Photointerrupter LTH-1650-01 Datasheet - Focal Distance 3mm - Infrared Cut-off Type - English Technical Document

1. Product Overview

The LTH-1650-01 is a compact, transmissive-type photointerrupter module. Its core function is to detect the interruption of an infrared light beam between its integrated infrared light-emitting diode (LED) and a silicon phototransistor. The primary design advantage is its integrated 3mm focal distance, which optimizes sensitivity for object detection at that specific gap. As an infrared cut-off type device, it is designed to minimize interference from ambient visible light, enhancing reliability in various sensing applications. The target market primarily includes office automation equipment, industrial control systems, and consumer electronics requiring non-contact position or object detection.

2. In-Depth Technical Parameter Analysis

2.1 Absolute Maximum Ratings

These parameters define the stress limits beyond which permanent damage to the device may occur. They are not intended for normal operation.

• Input LED Power Dissipation (P_D): Maximum 75 mW. This limits the combined thermal load from forward current and voltage drop.
• LED Peak Forward Current (I_CP): 1 A under pulsed conditions (300 pps, 10 µs pulse width). This allows for brief high-intensity pulses for enhanced signal detection.
• LED Continuous Forward Current (I_F): Maximum 60 mA DC. This is the safe limit for constant operation.
• LED Reverse Voltage (V_R): 5 V. Exceeding this can damage the LED junction.
• Phototransistor Power Dissipation (P_C): Maximum 100 mW, determined by collector current and collector-emitter voltage.
• Collector-Emitter Voltage (V_CEO): Maximum 30 V for the phototransistor.
• Collector Current (I_C): Maximum 20 mA for the output transistor.
• Operating Temperature (T_opr): -25°C to +85°C. The device is suitable for a wide range of industrial and commercial environments.
• Lead Soldering Temperature (T_S): 260°C for 5 seconds maximum, specified for leads 1.6mm from the case. This is critical for wave or reflow soldering processes.

2.2 Electrical & Optical Characteristics

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

• LED Forward Voltage (V_F): Typically 1.2V to 1.6V at I_F = 20 mA. This is used to calculate the required current-limiting resistor value.
• LED Reverse Current (I_R): Maximum 100 µA at V_R=5V, indicating the leakage current when the LED is reverse-biased.
• Collector-Emitter Dark Current (I_CEO): Maximum 100 nA at V_CE=10V with no light input. This is the phototransistor's leakage current, which affects the "off-state" signal level.
• Collector-Emitter Saturation Voltage (V_CE(SAT)): Typically 0.4V at I_C=0.05mA and I_F=20mA. This is the voltage across the transistor when it is fully "on," important for logic-level interfacing.

3. Binning System Explanation

The device features a performance binning system based on the On-State Collector Current (I_C(ON)), which is measured under standardized conditions (V_CE=5V, I_F=20mA, gap d=3.0mm). This current directly correlates with the sensitivity of the coupler.

• BIN A: I_C(ON) range from 100 µA to 300 µA. This is the standard sensitivity grade.
• BIN B: I_C(ON) range from 260 µA to 650 µA. This bin offers higher sensitivity.
• BIN C: I_C(ON) range from 400 µA to 1200 µA. This is the highest sensitivity grade available.

This binning allows designers to select a device with consistent sensitivity for their application, ensuring reliable triggering thresholds across production batches.

4. Performance Curve Analysis

The datasheet references typical characteristic curves which provide graphical insight into device behavior under varying conditions. While specific graphs are not detailed in the text, standard curves for such a device would typically include:

• Forward Current vs. Forward Voltage (I_F-V_F): Shows the non-linear relationship for the infrared LED, crucial for driver circuit design.
• Collector Current vs. Collector-Emitter Voltage (I_C-V_CE): Family of curves with LED forward current (I_F) as a parameter, illustrating the phototransistor's output characteristics.
• On-State Collector Current vs. Forward Current (I_C(ON)-I_F): Demonstrates the transfer characteristic and linearity of the optical coupling.
• On-State Collector Current vs. Ambient Temperature (I_C(ON)-T_A): Shows how sensitivity degrades with increasing temperature, a critical factor for thermal management in designs.
• Response Time Characteristics: The datasheet specifies Rise Time (T_R) of 3-15 µs and Fall Time (T_F) of 4-20 µs under test conditions (V_CE=5V, I_C=2mA, R_L=100Ω). These values define the maximum switching speed capability of the sensor.

5. Mechanical & Package Information

The package is a standard through-hole type. Key dimensional notes from the datasheet include:

• All dimensions are provided in millimeters, with inches in parentheses.
• The default tolerance is ±0.25mm (±0.010") unless a specific feature has a different callout.
• The focal distance (the optimal gap between the emitter and detector windows for maximum sensitivity) is specified as 3 mm.
• The package includes molded slots or features that aid in precise PCB mounting and alignment.
• Polarity is clearly marked on the package body, typically with a dot or a chamfered corner near the LED anode (or phototransistor collector) pin. Correct orientation is essential for circuit function.

6. Soldering & Assembly Guidelines

Proper handling is required to maintain device integrity.

• Soldering: The absolute maximum rating for lead soldering temperature is 260°C for 5 seconds, measured 1.6mm (0.063") from the plastic case. This guideline helps prevent thermal damage to the internal die bonds and the plastic encapsulation during wave or hand soldering.
• Cleaning: Use standard PCB cleaning solvents that are compatible with the device's plastic material. Avoid ultrasonic cleaning with excessive power or prolonged exposure.
• Storage: Devices should be stored in conditions within the Storage Temperature Range (T_stg) of -40°C to +100°C and in a low-humidity, anti-static environment to prevent moisture absorption and electrostatic discharge (ESD) damage.

7. Application Recommendations

7.1 Typical Application Scenarios

As indicated in the datasheet, primary applications include:

• Printers & Fax Machines: For paper-out detection, paper jam sensing, cover-open detection, and carriage position sensing.
• Optoelectronic Switches: Used in vending machines, industrial automation for counting, limit switching, and rotary encoder disk sensing.
• Consumer Electronics: Slot sensors in disk drives, tape decks, or other media handling systems.

7.2 Design Considerations

• Current Limiting Resistor (for LED): Must be calculated based on the supply voltage (V_CC), the LED forward voltage (V_F ~1.4V typ.), and the desired forward current (I_F). Do not exceed the continuous I_F rating of 60 mA. A typical operating point is 20 mA.
• Load Resistor (for Phototransistor): The value of the pull-up resistor (R_L) connected to the collector determines the output voltage swing and affects the switching speed. A smaller R_L gives faster fall time but reduces output voltage amplitude. The test condition uses R_L=100Ω.
• Electrical Noise Immunity: For long wire runs or noisy environments, consider adding a small bypass capacitor (e.g., 0.1µF) across the power supply pins near the device and using shielded cables.
• Optical Considerations: Keep the optical path (the 3mm gap) free of dust, dirt, or condensation. The infrared cut-off filter helps, but strong ambient infrared sources (like sunlight or incandescent bulbs) close to the sensor can cause false triggering.

8. Technical Comparison & Differentiation

Compared to basic phototransistors or photodiodes, this integrated photointerrupter offers key advantages:

• Aligned Optics: The emitter and detector are pre-aligned in a fixed, rigid package, eliminating the need for precise mechanical alignment during assembly, which is a significant advantage over discrete components.
• Optimized Gap: The 3mm focal distance is factory-set for peak sensitivity at that specific air gap.
• Ambient Light Rejection: The infrared cut-off filter over the phototransistor significantly reduces sensitivity to visible light, improving signal-to-noise ratio in typical indoor lighting conditions.
• Compact Form Factor: Provides a complete optical switch solution in a single, small package.

9. Frequently Asked Questions (FAQs)

Q: What is the purpose of the different bins (A, B, C)?
A: Bins categorize devices by their sensitivity (I_C(ON)). Choose a higher bin (B or C) for applications requiring detection of lower-contrast objects, longer life (as LED output degrades over time), or operation with higher dust levels. Bin A is sufficient for standard applications.

Q: Can I drive the LED with a voltage source directly?
A: No. An LED is a current-driven device. You must use a series current-limiting resistor to set the forward current (I_F) to a safe and consistent value, as shown in all application circuits.

Q: How do I interface the output with a microcontroller?
A: The phototransistor acts as a switch. Connect its emitter to ground, its collector to a digital input pin via a pull-up resistor (e.g., 10kΩ). When the beam is uninterrupted, the transistor is on, pulling the pin low. When interrupted, the transistor is off, and the pull-up resistor pulls the pin high. Ensure the microcontroller's input logic levels are compatible with the output voltage swing (near 0V for "on", near V_CC for "off").

Q: What affects the response time?
A> The intrinsic speed of the phototransistor, the value of the load resistor (R_L), and the capacitance of the circuit traces. For faster switching, use a smaller R_L as allowed by the desired output current and voltage levels.

10. Practical Use Case Example

Scenario: Paper-Out Sensor in a Desktop Printer.
The photointerrupter is mounted on the printer frame so that the paper stack in the tray sits within the 3mm optical gap, blocking the infrared beam. A lever or flag attached to the paper tray follower may be used. When paper is present, the beam is blocked, the phototransistor is off, and its output is high. When the last sheet of paper is fed, the follower moves, unblocking the beam. The phototransistor turns on, pulling the output low. This logic transition is detected by the printer's main controller, which then activates the "Paper Out" warning on the user interface. The infrared cut-off filter prevents false triggers from the printer's internal lighting or room lights.

11. Operating Principle

The device operates on the principle of modulated optical coupling. An internal infrared LED emits light when forward-biased with an appropriate current. Directly opposite, within the same package, is a silicon NPN phototransistor. The phototransistor's base region is exposed to light. When infrared photons from the LED strike the base-collector junction, they generate electron-hole pairs. This photogenerated current acts as base current, causing the transistor to conduct a much larger collector current (I_C), proportional to the light intensity. An object passing through the 3mm slot between them interrupts this light beam, causing the phototransistor to turn off. This provides a clean, electrically isolated switching signal based on a physical event.

12. Technology Trends

Photointerrupters remain fundamental components in position sensing. Current trends in the field include:

• Miniaturization: Development of even smaller surface-mount device (SMD) packages to save PCB space in compact consumer electronics.
• Integration: Incorporating additional circuitry on-chip, such as Schmitt triggers for hysteresis, amplifiers for weaker signals, or even digital interfaces (I2C) to provide a clean, processed digital output, simplifying microcontroller interfacing.
• Enhanced Performance: Improvements in LED efficiency and photodetector sensitivity allow operation at lower currents, reducing power consumption and heat generation.
• Specialized Variants: Devices with slotted wheels for rotary encoding, or reflective types where the emitter and detector face the same direction to sense reflective marks.

The core principle of optical interruption remains robust due to its non-contact nature, reliability, and simplicity, ensuring its continued relevance in mechatronic system design.