Photointerrupter LTH-301-05 Datasheet - Non-Contact Switching - English Technical Document

1. Product Overview

The LTH-301-05 is a reflective photointerrupter, a type of optoelectronic component that combines an infrared light-emitting diode (IR 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, making it a non-contact switch. The core advantage of this device lies in its reliability and longevity, as it eliminates mechanical wear and tear associated with traditional switches. It is designed for direct PCB (Printed Circuit Board) mounting or use with a dual-in-line socket, offering flexibility in assembly. The fast switching speed makes it suitable for applications requiring rapid detection, such as in printers, copiers, vending machines, and industrial automation equipment where position sensing, object counting, or edge detection is needed.

2. In-Depth Technical Parameter Analysis

2.1 Absolute Maximum Ratings

These ratings define the limits beyond which permanent damage to the device may occur. They are not for continuous operation. Key parameters include:

• IR Diode Continuous Forward Current (I_F): 60 mA. This is the maximum steady-state current that can flow through the LED.
• IR Diode Peak Forward Current: 1 A (for pulses of 10 μs width at 300 pulses per second). This allows for brief, high-intensity pulses for enhanced signal detection.
• Phototransistor Collector Current (I_C): 20 mA. The maximum current the output transistor can handle.
• Phototransistor Collector-Emitter Voltage (V_CEO): 30 V. The maximum voltage that can be applied across the phototransistor's collector and emitter.
• Operating Temperature Range: -25°C to +85°C. This defines the ambient temperature range for reliable operation.
• Lead Soldering Temperature: 260°C for 5 seconds at a distance of 1.6mm from the case. This is critical for assembly to prevent thermal damage.

2.2 Electrical & Optical Characteristics

These parameters are measured at an ambient temperature (T_A) of 25°C and define the device's typical performance.

2.2.1 Input (IR LED) Characteristics

• Forward Voltage (V_F): Typically 1.2V to 1.6V at a forward current (I_F) of 20 mA. This is the voltage drop across the LED when it is illuminated.
• Reverse Current (I_R): Maximum 100 μA at a reverse voltage (V_R) of 5V. This indicates the small leakage current when the LED is reverse-biased.

2.2.2 Output (Phototransistor) Characteristics

• Collector-Emitter Breakdown Voltage (V_(BR)CEO): Minimum 30V. The voltage at which the transistor breaks down when the base is open.
• Collector-Emitter Dark Current (I_CEO): Maximum 100 nA at V_CE=10V. This is the leakage current of the phototransistor when no light is incident (i.e., the "off" state current). A low value is desirable for good contrast between on and off states.

2.2.3 Coupler (Combined) Characteristics

These parameters describe the behavior of the LED and phototransistor working together.

• Collector-Emitter Saturation Voltage (V_CE(SAT)): Maximum 0.4V when the phototransistor is fully turned on (I_C=0.25mA, I_F=20mA). A low saturation voltage is good for digital logic interfacing.
• On-State Collector Current (I_C(ON)): Minimum 0.5 mA when the LED is driven (I_F=20mA) and V_CE=5V. This is the photocurrent generated, which determines the output signal strength.
• Response Time: This defines how quickly the output reacts to a change in the input light.
  • Rise Time (t_r): Typically 3 μs, maximum 15 μs. The time for the output current to rise from 10% to 90% of its final value when the LED is turned on.
  • Fall Time (t_f): Typically 4 μs, maximum 20 μs. The time for the output current to fall from 90% to 10% of its initial value when the LED is turned off.

3. Mechanical & Packaging Information

3.1 Package Dimensions

The device features a standard through-hole package with four leads. The exact dimensions are provided in the datasheet drawings. Key notes include:

• All dimensions are in millimeters, with inches in parentheses.
• The standard tolerance is ±0.25mm (±0.010") unless a specific note states otherwise.
• The package is designed for stability during wave soldering or manual soldering processes.

3.2 Polarity Identification

Correct orientation is crucial. The datasheet diagram clearly indicates the anode and cathode pins for the IR LED and the collector and emitter pins for the phototransistor. Mounting the device incorrectly can lead to non-operation or permanent damage.

4. Soldering & Assembly Guidelines

Proper handling ensures device reliability and longevity.

• Soldering: The leads can be soldered at a maximum temperature of 260°C, but this heat should only be applied for a maximum duration of 5 seconds. It is critical to maintain the specified distance (1.6mm / 0.063") from the plastic case body to prevent melting or deformation of the package.
• Cleaning: Use appropriate solvents that are compatible with the device's plastic material. Avoid ultrasonic cleaning with certain frequencies that might cause internal stress or cracking.
• Storage Conditions: To preserve performance, store the devices in an environment with a temperature range of -40°C to +100°C and low humidity, preferably in anti-static packaging to prevent electrostatic discharge (ESD) damage.

5. Application Suggestions

5.1 Typical Application Scenarios

• Paper Detection in Printers/Copiers: Detecting paper jams, end-of-paper, or multi-feed conditions.
• Object Counting: Counting items on a conveyor belt or through a chute.
• Position/Speed Sensing: Detecting slots in an encoder wheel to determine rotational position or speed of a motor.
• Vending Machines: Verifying coin passage or product dispensing.
• Security Systems: As part of a beam-break sensor for intrusion detection.

5.2 Design Considerations

• LED Current Limiting: Always use a series resistor with the IR LED to limit the forward current (I_F) to a safe value, typically between 10mA and 20mA for a balance between output signal strength and device lifespan. The resistor value can be calculated using R = (V_CC - V_F) / I_F.
• Phototransistor Biasing: A pull-up resistor is typically connected between the phototransistor's collector and the positive supply voltage (V_CC). The emitter is connected to ground. The value of this resistor (often between 1kΩ and 10kΩ) and the supply voltage determine the output voltage swing and the speed of response. A smaller resistor gives faster response but lower output voltage swing (and higher power consumption when on).
• Ambient Light Immunity: Since the device uses infrared light, it is somewhat immune to visible ambient light. However, strong sources of IR radiation (like sunlight or incandescent bulbs) can cause false triggering. Using a modulated IR signal and a demodulating circuit can greatly improve noise immunity.
• Gap and Reflectivity: The sensing distance and signal strength depend on the reflectivity of the target object and the width of the gap between the sensor and the object. Dark, non-reflective objects will produce a weaker signal.

6. Principle of Operation

The LTH-301-05 operates on a simple optical principle. The internal IR LED emits a beam of infrared light. Opposite the LED is a phototransistor. In the "uninterrupted" state, this light beam travels across a small gap and strikes the phototransistor, causing it to conduct (turn on). When an object is inserted into this gap, it blocks the infrared light. With no light incident on the phototransistor, it stops conducting (turns off). This change in the electrical state of the phototransistor (from conducting to non-conducting, or vice-versa) is detected by the external circuitry, registering the presence of the object. The phototransistor essentially acts as a current source controlled by light intensity.

7. Performance Curve Analysis

The datasheet includes typical characteristic curves which are invaluable for detailed design analysis. While specific graphs are not reproduced in text, they typically illustrate the following relationships:

• Forward Current vs. Forward Voltage (I_F-V_F) for the LED: Shows the non-linear relationship, helping to determine the exact voltage drop at different operating currents.
• Collector Current vs. Collector-Emitter Voltage (I_C-V_CE) for the Phototransistor: At different levels of incident light (or different LED drive currents), these curves show the transistor's output characteristics, similar to a bipolar transistor's output curves.
• Collector Current vs. Forward Current (I_C-I_F): This transfer characteristic curve is crucial. It shows how the output photocurrent (I_C) varies with the input LED current (I_F). It defines the current transfer ratio (CTR), which is a key efficiency parameter for the coupler.
• Temperature Dependence: Curves often show how parameters like forward voltage (V_F), dark current (I_CEO), and on-state current (I_C(ON)) vary with ambient temperature. This is critical for designing systems that operate over a wide temperature range.

8. Common Questions Based on Technical Parameters

1. Q: What is the typical sensing distance? A: The sensing distance is not a single fixed value in the datasheet. It depends on the specific mechanical design of the slot, the drive current to the LED (I_F), the sensitivity of the receiving circuit, and the reflectivity of the interrupting object. The designer must determine this based on the I_C(ON) parameter and application setup.
2. Q: Can I drive the LED directly from a microcontroller pin? A: Possibly, but you must check two things: a) The microcontroller pin's maximum current sourcing capability must be greater than your desired I_F (e.g., 20mA). b) You MUST include a current-limiting resistor in series as described in the design considerations. Never connect an LED directly to a voltage source.
3. Q: How do I interface the output with a digital input? A: The simplest method is to use a pull-up resistor on the collector. When the light path is clear, the phototransistor is on, pulling the collector voltage low (close to V_CE(SAT)). When the light is blocked, the transistor is off, and the pull-up resistor pulls the collector voltage high (to V_CC). This provides a clean logic-level signal.
4. Q: Why is the response time important? A: Fast response times (microseconds) allow the sensor to detect very fast-moving objects or rapid sequential events without missing counts. This is essential in high-speed machinery, encoder applications, or communication systems using pulsed light.
5. Q: What happens if I exceed the absolute maximum ratings? A: Exceeding these limits, even briefly, can cause immediate or latent damage to the device. This can include degradation of the LED's light output, increased dark current in the phototransistor, or complete failure (open or short circuit). Always design with a safety margin.

9. Practical Design and Usage Case

Case: RPM Measurement of a Small DC Motor

A designer needs to measure the rotational speed of a motor shaft. They attach a small slotted disk to the shaft. The LTH-301-05 is mounted such that the disk rotates through its sensing gap. Each time a slot passes through the gap, light reaches the phototransistor, causing a pulse in the output. The LED is driven with a constant 15mA current via a resistor. The phototransistor collector is connected to a 5V supply through a 4.7kΩ pull-up resistor and also to a microcontroller's interrupt-capable input pin.

The microcontroller firmware is programmed to count the number of pulses (rising or falling edges) received within a fixed time window (e.g., one second). Since the disk has, for example, 20 slots, the number of pulses per second divided by 20 gives the revolutions per second, which is easily converted to RPM. The fast rise and fall times of the sensor ensure that even at high motor speeds, the pulses are clean and accurately counted, without missing edges due to slow sensor response.

10. Development Trends

Photointerrupters like the LTH-301-05 represent a mature and reliable technology. Current trends in the broader field of optoelectronic sensors focus on:

• Miniaturization: Development of even smaller surface-mount device (SMD) packages to save board space in modern electronics.
• Integration:
  • Integrating the current-limiting resistor for the LED internally.
  • Including a Schmitt trigger or comparator in the package to provide a clean digital output directly, simplifying interface circuitry.
  • Adding ambient light rejection circuits or modulation/demodulation logic on-chip for superior noise immunity.
• Enhanced Performance: Improving the current transfer ratio (CTR) for lower power consumption or longer sensing distances, and reducing response times further for ultra-high-speed applications.
• Specialization: Creating variants with very narrow gaps for precise edge detection, or with different wavelengths for specific material detection (e.g., sensing transparent films).

Despite these advancements, the fundamental reflective photointerrupter remains a cost-effective and robust solution for a vast array of non-contact sensing applications, and understanding its detailed parameters as outlined in this datasheet is the first step towards a successful design.