Newer devices may feature surface-mount (SMD) packages for automated assembly, lower power consumption, and built-in signal conditioning ICs that provide digital outputs (I2C, PWM) or analog outputs with improved linearity. There is also a move towards using specific wavelengths or incorporating optical filters to improve immunity to ambient light. Furthermore, the development of materials and packaging techniques continues to improve the temperature range, humidity resistance, and long-term stability of these components. While advanced alternatives exist, the through-hole, discrete phototransistor-output reflective sensor remains a cost-effective and highly versatile solution for countless non-contact detection applications where simplicity, robustness, and proven performance are paramount.