LTH-306-09S Photointerrupter Datasheet - Mechanical Switch Replacement - English Technical Document

1. Product Overview

The LTH-306-09S is a photointerrupter, a type of optoelectronic device designed to detect the interruption of a light beam. It serves as a direct, solid-state replacement for traditional mechanical switches in various sensing applications. The core advantage lies in its non-contact operation, which eliminates issues related to mechanical wear, contact bounce, and physical degradation over time. This makes it highly reliable for applications requiring frequent actuation or operation in environments where dust, moisture, or vibration could compromise mechanical contacts. The device is suitable for a broad market, including industrial automation (position sensing, limit switches), consumer electronics (printer paper detection, disk tray sensing), and safety systems (door interlock detection).

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:
  • Power Dissipation: 75 mW. This is the maximum continuous power the LED can handle at the specified ambient temperature.
  • Peak Forward Current: 1 A (under pulsed conditions: 300 pps, 10 μs pulse width). This rating is crucial for driving the LED with short, high-intensity pulses.
  • Continuous Forward Current: 50 mA. The maximum DC current for reliable long-term operation.
  • Reverse Voltage: 5 V. Exceeding this can damage the LED junction.
• Output Phototransistor:
  • Power Dissipation: 100 mW.
  • Collector-Emitter Voltage (V_CE): 30 V. The maximum voltage that can be applied across the collector and emitter.
  • Emitter-Collector Voltage: 5 V.
  • Collector Current: 20 mA. The maximum current the phototransistor's output can sink.
• Environmental:
  • Operating Temperature Range: -25°C to +85°C. The ambient temperature range for normal device function.
  • Storage Temperature Range: -40°C to +100°C.
  • Lead Soldering Temperature: 260°C for 5 seconds (for leads 1.6mm from the case). This defines the reflow soldering profile constraint.

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.

• Input LED Characteristics:
  • Forward Voltage (V_F): Typically 1.2V to 1.6V at a forward current (I_F) of 20 mA. This is used to calculate the required current-limiting resistor value: R_limit = (V_supply - V_F) / I_F.
  • Reverse Current (I_R): Maximum 100 μA at a reverse voltage of 5V.
• Output Phototransistor Characteristics:
  • Collector-Emitter Dark Current (I_CEO): Maximum 100 nA at V_CE=10V. This is the leakage current when the LED is off (no light). A low value is desirable for good signal-to-noise ratio.
  • Collector-Emitter Saturation Voltage (V_CE(SAT)): Typically 0.4V at I_C=0.25mA and I_F=20mA. This is the voltage drop across the phototransistor when it is fully "on."
  • On-State Collector Current (I_C(ON)): Minimum 0.5 mA at V_CE=5V and I_F=20mA. This specifies the minimum output current when the light path is unobstructed.
• Coupler Characteristic:
  • Action Angle: 8° to 14°. This is a critical parameter defining the angular displacement of the interrupting object (e.g., a lever arm) required to reliably switch the output state. A smaller angle indicates higher sensitivity to movement.

3. Performance Curve Analysis

The datasheet references typical electrical/optical characteristic curves. While the specific graphs are not provided in the text, their standard purpose is analyzed below.

• Forward Current vs. Forward Voltage (I_F-V_F Curve): This graph shows the non-linear relationship between the LED's current and voltage. It helps designers understand the dynamic resistance of the LED and ensure stable current drive.
• Collector Current vs. Collector-Emitter Voltage (I_C-V_CE Curves): These curves, plotted for different LED drive currents (I_F), illustrate the phototransistor's output characteristics. They show the saturation region (where I_C is relatively constant) and the linear/active region, which is important for analog sensing applications.
• Current Transfer Ratio (CTR) vs. Forward Current: CTR is the ratio of phototransistor collector current (I_C) to LED forward current (I_F), typically expressed as a percentage. This curve shows how efficiency changes with drive current and is key for optimizing the drive circuit for desired output swing.
• Temperature Dependence Curves: Graphs showing how parameters like V_F, I_C(ON), and dark current vary with ambient temperature are essential for designing robust systems that operate across the specified temperature range.

4. Mechanical & Package Information

The datasheet includes a package dimensions drawing (not reproduced here). Key mechanical considerations include:

• Slot Dimensions: The critical gap through which the interrupting object passes. Its width and depth determine compatibility with the target object.
• Lead Spacing and Form: The pin layout (likely a standard 4-pin configuration: anode, cathode for the LED; collector, emitter for the phototransistor) and their spacing are vital for PCB footprint design.
• Overall Package Size: The external length, width, and height constrain the device's placement within an assembly.
• Polarity Identification: The package will have markings (such as a dot or a beveled edge) to identify Pin 1, which must be correctly aligned with the PCB footprint.
• Custom Lever Arms: A noted feature is the ability to design customized lever arms that attach to the interrupting object, allowing the sensor to be adapted for specific mechanical motions and increasing its application flexibility.

5. Soldering & Assembly Guidelines

Proper handling is crucial for reliability.

• Reflow Soldering: The specified limit is 260°C for 5 seconds, measured 1.6mm from the package body. This aligns with typical lead-free reflow profiles. Designers must ensure the thermal profile of their reflow oven does not exceed this limit to prevent damage to the internal epoxy or semiconductor junctions.
• Hand Soldering: If hand soldering is necessary, a temperature-controlled iron should be used, and the soldering time per lead should be minimized (typically < 3 seconds).
• Cleaning: Use appropriate, non-corrosive cleaning agents compatible with the device's plastic package.
• Storage Conditions: Store in a dry, anti-static environment within the specified -40°C to +100°C range to prevent moisture absorption (which can cause "popcorning" during reflow) and electrostatic discharge (ESD) damage.

6. Application Suggestions & Design Considerations

6.1 Typical Application Circuits

The most common configuration is a digital switch. The LED is driven with a constant current (e.g., 20mA via a series resistor). The phototransistor collector is connected to a pull-up resistor (R_pull-up) to the logic supply voltage (e.g., 5V), and the emitter is grounded. The output signal is taken from the collector node.

• Uninterrupted Beam (Object Absent): Light falls on the phototransistor base, causing it to conduct. The collector voltage is pulled low (close to V_CE(SAT)).
• Interrupted Beam (Object Present): The phototransistor turns off. The pull-up resistor pulls the collector voltage high (to the supply voltage).

The value of R_pull-up is a trade-off: a lower value provides faster rise times and better noise immunity but draws more current when the output is low. It should be chosen based on the required switching speed and the input characteristics of the following logic stage.

6.2 Design Considerations

• LED Current Selection: Operating at the typical 20mA provides good output current. Lower currents save power but reduce I_C(ON) and noise margin. Do not exceed the continuous forward current rating.
• Ambient Light Immunity: The device is sensitive to the specific wavelength of its internal LED. However, in environments with strong ambient light (especially sunlight containing IR), a modulated (pulsed) LED drive signal with synchronous detection in the receiving circuit can greatly improve immunity.
• Response Time: The switching speed (rise/fall time) is limited by the phototransistor's capacitance and the value of the pull-up resistor. For high-speed applications, consult specific dynamic characteristic graphs if available.
• Object Characteristics: The interrupting object's opacity, thickness, and color affect the amount of light blocked. For reliable operation, the object should be sufficiently opaque to reduce the phototransistor current below its threshold for the "off" state.
• Alignment: Precise mechanical alignment of the object within the sensor's slot is necessary for consistent operation, especially given the defined action angle.

7. Technical Comparison & Advantages

Compared to mechanical micro-switches, the LTH-306-09S photointerrupter offers several key advantages:

• Longevity & Reliability: No moving contacts to wear out, arc, or oxidize. Lifespan is typically orders of magnitude longer.
• High-Speed Operation: Can switch much faster than mechanical switches, which are limited by contact bounce and mechanical inertia.
• Consistent Performance: Contact resistance is not a factor. The output characteristics remain stable over time.
• Environmental Sealing: The plastic package can be more easily sealed against dust and moisture compared to a mechanical switch with an external actuator.
• Quiet Operation: Completely silent, unlike the audible click of a mechanical switch.

The trade-off is the need for supporting electronics (a current source for the LED and a pull-up resistor) and potential sensitivity to extreme ambient light or contamination of the optical path.

8. Frequently Asked Questions (Based on Technical Parameters)

• Q: Can I drive the LED directly from a 5V microcontroller pin? A: No. You must use a current-limiting resistor. For example, with V_CC=5V, V_F~1.4V, and desired I_F=20mA: R = (5V - 1.4V) / 0.02A = 180Ω. A 180Ω or 220Ω resistor is typical.
• Q: What does the "Action Angle" of 8-14 degrees mean for my design? A: It means the physical lever or flag that interrupts the beam must rotate or move through at least 8 degrees (and typically up to 14 degrees) as it passes through the slot to guarantee a reliable switch from the "on" to the "off" state. Your mechanical design must ensure this angular travel.
• Q: The output collector current (I_C(ON)) is only 0.5mA min. Is that enough to drive a logic input? A: Yes, for standard CMOS or TTL logic inputs, which have very high input impedance (requiring only microamps), a 0.5mA sink capability is more than sufficient. The voltage level (low = ~0.4V) is the critical parameter.
• Q: How do I protect the device from voltage spikes on the supply lines? A: Use standard board-level decoupling capacitors (e.g., 100nF ceramic) close to the device. For harsh environments, additional transient voltage suppression (TVS) diodes on the supply rail may be considered.

9. Practical Application Examples

• Printer Paper Detection: A flag attached to the paper tray lever rotates through the photointerrupter's slot. When paper is present, the flag is in one position (beam uninterrupted); when empty, it moves to the other position (beam interrupted), signaling the control system.
• Industrial Conveyor Belt Object Counting: Objects on the conveyor pass through a gate equipped with a photointerrupter. Each object breaks the beam, generating a pulse that is counted by a PLC or microcontroller.
• Safety Door Interlock: The photointerrupter is mounted on a door frame, and a tab is mounted on the door. When the door is properly closed, the tab enters the slot, allowing the beam to pass and signaling a "safe" condition. If the door is open, the beam is blocked, signaling an "unsafe" condition that can disable machinery.
• Rotary Encoder Disk Sensing: A slotted disk attached to a motor shaft rotates between the emitter and detector. The series of light pulses generated as the slots pass through is used to determine speed and position.

10. Operating Principle

A photointerrupter is an optocoupler with a physical gap between its emitter and detector. It consists of an infrared Light Emitting Diode (LED) on one side and a silicon Phototransistor on the opposite side, aligned across an open slot. When an electrical current is applied to the LED, it emits infrared light. This light travels across the gap and strikes the base region of the phototransistor. The photons generate electron-hole pairs in the base, effectively acting as a base current. This photogenerated current is then amplified by the transistor's gain, allowing a much larger collector current to flow. When an opaque object enters the slot, it blocks the light path. The photogenerated base current ceases, turning off the phototransistor and stopping the collector current. Thus, the presence or absence of an object in the slot digitally controls the conductivity of the output phototransistor.

11. Technology Trends

The fundamental technology of photointerrupters is mature. Current trends focus on integration and miniaturization. Devices are becoming smaller in package size (SMD types) while maintaining or improving performance. There is also a trend towards integrating additional circuitry on-chip, such as Schmitt triggers for hysteresis (to provide clean digital switching without external components), amplifiers for analog output, or even full digital interfaces (I2C). This reduces external component count and simplifies design. Furthermore, devices with higher sensitivity allow for operation with lower LED currents, reducing overall system power consumption, which is critical for battery-powered applications. The development of materials for the optical path (lenses, filters) also continues to improve ambient light rejection and sensing accuracy.