Photointerrupter Sensor LTH-309-08 Datasheet - English Technical Document

1. Product Overview

The LTH-309-08 is a reflective photointerrupter, a type of optoelectronic sensor that combines an infrared light-emitting diode (LED) and a phototransistor in a single, compact package. Its primary function is to detect the presence or absence of an object without physical contact by sensing the interruption of the infrared light beam reflected from a surface. This device is designed for direct PCB (Printed Circuit Board) mounting or insertion into a standard dual-in-line socket, making it highly versatile for automated assembly processes.

The core advantage of this sensor lies in its non-contact switching capability, which eliminates mechanical wear and tear, ensuring high reliability and a long operational lifespan. It is particularly suited for applications requiring fast response times and precise object detection in constrained spaces. Typical target markets include office automation equipment (printers, copiers), industrial automation (conveyor belt counters, position sensing), consumer electronics, and various instrumentation devices where reliable object detection is critical.

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: The continuous forward current must not exceed 50 mA, with a peak forward current of 1 A allowed under pulsed conditions (300 pps, 10 µs pulse width). The maximum power dissipation for the LED is 75 mW. A reverse voltage greater than 5 V must be avoided.
• Output Phototransistor: The collector current is limited to 20 mA. The collector-emitter voltage can withstand up to 30 V, while the emitter-collector voltage is limited to 5 V. The phototransistor's power dissipation must not exceed 100 mW.
• Environmental Limits: The device is rated for operation within an ambient temperature range of -25°C to +85°C. Storage can be from -55°C to +100°C. For soldering, leads can withstand 260°C for 5 seconds when measured 1.6mm from the package body.

2.2 Electrical & Optical Characteristics

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

• Input LED Forward Voltage (V_F): Typically 1.2V to 1.6V when driven with a forward current (I_F) of 20 mA. This parameter is crucial for designing the current-limiting resistor in the drive circuit.
• Output Phototransistor Dark Current (I_CEO): The leakage current when no light is incident on the sensor, specified as a maximum of 100 nA at V_CE=10V. A low dark current is essential for good signal-to-noise ratio, especially in low-light or high-gain applications.
• On-State Collector Current (I_C(ON)): The minimum collector current is 0.5 mA when the LED is driven at I_F=20mA and V_CE=5V. This parameter indicates the sensitivity of the phototransistor.
• Collector-Emitter Saturation Voltage (V_CE(SAT)): The voltage drop across the phototransistor when it is fully "on," typically 0.4V at I_C=0.25mA and I_F=20mA. A low saturation voltage is desirable for interfacing with low-voltage logic circuits.
• Response Time: The sensor's switching speed is characterized by rise time (T_R) and fall time (T_F). Typical values are 3-15 µs for rise time and 4-20 µs for fall time under test conditions of V_CE=5V, I_C=2mA, and R_L=100Ω. This fast switching enables detection of rapidly moving objects.

3. Performance Curve Analysis

The datasheet references typical electrical/optical characteristic curves. While the specific graphs are not provided in the text, their general purpose and the insights they offer can be explained.

These curves typically plot key parameters against variables like temperature or drive current. For instance, a curve showing I_C(ON) versus I_F (LED forward current) would help a designer understand the relationship between input power and output signal strength, allowing optimization of the LED drive for desired sensitivity and power consumption. Another common curve is I_C(ON) versus ambient temperature, which is critical for understanding how the sensor's performance degrades or varies at temperature extremes, ensuring reliable operation across the specified -25°C to +85°C range. These graphs are essential for robust system design beyond the nominal 25°C point specifications.

4. Mechanical & Packaging Information

The LTH-309-08 is designed for compact integration. The package dimensions are provided in the datasheet with all measurements in millimeters (and inches in parentheses). Key mechanical notes include:

• A general tolerance of ±0.25mm (±0.010") applies unless otherwise specified.
• Lead spacing is measured at the point where the leads exit the plastic package body, which is critical for PCB footprint design.
• The package is a standard through-hole type, facilitating both manual and wave soldering processes.

Proper polarity identification is implied by the standard pinout for such devices: the LED anode and cathode are on one side, and the phototransistor collector and emitter are on the other. Designers must consult the dimensional drawing to confirm the exact pin arrangement and orientation for correct PCB layout.

5. Soldering & Assembly Guidelines

The datasheet specifies a lead soldering temperature limit of 260°C for 5 seconds, measured 1.6mm (0.063 inches) from the package body. This is a critical parameter for process control during wave soldering or hand soldering.

• Reflow Soldering: While primarily a through-hole device, if used in a mixed-technology board, extreme care must be taken during reflow. The plastic package has a lower thermal tolerance than surface-mount components. It is generally not recommended for standard infrared or convection reflow profiles unless specifically qualified.
• Hand Soldering: Use a temperature-controlled iron. Apply heat to the lead/pad junction quickly and efficiently to minimize heat transfer to the sensitive semiconductor die inside the package. Do not apply solder directly to the iron tip on the component lead for an extended period.
• Cleaning: Use cleaning solvents that are compatible with the package plastic to avoid cracking or degradation.
• Storage Conditions: Store in a dry, anti-static environment within the specified temperature range of -55°C to +100°C to prevent moisture absorption (which can cause "popcorning" during soldering) and electrostatic discharge damage.

6. Application Suggestions

6.1 Typical Application Scenarios

• Paper Detection in Printers/Copiers: Detecting paper jams, tray empty conditions, or paper presence at specific points along the paper path.
• Object Counting on Conveyors: Counting products, bottles, or components as they pass a fixed point.
• Position Sensing: Detecting the home position of a moving carriage (like in a scanner or plotter) or the open/closed state of a door or lid.
• Rotary Encoder Disk Sensing: Used in conjunction with a slotted wheel to create a low-resolution optical encoder for speed or position feedback.

6.2 Design Considerations

• LED Current Drive: Use a constant current source or a current-limiting resistor in series with the LED to maintain a stable I_F, typically around 20 mA as per test conditions, for consistent output. Pulsing the LED at a higher current can increase sensing distance but must stay within absolute maximum ratings.
• Phototransistor Biasing: A pull-up resistor (R_L) is connected between the collector and the supply voltage (V_CC). The value of R_L affects both the output voltage swing and the response time. A smaller R_L gives faster response but a smaller output voltage change. The emitter is typically connected to ground.
• Output Interface: The phototransistor output can be fed directly into a Schmitt-trigger input of a microcontroller for digital sensing, or into an analog input for measuring reflected light intensity. For noisy environments, adding a small capacitor across the phototransistor's collector and emitter can help filter high-frequency noise.
• Target Surface: Reflective sensing performance depends heavily on the target's reflectivity, color, and distance. For consistent operation, calibrate the detection threshold based on the specific target material. The sensing gap should be minimized for best signal strength.
• Ambient Light Immunity: As the sensor uses infrared light, it is somewhat immune to visible ambient light. However, strong sources of infrared light (like sunlight or incandescent bulbs) can cause false triggering. Using a modulated LED signal and synchronous detection in the receiver circuit can greatly enhance immunity to ambient light.

7. Technical Comparison & Differentiation

Compared to mechanical limit switches, the LTH-309-08 offers clear advantages: no moving parts, higher reliability, faster response, and silent operation. Within the photointerrupter category, its key differentiators are derived from its specified parameters. The fast switching speed (3-15 µs rise time) makes it suitable for higher-speed applications than slower phototransistors. The relatively low saturation voltage (0.4V) allows for better compatibility with modern 3.3V logic systems compared to devices with higher V_CE(SAT). The standard through-hole DIP package offers robustness and ease of prototyping, though it occupies more board space than surface-mount alternatives. Designers would choose this part for applications requiring a balance of speed, sensitivity, and proven reliability in a standard package format.

8. Frequently Asked Questions (Based on Technical Parameters)

• Q: Can I drive the LED with 3.3V logic? A: Yes, but you must calculate the series resistor carefully. With a typical V_F of 1.6V at 20mA, the resistor value would be (3.3V - 1.6V) / 0.02A = 85Ω. Use the maximum V_F from the datasheet for a safe design.
• Q: What is the maximum sensing distance? A: The datasheet does not specify a distance. This depends on LED drive current, target reflectivity, and the required I_C(ON). It is best determined empirically for your specific target. Generally, reflective sensors work best at short ranges (a few millimeters).
• Q: How do I protect the phototransistor from voltage spikes? A: Although it has a 30V V_(BR)CEO, for reliability in inductive environments, a small transient voltage suppressor (TVS) diode or a regular diode in reverse bias across the collector-emitter can be added.
• Q: Can I use this in a dusty environment? A: Dust accumulation on the lens will attenuate the light beam, reducing sensitivity and potentially causing failures. The device is not sealed. For harsh environments, consider a device with a sealed slot or provide external protection.

9. Practical Application Case Study

Scenario: Paper-Out Sensor in a Desktop Printer. The LTH-309-08 is mounted on the main PCB near the paper feed tray. A white plastic flag, attached to the paper tray mechanism, moves into the sensor's detection gap when the paper stack is depleted. In the "paper present" state, the flag is out of the gap, allowing the infrared light from the LED to reflect off a fixed surface inside the printer back to the phototransistor, generating a high I_C(ON) and a logic LOW output at the collector (with a pull-up resistor). When the paper runs out, the flag moves into the gap, blocking the light path. The phototransistor turns off, causing the collector voltage to be pulled HIGH by the resistor. The printer's microcontroller detects this HIGH signal and triggers a "Paper Out" warning on the display. The fast response time ensures immediate detection, while the non-contact nature guarantees the sensor will not wear out over the printer's lifetime.

10. Operating Principle Introduction

A photointerrupter operates on the principle of modulated light detection. The internal infrared LED emits light when forward biased. Opposite the LED is a phototransistor. In a reflective type like the LTH-309-08, both elements face the same direction. The emitted light travels out of the package, strikes a target surface, and some fraction is reflected back into the package where it is incident on the phototransistor. The phototransistor acts as a light-controlled switch. When photons strike its base region, they generate electron-hole pairs, effectively providing base current. This causes the transistor to turn "on," allowing a collector current (I_C) to flow that is proportional to the intensity of the received light. When the light path is blocked (e.g., by an object), the phototransistor turns "off," and only a small dark current flows. This on/off change in collector current is used to generate a digital signal indicating the presence or absence of the object interrupting the light path.

11. Technology Trends

The trend in optoelectronic sensors like photointerrupters is towards miniaturization, higher integration, and enhanced functionality. Surface-mount device (SMD) packages are becoming the norm to save PCB space and enable automated pick-and-place assembly. There is also a move towards integrating the sensor with signal conditioning circuitry (amplifiers, Schmitt triggers, logic outputs) on a single chip, creating digital output sensors that are easier to interface directly with microcontrollers. Furthermore, advancements are being made in improving ambient light rejection through optical filtering and smarter modulation techniques. While the fundamental principle remains unchanged, these trends focus on making sensors smaller, smarter, more reliable, and easier to implement in modern electronic designs.