LTH-301-32 Photointerrupter Datasheet - Slotted Optical Switch - 30V Collector-Emitter Voltage - English Technical Document

1. Product Overview

The LTH-301-32 is a slotted optical switch, commonly known as a photointerrupter. It is a non-contact sensing device that combines an infrared light-emitting diode (IR LED) and a phototransistor in a single package, separated by a physical gap. The core function is to detect the presence or absence of an object (like a vane or flag) that passes through this slot, interrupting the infrared light beam. This makes it ideal for applications requiring position sensing, limit switching, or object detection without physical contact, thereby eliminating mechanical wear and enabling high-speed operation.

The device is designed for direct mounting onto printed circuit boards (PCBs) or into standard dual-in-line (DIP) sockets, offering flexibility in assembly and integration. Its primary advantages include reliable non-contact switching, immunity to mechanical bounce, and a fast response time suitable for digital systems.

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.

• IR Diode Continuous Forward Current (I_F): 60 mA. This is the maximum steady-state current that can be passed through the infrared LED.
• IR Diode Reverse Voltage (V_R): 5 V. Exceeding this reverse bias voltage across the LED can cause breakdown.
• Transistor Collector Current (I_C): 20 mA. The maximum continuous current the output phototransistor's collector can handle.
• Transistor Power Dissipation (P_D): 75 mW. The maximum power the phototransistor can dissipate, calculated as V_CE * I_C.
• IR Diode Peak Forward Current: 1 A (pulse width = 10 μs, 300 pps). This allows for brief, high-current pulses to achieve higher instantaneous light output, useful for noise immunity, but the duty cycle must be strictly observed.
• Diode Power Dissipation: 100 mW. The maximum power the IR LED can dissipate (V_F * I_F).
• Phototransistor Collector-Emitter Voltage (V_CEO): 30 V. The maximum voltage that can be applied between the collector and emitter of the phototransistor.
• Phototransistor Emitter-Collector Voltage (V_ECO): 5 V. The maximum reverse voltage between emitter and collector.
• Operating Temperature Range: -25°C to +85°C. The ambient temperature range for reliable operation.
• Storage Temperature Range: -40°C to +100°C.
• Lead Soldering Temperature: 260°C for 5 seconds, measured 1.6mm from the case. This defines the reflow or hand-soldering profile limits.

2.2 Electrical & Optical Characteristics

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

2.2.1 Input LED Characteristics

• Forward Voltage (V_F): 1.2V (Min), 1.6V (Typ) at I_F = 20mA. This is the voltage drop across the IR LED when driven with the typical operating current. A current-limiting resistor is required in series with the LED.
• Reverse Current (I_R): 100 μA (Max) at V_R = 5V. The small leakage current when the LED is reverse-biased.

2.2.2 Output Phototransistor Characteristics

• Collector-Emitter Breakdown Voltage (V_(BR)CEO): 30V (Min). Correlates with the absolute maximum rating.
• Emitter-Collector Breakdown Voltage (V_(BR)ECO): 5V (Min).
• Collector-Emitter Dark Current (I_CEO): 100 nA (Max) at V_CE=10V. This is the leakage current of the phototransistor when no light is incident (i.e., the slot is blocked). It determines the "off-state" signal level.

2.2.3 Coupler (System) Characteristics

These parameters describe the combined behavior of the LED and phototransistor.

• Collector-Emitter Saturation Voltage (V_CE(SAT)): 0.4V (Max) at I_C=0.2mA and I_F=20mA. This is the voltage across the phototransistor when it is fully turned "on" (light unobstructed). A lower V_CE(SAT) is better for interfacing with logic circuits.
• On-State Collector Current (I_C(ON)): 0.6 mA (Min) at V_CE=5V and I_F=20mA. This is the minimum photocurrent generated when the light path is clear. The actual current can be higher and is dependent on the LED drive current and device gain.
• Response Time: This defines the switching speed.
  • Rise Time (t_r): 3 μS (Typ), 15 μS (Max). Time for the output to go from 10% to 90% of its final value when the light beam is unblocked.
  • Fall Time (t_f): 4 μS (Typ), 20 μS (Max). Time for the output to go from 90% to 10% of its final value when the light beam is blocked.
  These times are measured under specific conditions (V_CE=5V, I_C=2mA, R_L=100Ω) and determine the maximum frequency of object detection.

3. Performance Curve Analysis

The datasheet references typical performance curves which graphically illustrate key relationships. While the specific graphs are not provided in the text, their typical content and interpretation are as follows:

3.1 Transfer Characteristics

A graph of Output Collector Current (I_C) vs. Input LED Forward Current (I_F) at a constant collector-emitter voltage (e.g., V_CE=5V). This curve shows the current transfer ratio (CTR) trend, which is the ratio I_C / I_F. It helps designers select the appropriate LED drive current to achieve the desired output current level for a given load or logic threshold.

3.2 Temperature Dependence

Curves showing how parameters like I_C(ON) and dark current (I_CEO) vary over the operating temperature range (-25°C to +85°C). Phototransistor gain typically decreases with increasing temperature, while dark current increases. Understanding these shifts is critical for designing stable systems across the full temperature range, often requiring margin in the chosen I_F and threshold detection levels.

3.3 Output Saturation Voltage

A plot of V_CE(SAT) vs. I_C for different I_F values. This is essential for determining the minimum voltage drop when the transistor is on, ensuring compatibility with low-voltage logic families.

4. Mechanical & Package Information

4.1 Package Dimensions

The LTH-301-32 comes in a standard, compact DIP-style package. Key dimensional notes from the datasheet:

• 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 package features a molded body with a precise slot. The leads are on a standard 0.1" (2.54mm) pitch, compatible with DIP sockets and PCB layouts. The exact length, width, height, slot width, and lead positioning are defined in the dimensioned drawing referenced in the datasheet.

4.2 Polarity Identification

For proper operation, correct pin identification is crucial. The package uses standard marking: the cathode of the IR LED and the emitter of the phototransistor are typically connected to a common pin or are adjacent. The datasheet's pinout diagram must be consulted to identify:

1. Anode of the IR LED.
2. Cathode of the IR LED.
3. Collector of the phototransistor.
4. Emitter of the phototransistor.

Incorrect connection can prevent operation or damage the device.

5. Soldering & Assembly Guidelines

5.1 Soldering Profile

The absolute maximum rating specifies lead soldering at 260°C for 5 seconds, measured 1.6mm from the plastic case. This is a critical parameter for wave soldering or hand soldering.

• Reflow Soldering: If used in a reflow process, a profile with a peak temperature not exceeding 260°C and a time above 240°C (T_L) of less than 10 seconds is generally recommended. The plastic body is sensitive to thermal stress.
• Hand Soldering: Use a temperature-controlled iron. Apply heat to the lead, not the body, and complete the joint within 3-5 seconds per lead to avoid heat soaking into the package.

5.2 Cleaning & Handling

Standard PCB cleaning processes using isopropyl alcohol or similar solvents are typically acceptable. Avoid ultrasonic cleaning unless verified, as it may cause micro-cracks in the plastic or the internal die bond. Handle the device by the body, not the leads, to prevent mechanical stress on the seal.

5.3 Storage Conditions

Store in a dry, anti-static environment within the specified storage temperature range (-40°C to +100°C). Moisture Sensitivity Level (MSL) is not explicitly stated in the provided text, but for long-term storage, keeping components in their original moisture-barrier bags is good practice.

6. Application Suggestions

6.1 Typical Application Circuits

The most common configuration is to use the photointerrupter as a digital switch.

1. LED Drive Circuit: A current-limiting resistor (R_LIMIT) is connected in series with the IR LED. R_LIMIT = (V_CC - V_F) / I_F. For a 5V supply and I_F=20mA, R_LIMIT ≈ (5V - 1.6V) / 0.02A = 170Ω (use 180Ω standard value).
2. Phototransistor Output Circuit: The phototransistor can be used in two common configurations:
   • Pull-up Resistor Configuration: Connect a resistor (R_LOAD) from the collector to V_CC. The emitter is connected to ground. The output is taken from the collector. When light is blocked, the transistor is off, and the output is pulled high (V_CC). When light is present, the transistor turns on, pulling the output low (near V_CE(SAT)). R_LOAD value is chosen based on desired I_C and speed; 1kΩ to 10kΩ is common.
   • Current-to-Voltage Configuration: Connect the phototransistor in a common-emitter configuration with an operational amplifier in a transimpedance setup to convert the photocurrent into a precise voltage. This is used for analog sensing.

6.2 Design Considerations

• Noise Immunity: For environments with ambient light (especially infrared), use a modulated LED drive signal and synchronous detection, or ensure the slot is physically shrouded.
• Debouncing: While the device itself has no mechanical bounce, the output signal may need software debouncing if the sensed object can chatter in the slot.
• Object Material: The object interrupting the beam must be opaque to infrared light. Thin or translucent materials may not be reliably detected.
• Alignment: Precise mechanical alignment of the object passing through the slot is necessary for consistent operation.

6.3 Common Application Scenarios

• Printers & Copiers: Paper-out detection, toner level sensing, carriage position homing.
• Industrial Automation: Limit switches on linear actuators, part presence detection on conveyor belts, vane sensing on rotating shafts (tachometer).
• Consumer Electronics:
• Security Systems: Door/window position sensing.
• Vending Machines: Coin or product dispensing verification.

7. Technical Comparison & Selection Guide

When selecting a photointerrupter, key differentiating factors include:

• Slot Width & Gap: Determines the size of the object that can be sensed. The LTH-301-32 has a specific slot dimension.
• Output Type: Phototransistor (as here) vs. Photodarlington (higher gain, slower speed) vs. Logic Output (built-in Schmitt trigger).
• Current Transfer Ratio (CTR): Higher CTR provides more output current for a given input current, allowing for higher value pull-up resistors or longer cable runs.
• Speed (t_r, t_f): Critical for high-speed counting or encoding applications.
• Package & Mounting: Through-hole (DIP) vs. surface-mount (SMD). The LTH-301-32 is a through-hole device.
• Operating Voltage: The V_(BR)CEO of 30V allows it to interface with a wide range of supply voltages, from 3.3V to 24V systems.

The LTH-301-32 positions itself as a general-purpose, reliable device with a balanced set of characteristics suitable for a broad range of medium-speed digital sensing applications.

8. Frequently Asked Questions (Based on Technical Parameters)

8.1 What is the purpose of the peak forward current rating for the LED?

The 1A peak rating allows the LED to be pulsed with a much higher current than its DC rating (60mA). This can be used to generate a brighter light pulse, improving signal-to-noise ratio in noisy environments or allowing for a lower duty cycle to save power. The strict limits on pulse width (10μs) and repetition rate (300 pps) must be followed to prevent overheating.

8.2 How do I choose the value of the pull-up resistor (R_LOAD)?

The choice involves a trade-off between power consumption, switching speed, and noise immunity. A smaller resistor (e.g., 1kΩ) provides faster rise times (less RC time constant) and better noise immunity but draws more current when the transistor is on (I_C = V_CC/R_LOAD). A larger resistor (e.g., 10kΩ) saves power but is slower and more susceptible to noise. Ensure the chosen R_LOAD, at the minimum supply voltage, still allows enough I_C to pull the output below the logic-low threshold of the receiving circuit, considering the minimum I_C(ON) specification.

8.3 Why is the response time specified with a load resistor (R_L=100Ω)?

The phototransistor's switching speed is limited by the capacitance of its junction and the resistance through which it charges/discharges. Specifying it with a small load resistor (100Ω) shows the device's intrinsic speed limit. In a real circuit with a larger pull-up resistor, the rise time will be slower due to the larger RC constant (t_rise ≈ R_LOAD * C). The fall time is primarily governed by the device's internal carrier recombination and is less dependent on the external resistor.

8.4 How does temperature affect operation?

As temperature increases:

• The phototransistor's gain (and thus I_C(ON)) decreases. You may need to increase I_F to compensate.
• The dark current (I_CEO) increases. This raises the "off" voltage level, potentially causing false triggering if the detection threshold is set too tightly.
• The LED's forward voltage (V_F) decreases slightly.

Designs for wide temperature ranges must account for these shifts, often by derating the usable I_C(ON) and allowing margin for I_CEO.

9. Operational Principle

A photointerrupter operates on the principle of optoelectronic coupling. The device contains two separate components in one housing: an infrared light-emitting diode (IR LED) and a silicon phototransistor. They face each other across an air gap (the slot). When power is applied to the IR LED, it emits invisible infrared light. This light travels across the slot and strikes the base region of the phototransistor. The photons generate electron-hole pairs in the base, which act as base current, turning the transistor on. This allows a much larger collector current to flow, limited by the external circuit.

When an opaque object is inserted into the slot, it blocks the light path. The photogeneration of base current ceases, and the phototransistor turns off, stopping the collector current. Thus, the electrical state of the output (on/off) is directly controlled by the mechanical state of the slot (clear/blocked), without any electrical contact between the input (LED side) and output (transistor side). This provides excellent electrical isolation, typically in the range of hundreds to thousands of volts.

10. Industry Trends & Context

Photointerrupters like the LTH-301-32 represent a mature and fundamental sensing technology. Key trends influencing this sector include:

• Miniaturization: Strong demand for smaller surface-mount device (SMD) packages to save PCB space in modern electronics.
• Integration:
• Higher Speed: Development of devices with faster response times (nanosecond range) for high-resolution encoders and data communication applications.
• Improved Precision: Tighter tolerances on slot dimensions and optical alignment for more accurate position sensing.
• Alternative Technologies: Photointerrupters face competition from other non-contact sensors like Hall effect sensors (for magnetic sensing), capacitive sensors, and miniature ultrasonic sensors. The choice depends on the object material, required precision, environmental conditions, and cost.

Despite these trends, the basic through-hole slotted optical switch remains a highly cost-effective, reliable, and easy-to-use solution for countless applications where robustness, electrical isolation, and simple digital output are paramount.