LTH-301-23 Photointerrupter Datasheet - Dimensions 4.0x3.2x2.5mm - Forward Voltage 1.6V - English Technical Document

1. Product Overview

The LTH-301-23 is a compact, through-hole photointerrupter module designed for non-contact switching applications. It integrates an infrared light-emitting diode (IR LED) and a phototransistor within a single housing, separated by a physical gap. The core principle of operation involves the interruption of the infrared light beam between the emitter and detector, which causes a corresponding change in the phototransistor's output state. This makes it ideal for applications requiring position sensing, object detection, or limit switching without physical contact, thereby eliminating mechanical wear and enabling high reliability and fast switching speeds.

Its primary advantages include non-contact operation, which provides long operational life, fast response times suitable for counting or speed detection, and a design compatible with direct PCB mounting or standard dual-in-line sockets for easy integration. The target markets and applications are broad, encompassing office automation equipment (printers, copiers), industrial automation (conveyor belt object detection, position sensing), consumer electronics, and various instrumentation and control 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. Key limits include:

• 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 Peak Forward Current: 1 A for pulses of 10 μs width at 300 pulses per second. This allows for brief, high-intensity pulses for applications requiring stronger signal bursts.
• Phototransistor Collector-Emitter Voltage (V_CEO): 30 V. The maximum voltage that can be applied across the collector and emitter of the output transistor.
• Operating Temperature Range: -25°C to +85°C. This defines the ambient temperature range for reliable device operation.
• Lead Soldering Temperature: 260°C for 5 seconds at a distance of 1.6mm from the case. This is critical for assembly process control to prevent thermal damage.

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 (IR 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 current-limiting resistor value for the LED driver circuit.
• Reverse Current (I_R): Maximum 100 μA at a reverse voltage (V_R) of 5V. This indicates the LED's leakage current when reverse-biased, which is very low.

2.2.2 Output (Phototransistor) Characteristics

• Collector-Emitter Breakdown Voltage (V_(BR)CEO): Minimum 30V. This ensures the transistor can withstand typical circuit voltages.
• 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), determining the "off-state" signal level.
• Collector-Emitter Saturation Voltage (V_CE(SAT)): Maximum 0.4V at I_C=0.2mA and I_F=20mA. This is the voltage drop across the transistor when it is fully "on," important for logic-level interfacing.
• On-State Collector Current (I_C(ON)): Minimum 0.4 mA at V_CE=5V and I_F=20mA. This specifies the minimum output current available when the beam is unblocked, defining the sensor's sensitivity.

2.2.3 Coupler (System) Characteristics

• Rise Time (t_r): 3 μs (Typical) to 15 μs (Maximum) under test conditions of V_CE=5V, I_C=2mA, and R_L=100Ω.
• Fall Time (t_f): 4 μs (Typical) to 20 μs (Maximum) under the same conditions.

These response times define how quickly the output can switch from off to on (rise) and on to off (fall). The fast switching speed (microsecond range) enables detection of rapidly moving objects or high-speed counting applications.

3. Performance Curve Analysis

The datasheet references typical electrical/optical characteristic curves. While the specific graphs are not detailed in the provided text, standard curves for such a device would typically include:

• Forward Current vs. Forward Voltage (I_F-V_F) for the IR LED: Shows the non-linear relationship, crucial for designing the driving circuit.
• Collector Current vs. Collector-Emitter Voltage (I_C-V_CE) for the Phototransistor: At different levels of irradiance (LED current), these output curves show the transistor's operating regions (cut-off, active, saturation).
• Current Transfer Ratio (CTR) vs. Forward Current: CTR is the ratio of phototransistor collector current (I_C) to LED forward current (I_F). This curve shows the efficiency of the optical coupling and how it varies with drive current.
• Temperature Dependence of Dark Current (I_CEO) and On-State Current (I_C(ON)): These curves illustrate how performance degrades at temperature extremes, which is vital for designing robust systems operating across the specified temperature range.

These curves allow designers to optimize operating points, understand performance trade-offs, and ensure reliable operation under all specified conditions.

4. Mechanical & Package Information

4.1 Package Dimensions

The LTH-301-23 is housed in a standard through-hole package. Key dimensional notes from the datasheet:

• All dimensions are provided in millimeters, with inches in parentheses.
• The standard tolerance is ±0.25mm (±0.010") unless a specific feature note states otherwise.
• The package is designed for direct PCB mounting or insertion into a standard dual-in-line socket, providing flexibility in assembly and prototyping.

The physical gap between the emitter and detector is fixed within the housing, defining the slot where the interrupting object passes. The exact width of this gap is a critical mechanical specification found in the dimensioned drawing.

4.2 Polarity Identification & Pinout

For proper operation, correct pin identification is essential. The device has four leads. Typically, the two leads on one side belong to the infrared LED (anode and cathode), and the two on the other side belong to the phototransistor (collector and emitter). The datasheet's package drawing will clearly indicate pin 1, often with a notch, dot, or beveled edge on the case. The electrical characteristics table confirms the anode is positive for the LED, and the collector is positive for the NPN phototransistor when used in a common-emitter configuration.

5. Soldering & Assembly Guidelines

The Absolute Maximum Ratings provide the key guideline for soldering: the lead soldering temperature must not exceed 260°C for a duration of 5 seconds, measured at a point 1.6mm (0.063") away from the plastic case. This is a standard precaution to prevent the internal epoxy or the semiconductor dies from being damaged by excessive heat during wave soldering or hand-soldering processes.

Recommendations:

• Use a temperature-controlled soldering iron.
• Minimize contact time between the iron and the lead.
• For wave soldering, ensure the profile (preheat, soak, peak temperature, time above liquidus) is controlled to meet this requirement.
• Avoid applying mechanical stress to the leads during or after soldering.

Storage Conditions: The device should be stored within the specified storage temperature range of -40°C to +100°C, preferably in a dry, anti-static environment to prevent moisture absorption (which can cause "popcorning" during reflow) and electrostatic discharge damage.

6. Application Suggestions

6.1 Typical Application Circuits

The most common configuration is a common-emitter switch. The IR LED is driven through a current-limiting resistor (R_limit) connected to a voltage source. The value is calculated as R_limit = (V_CC - V_F) / I_F. The phototransistor's collector is connected to a pull-up resistor (R_pull-up) and the supply voltage, while the emitter is grounded. The output signal is taken from the collector node. When the beam is uninterrupted, the transistor turns on, pulling the output voltage low (near V_CE(SAT)). When the beam is blocked, the transistor turns off, and the pull-up resistor pulls the output voltage high (to V_CC).

6.2 Design Considerations

• Current Setting: Choose I_F based on required sensitivity and power consumption. Higher I_F gives higher I_C(ON) but increases power dissipation.
• Output Load Resistor (R_pull-up): Its value affects switching speed and output current capability. A smaller resistor provides faster rise times (shorter RC time constant) and higher sink current but consumes more power when the transistor is on.
• Ambient Light Immunity: Since it uses modulated infrared light, it has good immunity to most ambient visible light. However, strong sources of infrared light (e.g., sunlight, incandescent bulbs) can cause false triggering. Using a modulated LED drive signal and a synchronized detector circuit can greatly enhance noise immunity.
• Object Characteristics: The sensor detects any object opaque to the infrared wavelength. The object's size, speed, and material will affect the signal's integrity.
• Alignment: Precise mechanical alignment of the interrupting object with the sensor slot is necessary for reliable operation.

7. Technical Comparison & Differentiation

Compared to mechanical micro-switches, the LTH-301-23 offers superior life expectancy (millions vs. thousands of cycles), faster response, and silent operation. Compared to reflective optical sensors, transmissive photointerrupters like this one are generally more reliable and less sensitive to variations in the color or reflectivity of the target object, as they rely on beam interruption rather than reflection. Its key differentiators within the photointerrupter category are its specific combination of package size, slot width, electrical sensitivity (I_C(ON)), and fast switching speed, making it suitable for space-constrained, high-speed applications.

8. Frequently Asked Questions (FAQ)

Q1: What is the typical operating current for the IR LED?

A1: The datasheet uses I_F = 20 mA for most test conditions, which is a common and reliable operating point. It can be driven lower to save power or briefly higher (within absolute limits) for increased signal strength.

Q2: How do I interface the output with a microcontroller?

A2: The digital output (low when beam present, high when blocked) can be connected directly to a microcontroller's digital input pin. Ensure the output voltage levels (V_CC for high, V_CE(SAT) for low) are compatible with the MCU's logic levels. A pull-up resistor is typically required.

Q3: Can it detect transparent objects?

A3: Standard photointerrupters using infrared light may not reliably detect objects that are transparent to infrared wavelengths (e.g., some plastics). For such applications, a sensor with a different wavelength or a different sensing principle may be needed.

Q4: What is the significance of the rise and fall times?

A4: These times limit the maximum switching frequency. The maximum theoretical frequency is approximately 1/(t_r + t_f). With typical times of 3μs and 4μs, the device can handle frequencies well into the tens of kHz, suitable for high-speed counting or encoder applications.

9. Operational Principle

A photointerrupter is a transmissive optoelectronic device. It consists of an infrared light source (an LED) and a light detector (a phototransistor) facing each other inside a housing with a precise gap between them. When an electric current flows through 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, which effectively acts as a base current, turning the transistor on and allowing a collector current to flow. When an opaque object enters the gap, it blocks the light path. The photogenerated base current ceases, turning the transistor off, and the collector current drops to a very low value (the dark current). This on/off change in output current is used as a switching signal.

10. Industry Trends

The trend in optoelectronic sensing is towards miniaturization, higher integration, and improved performance. Surface-mount device (SMD) versions are becoming increasingly popular for automated assembly and space savings. There is also a move towards devices with built-in signal conditioning, such as Schmitt triggers for clean digital outputs, or analog amplifiers for distance/proximity sensing. Furthermore, increasing emphasis is placed on achieving higher immunity to electromagnetic interference (EMI) and ambient light, as well as extending the operating temperature range for automotive and industrial applications. While fundamental devices like the LTH-301-23 remain widely used for their simplicity and cost-effectiveness, newer designs often incorporate these advanced features for more demanding applications.