LTH-306-04 Photointerrupter Datasheet - Slotted Optical Switch - English Technical Document

1. Product Overview

The LTH-306-04 is a slotted optical switch, commonly known as a photointerrupter. It is a non-contact sensing device that combines an infrared light-emitting diode (LED) and a phototransistor in a single, compact housing. The core function is to detect the presence or absence of an object by interrupting the light path between the emitter and the detector. This device is designed for direct PCB mounting or use with a dual-in-line socket, offering a reliable solution for position sensing, limit switching, and object detection in various electronic applications.

1.1 Core Advantages

• Non-Contact Operation: Eliminates mechanical wear and tear, ensuring long-term reliability and silent operation.
• Fast Switching Speed: Enables detection of high-speed events, suitable for counting and timing applications.
• Compact Form Factor: The standardized package allows for easy integration into space-constrained designs.
• Electrical Isolation: The input (LED) and output (phototransistor) are electrically isolated, providing noise immunity and safety.

1.2 Target Market and Applications

This component is widely used across industries requiring precise, reliable object detection without physical contact. Typical applications include:

• Consumer Electronics: Paper detection in printers, scanners, and copiers; disk tray position sensing in CD/DVD players.
• Industrial Automation: Limit switches on linear actuators, rotary encoder disks, conveyor belt object counting, and robotic arm position feedback.
• Office Equipment: Detecting paper jams, toner levels, and cover open/closed status.
• Instrumentation: Tachometers, flow meters, and other devices requiring rotational or linear speed measurement.

2. In-Depth Technical Parameter Analysis

The performance of the photointerrupter is defined by its electrical and optical characteristics, which must be carefully considered during circuit design.

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
  • Continuous Forward Current (I_F): 60 mA
  • Peak Forward Current (300 pps, 10 μs pulse): 1 A
  • Reverse Voltage: 5 V
• Output Phototransistor:
  • Power Dissipation: 100 mW
  • Collector-Emitter Voltage (V_CE): 30 V
  • Collector Current (I_C): 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

2.2 Electrical & Optical Characteristics (T_A = 25°C)

These are the typical operating parameters under specified test conditions.

• Input LED Forward Voltage (V_F): 1.2V (Min), 1.6V (Typ) at I_F = 20mA. This parameter is crucial for selecting the current-limiting resistor for the LED.
• Output Phototransistor Dark Current (I_CEO): Max 100 nA at V_CE = 10V. This is the leakage current when the LED is off, affecting the "off-state" signal level.
• On-State Collector Current (I_C(ON)): 0.5mA (Min), 2mA (Typ) at V_CE = 5V and I_F = 20mA. This defines the output signal strength when the light path is unobstructed.
• Collector-Emitter Saturation Voltage (V_CE(SAT)): Typ 0.4V at I_C = 0.25mA and I_F = 20mA. A low saturation voltage is desirable for clean digital signal output.
• Response Time:
  • Rise Time (t_r): 3 μS (Typ), 15 μS (Max)
  • Fall Time (t_f): 4 μS (Typ), 20 μS (Max)
  Measured at V_CE=5V, I_C=2mA, R_L=100Ω. These times limit the maximum switching frequency of the device.

3. Performance Curve Analysis

While the specific curves are not detailed in the provided text, typical performance graphs for such devices provide essential design insights.

3.1 Forward Current vs. Forward Voltage (I_F-V_F)

This curve shows the non-linear relationship between the LED current and voltage. It helps in designing an efficient drive circuit, ensuring the LED operates within its safe operating area while providing sufficient optical output.

3.2 Collector Current vs. Forward Current (I_C-I_F)

This graph, often called the transfer characteristic or current transfer ratio (CTR) curve, is fundamental. It illustrates how the phototransistor's output current changes with the LED's input current. The slope represents the CTR, a key efficiency parameter. Designers use this to determine the required LED drive current to achieve a desired output current swing.

3.3 Temperature Dependence

Performance curves at different temperatures (e.g., -25°C, 25°C, 85°C) are critical for understanding device behavior in non-ambient conditions. Typically, the LED's forward voltage decreases with increasing temperature, while the phototransistor's sensitivity may also vary. These effects must be compensated for in precision or wide-temperature-range applications.

4. Mechanical & Package Information

4.1 Package Dimensions

The LTH-306-04 features a standard through-hole package. Key dimensional notes include:

• All dimensions are in millimeters (inches).
• Tolerance is ±0.25mm (.010") unless otherwise specified.
• Lead spacing is measured where the leads emerge from the package body, which is critical for PCB layout.

The slot width, depth, and overall package footprint determine the size of the object that can be detected and the mounting requirements.

4.2 Polarity Identification

For correct operation, proper lead identification is essential. The longer lead typically denotes the anode of the LED. The phototransistor's collector and emitter must also be connected correctly based on the datasheet pinout diagram (implied but not detailed in the excerpt). Incorrect polarity can prevent operation or damage the device.

5. Soldering & Assembly Guidelines

5.1 Hand Soldering

When hand soldering, care must be taken to avoid excessive heat. The absolute maximum rating specifies that leads can be soldered at 260°C for 5 seconds, measured 1.6mm (0.063") from the plastic case. Exceeding this can melt the housing or damage the internal semiconductor die.

5.2 Wave Soldering

For wave soldering, standard profiles for through-hole components are generally applicable. Preheating is recommended to minimize thermal shock. The device should not be immersed in the solder wave for longer than necessary.

5.3 Cleaning

If cleaning is required after soldering, use solvents compatible with the device's plastic material. Harsh chemicals or ultrasonic cleaning with inappropriate frequencies may damage the package or internal bonds.

6. Application Design Considerations

6.1 Driving the Input LED

The LED requires a constant current source or a voltage source with a series current-limiting resistor. Using a resistor is the most common method. The resistor value (R_LIMIT) is calculated as: R_LIMIT = (V_CC - V_F) / I_F. Use the maximum V_F from the datasheet to ensure the current does not exceed the chosen I_F under all conditions. For example, with V_CC = 5V, V_F = 1.6V, and desired I_F = 20mA: R_LIMIT = (5 - 1.6) / 0.02 = 170 Ω. A standard 180 Ω resistor would be suitable.

6.2 Interfacing the Output Phototransistor

The phototransistor can be used in two common configurations:

• Common-Emitter (Switching Mode): The collector is connected to V_CC via a pull-up resistor (R_L), and the emitter is grounded. The output is taken from the collector. When light hits the transistor, it turns on, pulling the collector voltage low (near V_CE(SAT)). When blocked, it turns off, and the pull-up resistor pulls the voltage high to V_CC. This provides a logic-level output.
• Common-Collector (Emitter Follower): The collector is connected directly to V_CC, and the emitter is connected to ground via a resistor. The output is taken from the emitter. This configuration provides current gain but not voltage inversion.

The value of the load resistor (R_L) affects both the output voltage swing and the response time. A smaller R_L provides faster switching (as indicated in the test condition R_L=100Ω) but reduces the output voltage swing for a given photocurrent. A larger R_L gives a larger swing but slower response.

6.3 Environmental Considerations

• Ambient Light: The device uses an infrared LED, which reduces interference from visible ambient light. However, strong IR sources (sunlight, incandescent bulbs) can cause false triggering. Using a modulated LED signal and synchronous detection can greatly improve immunity.
• Contaminants: Dust, oil, or other contaminants on the lens or in the slot can attenuate the light signal, reducing sensitivity. The application should consider the operating environment.
• Object Characteristics: The object being detected should be opaque to the infrared wavelength. Translucent or reflective materials may not reliably interrupt the beam.

7. Technical Comparison & Differentiation

Compared to mechanical switches and other sensing technologies, the LTH-306-04 photointerrupter offers distinct advantages:

• vs. Mechanical Microswitches: No contact bounce, virtually infinite lifespan (no moving parts to wear out), faster response, and silent operation.
• vs. Reflective Sensors: Slotted sensors are immune to the color and reflectivity of the target object. They provide a more consistent and reliable signal when the sole requirement is to detect the presence of an object in a specific gap.
• vs. Hall Effect Sensors: Photointerrupters do not require a magnetic field, making them suitable for applications involving non-ferrous materials or where magnetic fields are undesirable.

Its key differentiators within the photointerrupter category would be its specific package size, slot dimensions, current transfer ratio (CTR), and switching speed, which should be compared against datasheets of competing models for a given application.

8. Frequently Asked Questions (FAQs)

8.1 What is the typical operating life of this device?

Since there are no moving parts, the life is primarily determined by the LED's gradual decrease in light output (lumen depreciation). When operated within its specified ratings, especially current and temperature, it can typically operate for tens of thousands of hours.

8.2 How do I choose the load resistor (R_L) value?

The choice involves a trade-off. For a digital on/off signal, select R_L so that the voltage drop across it when the phototransistor is fully on (I_C(ON) * R_L) is a significant portion of your supply voltage (e.g., > 2.5V for a 5V system to ensure a good logic low). Then verify that the resulting response time meets your speed requirements. Start with the test condition value (100Ω) as a reference.

8.3 Can I use this outdoors?

The operating temperature range (-25°C to +85°C) allows for many outdoor environments. However, direct sunlight contains strong IR and can saturate the sensor. Additionally, moisture, condensation, or dirt blocking the slot will impair function. A protective housing or careful sealing is necessary for reliable outdoor use.

8.4 Why is my output signal noisy or unstable?

Common causes include: 1) Insufficient LED drive current, resulting in a weak signal. 2) Electrical noise pickup on the high-impedance phototransistor output. Use a shorter wire, add a small capacitor (e.g., 10nF to 100nF) from the output to ground, or use a shielded cable. 3) Interference from ambient light. 4) The object being detected is not fully opaque to IR.

9. Practical Application Examples

9.1 Rotary Encoder Disk

A slotted wheel attached to a motor shaft rotates between the emitter and detector. As the slots pass through, they create a pulsed output. By counting these pulses, the rotation speed can be measured. Using two photointerrupters slightly offset creates a quadrature output, allowing direction detection as well.

9.2 Paper-End Detection in a Printer

The photointerrupter is mounted so the paper tray flag passes through its slot. When paper is present, the flag is pushed out, interrupting the beam and changing the output state. The microcontroller monitors this signal to alert the user when the paper supply is low.

9.3 Safety Interlock

In equipment with moving parts or high voltage, a photointerrupter can be used as a safety interlock on a protective cover. When the cover is opened, an attached vane enters the slot, breaking the beam and sending a signal to immediately cut power to the dangerous subsystem.

10. Operating Principle

The device operates on the principle of optoelectronic transduction. An electrical current applied to the input side causes the infrared LED to emit light. This light travels across a small air gap within the device's housing. On the output side, a silicon phototransistor is positioned to receive this light. When photons strike the base region of the phototransistor, they generate electron-hole pairs, which act as a base current. This photogenerated base current is amplified by the transistor's gain, resulting in a much larger collector current that can be used as an electrical output signal. When an opaque object is placed in the slot, it blocks the light path. The photogeneration of base current ceases, and the phototransistor turns off, causing the collector current to drop to a very low value (the dark current). This on/off change in output current constitutes the switching action.

11. Industry Trends

The fundamental technology of slotted photointerrupters is mature and stable. However, trends in the broader optoelectronics and sensing field influence their application and evolution:

• Miniaturization: There is a continuous drive for smaller package sizes to fit into increasingly compact consumer and medical devices.
• Surface-Mount Technology (SMT): While through-hole versions remain popular for prototyping and certain applications, SMT photointerrupters are becoming more prevalent for automated, high-volume assembly.
• Integration: Some modern variants integrate the current-limiting resistor for the LED or even a Schmitt-trigger buffer on the output side, simplifying external circuitry and providing a clean digital signal directly.
• Enhanced Performance: Developments in LED and photodetector materials can lead to devices with higher sensitivity, faster response times, and better temperature stability.
• Application-Specific Designs: Sensors are being tailored for specific markets, such as automotive (with wider temperature ranges) or industrial (with higher protection ratings against dust and moisture).

Despite these trends, the basic through-hole slotted photointerrupter, as represented by the LTH-306-04, remains a highly reliable, cost-effective, and easy-to-use solution for a vast array of non-contact sensing tasks, ensuring its continued relevance in electronic design.