LTH-301-07 Photo Interrupter Datasheet - Slot Type - Dimensions 4.0x3.2x2.5mm - Forward Voltage 1.2V - Power Dissipation 80mW - English Technical Document

1. Product Overview

The LTH-301-07 is a compact, slot-type photo interrupter module designed for non-contact switching applications. It integrates an infrared light-emitting diode (LED) and a phototransistor within a single housing, separated by a physical gap. The fundamental operating principle involves the interruption of the infrared light beam passing from the emitter to the detector. When an opaque object enters the slot, it blocks the light path, causing the phototransistor's output state to change. This provides a reliable, wear-free sensing mechanism compared to mechanical switches.

Its core advantages include high reliability due to the absence of moving parts, fast switching speeds suitable for detecting rapid motion, and accurate position sensing. The device is engineered for direct PCB mounting or use with a dual-in-line socket, offering flexibility in assembly. Typical target markets and applications encompass office automation equipment such as facsimile machines, copiers, printers, and scanners, where it is used for paper detection, edge sensing, and position encoding.

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: Maximum continuous forward current is 50 mA. Peak forward current can reach 1 A under pulsed conditions (300 pps, 10 μs pulse width). The maximum power dissipation is 80 mW, and the reverse voltage withstand is limited to 5 V.
• Output Phototransistor: The collector-emitter voltage rating is 30 V, while the emitter-collector voltage is 5 V. Maximum collector current is 20 mA, with a power dissipation limit of 100 mW.
• Thermal Limits: The operating temperature range is specified from -25°C to +85°C, with a wider storage range of -40°C to +100°C. Lead soldering temperature must not exceed 260°C for 5 seconds when measured 1.6mm from the case.

2.2 Electrical and Optical Characteristics

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

• Input LED Forward Voltage (V_F): Typically 1.2 V with a maximum of 1.6 V when driven at a forward current (I_F) of 20 mA. This low voltage is suitable for low-power logic circuits.
• Output Phototransistor Dark Current (I_CEO): The leakage current when no light is incident is guaranteed to be less than 100 nA at V_CE=10V, ensuring a good \"off\" state.
• Coupler Performance: The key parameter is the On-State Collector Current (I_C(ON)), which is guaranteed to be at least 0.6 mA when the LED is driven with I_F=20mA and V_CE=5V. The Collector-Emitter Saturation Voltage (V_CE(SAT)) is a maximum of 0.4 V under these conditions, indicating a good low-resistance \"on\" state.
• Switching Speed: The response time is characterized by Rise Time (T_r) and Fall Time (T_f). Typical values are 3 μs and 4 μs respectively, with maximums of 15 μs and 20 μs. This speed is sufficient for many medium-speed sensing and counting applications.

3. Mechanical and Package Information

The device features a standard through-hole package. The outline dimensions are provided in the datasheet with all measurements in millimeters. The primary body dimensions are approximately 4.0mm in length, 3.2mm in width, and 2.5mm in height, excluding leads. The slot gap width is a critical dimension for determining the size of the object that can be detected. The leads are spaced for standard dual-in-line mounting. Polarity is indicated by the physical shape of the housing and/or marking; the longer lead typically corresponds to the anode of the LED. It is crucial to consult the dimensional drawing for precise placement of the slot relative to the PCB edge and other components.

4. Soldering and Assembly Guidelines

4.1 Soldering Process

Proper soldering is critical to prevent damage to the plastic housing and internal components. The housing must not be dipped into solder. No external stress should be applied to the leads during soldering while the device is hot.

• Hand Soldering (Iron): The recommended maximum temperature is 350°C, with a soldering time not exceeding 3 seconds per lead. The iron tip should be applied no closer than 2mm from the base of the housing.
• Wave Soldering: A specific profile is recommended. Pre-heat temperature should not exceed 100°C for up to 60 seconds. The solder wave temperature should be a maximum of 260°C, with a contact time of 5 seconds or less. The dipping position must ensure the solder does not rise within 2mm of the housing base.

4.2 Storage Conditions and Shelf Life

To maintain solderability and device integrity, strict storage conditions are mandated. The ideal storage ambient is below 30°C temperature and below 70% relative humidity. Components should be assembled within 3 months of the delivery date. For longer storage in original packing, they should be kept in a sealed container with desiccant or in a nitrogen ambient desiccator, but not for over one year. Once the moisture barrier bag is opened, the components must be used within 3 months in a controlled environment of <25°C and <60% RH. Rapid temperature changes in high humidity should be avoided to prevent condensation, which can lead to pin oxidation. If storage conditions are not met, solderability assessment is required before use.

5. Application Notes and Design Considerations

5.1 Typical Application Circuits

The most common configuration is to use the photo interrupter as a digital switch. A current-limiting resistor is placed in series with the input LED, calculated based on the supply voltage (V_CC), the desired forward current (I_F, e.g., 20mA), and the LED's forward voltage (V_F ~1.2V): R_limit = (V_CC - V_F) / I_F. The output phototransistor is typically connected with a pull-up resistor (R_L) from the collector to V_CC. The emitter is connected to ground. When the light path is unobstructed, the phototransistor conducts, pulling the collector output voltage low (near V_CE(SAT)). When interrupted, the phototransistor turns off, and the output is pulled high by R_L. The value of R_L affects both the output voltage swing and the switching speed; a lower value provides faster speed but higher current consumption.

5.2 Design Considerations

• Ambient Light Immunity: As the device uses modulated infrared light (implied by its fast switching), it offers good rejection of steady ambient light. However, for critical applications, additional shielding or housing design may be necessary to block direct sunlight or other strong IR sources.
• Object Characteristics: The sensing reliability depends on the object's opacity to the infrared wavelength. Transparent or highly reflective materials may not reliably interrupt the beam.
• Alignment: Precise mechanical alignment of the object path with the slot is necessary for consistent operation. The slot width defines the minimum object size for reliable triggering.
• Debouncing: The electrical output may require software or hardware debouncing, especially if used with mechanical parts that may chatter or vibrate.

6. Performance Curves and Graphical Data

The datasheet references typical characteristic curves which are essential for detailed design analysis. While the specific graphs are not reproduced in the text, they typically include:

• Forward Current vs. Forward Voltage (I_F-V_F): Shows the relationship for the input LED, useful for calculating the exact voltage drop at different drive currents.
• Collector Current vs. Collector-Emitter Voltage (I_C-V_CE): Family of curves for the output phototransistor with incident light intensity (or LED drive current) as a parameter. This graph is crucial for determining the operating point and the value of the load resistor.
• Current Transfer Ratio (CTR) vs. Forward Current: CTR is the ratio of output collector current to input LED current (I_C/I_F). This curve shows how efficiency varies with drive current, helping to optimize the design for power consumption and output signal strength.
• Temperature Dependence: Curves showing how parameters like forward voltage, collector current, or CTR vary over the operating temperature range. This is vital for ensuring reliable operation in non-ambient environments.

7. Frequently Asked Questions (FAQ)

7.1 What is the difference between a photo interrupter and a photo reflector?

A photo interrupter (or transmissive sensor) has the emitter and detector facing each other across a gap. An object is detected when it blocks the light beam. A photo reflector (or reflective sensor) has the emitter and detector side-by-side, facing the same direction. An object is detected when it reflects the emitted light back to the detector. The LTH-301-07 is a slot-type photo interrupter.

7.2 Can I drive the LED with a voltage directly without a current-limiting resistor?

No. An LED is a current-driven device. Connecting it directly to a voltage source exceeding its forward voltage will cause excessive current to flow, potentially destroying it. A series resistor is mandatory to set the operating current.

7.3 Why is the storage humidity condition so important?

The plastic packaging of electronic components can absorb moisture from the air. During the high-temperature soldering process, this absorbed moisture can rapidly expand, causing internal delamination, cracking, or \"popcorning,\" which damages the device. The specified storage conditions and baking requirements (if exposed) are designed to prevent this.

7.4 How do I choose the value of the pull-up resistor (R_L) on the phototransistor?

The choice involves a trade-off. A smaller R_L provides a faster rise time (as it charges the circuit capacitance faster) and a stronger \"low\" signal, but it consumes more power when the transistor is on. A larger R_L saves power but slows down the switching speed and results in a weaker pull-up. A common starting point is between 1kΩ and 10kΩ, but the datasheet's test condition of R_L=100Ω for speed measurement indicates it can drive relatively low impedances.