Photointerrupter LTH-872-N55T1 Datasheet - English Technical Document

1. Product Overview

The LTH-872-N55T1 is a reflective photointerrupter, a type of optoelectronic component that combines an infrared light-emitting diode (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 by sensing the interruption of the light beam reflected from the object back to the sensor. This device is engineered for applications requiring reliable, fast, and non-invasive object detection or position sensing.

1.1 Core Advantages

The key advantages of this photointerrupter stem from its fundamental operating principle and design. Non-contact switching eliminates mechanical wear and tear, significantly enhancing the operational lifespan and reliability compared to mechanical switches. This is crucial in high-cycle applications. Furthermore, it offers fast switching speed, with typical rise and fall times in the microsecond range, enabling it to detect rapidly moving objects or high-frequency events. The integrated package ensures precise alignment between the emitter and detector, simplifying assembly and improving consistency.

1.2 Target Market and Application

The primary target markets for this component are office automation and precision instrumentation. Its main documented application is within scanners and printers. In these devices, photointerrupters are commonly used for functions such as paper presence detection (e.g., sensing the leading edge of a sheet), paper jam detection, carriage or print head position sensing, and detecting the home position of moving mechanisms. The fast response time is essential for maintaining the high throughput of modern scanning and printing equipment.

2. In-Depth Technical Parameter Analysis

Understanding the electrical and optical characteristics is critical for proper circuit design and ensuring reliable operation within the device's specified limits.

2.1 Absolute Maximum Ratings

These ratings define the stress limits beyond which permanent damage to the device may occur. They are not intended for normal operation.

• Input LED:
  • Power Dissipation (P_D): 75 mW maximum.
  • Continuous Forward Current (I_F): 50 mA maximum. This is the absolute maximum current that can flow through the LED.
  • Reverse Voltage (V_R): 5 V maximum. Exceeding this can break down the LED junction.
• Output Phototransistor:
  • Power Dissipation (P_C): 100 mW maximum.
  • Collector-Emitter Voltage (V_CEO): 30 V maximum. This is the maximum voltage that can be applied across the phototransistor's collector and emitter when the base is open (dark condition).
  • Emitter-Collector Voltage (V_ECO): 5 V maximum (reverse voltage rating).
  • Collector Current (I_C): 20 mA maximum.
• Environmental:
  • Operating Temperature (T_opr): -25°C to +85°C.
  • Storage Temperature (T_stg): -55°C to +100°C.
  • Lead Soldering Temperature (T_sol): 260°C for 5 seconds maximum (for leads 1.6mm from the case).

2.2 Electrical & Optical Characteristics

These parameters are specified at an ambient temperature (T_A) of 25°C and define the device's performance under normal operating conditions.

• Input 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 illuminated.
  • Reverse Current (I_R): Maximum 100 µA at a reverse voltage (V_R) of 5V. This is the small leakage current when the LED is reverse-biased.
• Output Phototransistor Characteristics:
  • Collector-Emitter Dark Current (I_CEO): Maximum 100 nA at V_CE=10V. This is the leakage current when the phototransistor is in complete darkness (no light from the LED). A low value is desirable for good signal-to-noise ratio.
  • Collector-Emitter Saturation Voltage (V_CE(SAT)): Maximum 0.4V at I_C=0.25mA and I_F=20mA. This is the voltage across the transistor when it is fully "on" (saturated). A low saturation voltage minimizes power loss in the switching element.
  • 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 LED is driven and an object is not interrupting the beam (reflective mode assumed).
• Coupler (System) Response Time:
  • Rise Time (T_R): 3 µs (typical) to 15 µs (maximum). This is the time for the phototransistor's output to rise from 10% to 90% of its final value when the LED is turned on.
  • Fall Time (T_F): 4 µs (typical) to 20 µs (maximum). This is the time for the output to fall from 90% to 10% when the LED is turned off. These fast times are critical for the stated "fast switching speed" feature.
  • Test Conditions: V_CE=5V, I_C=2mA, R_L=100 Ω.

3. Performance Curve Analysis

The datasheet references typical electrical/optical characteristic curves. While the specific graphs are not provided in the text, their purpose is to illustrate the relationship between key parameters under varying conditions, which is essential for robust design.

3.1 Inferred Curve Information

Based on standard practice for such components, the typical curves would likely include:

• Forward Current vs. Forward Voltage (I_F-V_F): This curve shows the non-linear relationship between the current through the LED and the voltage across it. It helps determine the required series resistor value to achieve a desired drive current from a given supply voltage.
• Collector Current vs. Collector-Emitter Voltage (I_C-V_CE): For the phototransistor, this family of curves would be plotted for different levels of incident light (or different LED drive currents, I_F). It defines the transistor's operating regions (cutoff, active, saturation) under illuminated conditions.
• Current Transfer Ratio (CTR) vs. Forward Current: CTR is the ratio of the phototransistor's output collector current (I_C) to the LED's input forward current (I_F), typically expressed as a percentage. This curve shows how efficiency changes with drive current and is crucial for designing the interface circuit to ensure sufficient output signal swing.
• Temperature Dependence: Curves showing how parameters like forward voltage (V_F), dark current (I_CEO), and CTR vary with ambient temperature. This is vital for ensuring stable operation across the full specified temperature range (-25°C to +85°C).

4. Mechanical & Package Information

The package dimensions are referenced but not detailed in the provided text. The notes specify that all dimensions are in millimeters (with inches in parentheses) and the general tolerance is ±0.25mm unless otherwise stated. The part number LTH-872-N55T1 suggests a specific package style common to reflective photointerrupters, which typically features a molded plastic body with a slot. The emitter and detector face the same direction across this slot, allowing them to detect an object that reflects the emitted light back.

4.1 Polarity Identification and Pinout

While the exact pinout is not listed, standard photointerrupter packages have 4 pins: two for the anode and cathode of the infrared LED, and two for the collector and emitter of the NPN phototransistor. The datasheet would typically include a diagram showing the top view and pin numbering (e.g., 1: Anode, 2: Cathode, 3: Collector, 4: Emitter). Correct polarity connection for the LED is mandatory to prevent damage.

5. Soldering & Assembly Guidelines

The datasheet provides a critical parameter for assembly: the maximum lead soldering temperature. For leads positioned 1.6mm (0.063 inches) from the plastic case, the temperature must not exceed 260°C for 5 seconds. This is a standard rating for wave or hand soldering. For reflow soldering, the component must be compatible with the specific reflow profile used, which typically has a peak temperature around 240-250°C. Exceeding these thermal limits can cause internal damage to the semiconductor junctions or deform the plastic package, affecting optical alignment and performance.

6. Application Suggestions & Design Considerations

6.1 Typical Application Circuit

A basic interface circuit involves two main parts:

1. LED Driver: A current-limiting resistor is connected in series with the LED. The resistor value (R_series) is calculated as: R_series = (V_CC - V_F) / I_F. Using the typical V_F of 1.4V and a desired I_F of 20mA with a 5V supply gives R_series = (5 - 1.4) / 0.02 = 180 Ω. A standard 180Ω or 220Ω resistor would be suitable. Driving the LED with a constant current, rather than a constant voltage, provides more stable light output.
2. Phototransistor Output: The phototransistor is typically used in a common-emitter configuration. A load resistor (R_L) is connected between the collector and the positive supply (V_CC). The emitter is connected to ground. When light falls on the transistor, it turns on, pulling the collector voltage low (towards V_CE(SAT)). When dark, the transistor is off, and the collector voltage is pulled high to V_CC by R_L. The value of R_L determines the output voltage swing and speed; a smaller R_L gives faster response but a smaller swing. The datasheet tests with R_L=100Ω.

6.2 Design Considerations

• Ambient Light Immunity: As a reflective sensor, it can be susceptible to ambient light (especially sunlight or bright indoor lighting containing infrared). Using a modulated LED drive signal and synchronous detection in the receiver circuit can greatly improve immunity to such interference.
• Object Reflectivity: The effective sensing distance and signal strength depend heavily on the reflectivity of the target object. Highly reflective surfaces (like white paper) work best, while dark or matte surfaces may not reflect enough light.
• Alignment and Gap: The optimal sensing distance (gap between the sensor and the reflective object) is usually specified in the full datasheet. Mechanical design must ensure this gap is maintained consistently.
• Electrical Noise: For long cable runs or noisy environments, proper shielding and filtering of the output signal may be necessary, as the phototransistor output is a high-impedance node when off and can be sensitive to pickup.

7. Technical Comparison & Differentiation

Compared to other sensing technologies, this photointerrupter offers specific advantages:

• vs. Mechanical Switches: No contact bounce, much longer lifespan (millions vs. thousands of cycles), faster response, and silent operation.
• vs. Transmissive Photointerrupters (Slotted Optocouplers): Reflective types like the LTH-872-N55T1 do not require an object to pass through a slot; they can sense objects at a distance. This simplifies mechanical design for applications like paper detection where the paper runs along a surface.
• vs. Modern Sensors (e.g., Hall Effect, Ultrasonic): Photointerrupters are generally simpler and lower cost for basic presence/absence detection. They do not require magnets (like Hall sensors) and are less complex than ultrasonic sensors, though they may be less effective on non-reflective targets.

8. Frequently Asked Questions (Based on Technical Parameters)

Q: What is the purpose of the dark current (I_CEO) specification?
A: Dark current is the small leakage current that flows through the phototransistor when it is completely dark (no light from the LED and no ambient light). In the "off" state, this current flowing through the load resistor (R_L) creates a small voltage drop. A high dark current could result in an output voltage that is not fully at the "high" logic level, potentially causing misinterpretation by the following circuitry. The specified max of 100 nA is very low, ensuring a clean off-state signal.

Q: How do I choose the right LED drive current (I_F)?
A: The drive current affects light output, which directly affects the output current of the phototransistor (I_C(ON)) and the device's sensitivity. Operating at the typical test condition of 20mA is a good starting point. You can reduce current to save power if the application has high reflectivity and short distance. Increasing current may improve signal strength for difficult targets but will increase power dissipation and must stay below the 50mA absolute maximum. Refer to the typical CTR vs. I_F curve for guidance.

Q: Can I use this sensor outdoors?
A: The operating temperature range (-25°C to +85°C) allows for use in many environments. However, direct sunlight contains strong infrared radiation that can saturate the phototransistor, causing constant "on" detection. For outdoor use, optical filtering (an IR-pass filter that blocks visible light but passes the LED's wavelength) and/or signal modulation techniques are strongly recommended to reject ambient IR light.

9. Operating Principle

The LTH-872-N55T1 operates on the principle of internal reflection modulation. An infrared LED emits light. In the absence of a reflective target within the sensing field, most of this light dissipates. When a suitably reflective object enters the field, a portion of the emitted light is reflected back towards the device. The integrated phototransistor, which is sensitive to the same infrared wavelength, detects this reflected light. The incident photons generate electron-hole pairs in the phototransistor's base region, effectively providing base current. This causes the transistor to turn on, allowing a collector current (I_C) to flow that is proportional to the intensity of the reflected light. This change in output current/voltage is then used by external circuitry to signal the object's presence.

10. Industry Trends

While fundamental photointerrupter technology is mature, trends focus on miniaturization, integration, and enhanced functionality. Newer devices may feature:
- Surface-Mount (SMD) Packages: Smaller footprints for high-density PCB assembly.
- Integrated ICs: Some modern photointerrupters include amplification, Schmitt triggers for hysteresis, and even digital output (e.g., I2C) on-chip, simplifying interface design.
- Higher Speed: Development continues for even faster response times to keep pace with increasing machine speeds.
- Improved Ambient Light Rejection: Advanced optical designs and modulation schemes are being employed to make sensors more robust in challenging lighting environments. The core reflective sensing principle, as embodied in components like the LTH-872-N55T1, remains a reliable and cost-effective solution for a wide range of non-contact detection tasks.