LTH-301-27P1 Photointerrupter Datasheet - Non-Contact Switching - English Technical Document

1. Product Overview

The LTH-301-27P1 is a reflective photointerrupter, a type of optoelectronic sensor. Its core function is to detect the presence or absence of an object without physical contact. It achieves this by combining an infrared light-emitting diode (IR LED) and a phototransistor within a single, compact housing. When an object enters the gap between the emitter and detector, it interrupts the infrared light beam, causing a change in the phototransistor's output state. This makes it ideal for applications requiring reliable, non-mechanical sensing such as position detection, limit switching, and object counting.

The device is designed for direct mounting onto printed circuit boards (PCBs) or into standard dual-in-line sockets, facilitating easy integration into electronic assemblies. Its primary advantages include immunity to contact bounce, long operational life due to the absence of moving parts, and fast switching speeds suitable for high-speed counting or timing applications.

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): 50 mA. This is the maximum steady-state current that can be applied to the infrared LED.
• IR Diode Reverse Voltage (V_R): 5 V. Exceeding this reverse bias voltage across the LED can cause breakdown.
• Phototransistor Collector Current (I_C): 40 mA. The maximum current the phototransistor's collector can handle.
• Phototransistor Collector-Emitter Voltage (V_CEO): 30 V. The maximum voltage that can be applied between the collector and emitter of the phototransistor.
• Operating Temperature Range: -35°C to +65°C. 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 wave or reflow soldering processes to prevent thermal damage.

Note on Power Derating: Both the transistor power dissipation (100 mW) and diode power dissipation (75 mW) must be derated linearly at a rate of 1.33 mW/°C for ambient temperatures above 25°C. This means the allowable power decreases as temperature increases to prevent overheating.

2.2 Electrical & Optical Characteristics

These parameters are measured at 25°C and define the device's typical performance under specified test conditions.

2.2.1 Input IR LED Characteristics

• Forward Voltage (V_F): Typically 1.6V (max 1.6V) at a forward current (I_F) of 20 mA. This is used to calculate the current-limiting resistor value: R = (V_supply - V_F) / I_F.
• 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.

2.2.2 Output Phototransistor Characteristics

• Collector-Emitter Breakdown Voltage (V_(BR)CEO): Minimum 30V. This is the voltage at which the phototransistor 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 when the phototransistor is in the "off" state (no light incident). A low value is desirable for good signal-to-noise ratio.

2.2.3 Coupler (System) Characteristics

These parameters describe the performance of the combined LED-phototransistor pair.

• Collector-Emitter Saturation Voltage (V_CE(SAT)): Maximum 0.4V when the phototransistor is driven into saturation (I_C=0.25mA, I_F=20mA). A low saturation voltage is key for interfacing with logic circuits.
• On-State Collector Current (I_C(ON)): Minimum 1.5 mA when the phototransistor is illuminated (V_CE=5V, I_F=20mA). This is the photocurrent generated and defines the sensor's sensitivity. The actual current can be higher depending on the reflectivity of the interrupting object and the alignment.

3. Mechanical & Packaging Information

The LTH-301-27P1 is housed in a standard 4-pin dual-in-line package. The exact dimensions are provided in the package drawing within the datasheet. Key mechanical notes include:

• All dimensions are in millimeters, with a standard tolerance of ±0.25mm unless otherwise specified.
• The package features a slot or gap between the IR emitter and photodetector. The object to be detected passes through this gap.
• Polarity is clearly marked. The IR LED anode and cathode pins are identified, as are the phototransistor's collector and emitter pins. Correct orientation during PCB mounting is essential.
• The device is suitable for both PCB mounting and socket mounting, offering flexibility in assembly and potential for field replacement.

4. Soldering & Assembly Guidelines

Proper handling is crucial for reliability.

• Soldering: The leads can withstand a temperature of 260°C for a maximum of 5 seconds, measured 1.6mm from the plastic case body. This guideline is critical for wave soldering processes. For reflow soldering, a standard profile with a peak temperature below 260°C is recommended.
• Cleaning: Use mild cleaning agents compatible with the plastic housing. Avoid ultrasonic cleaning with excessive power, as it may damage internal components.
• Storage: Store in an environment within the specified storage temperature range of -40°C to +100°C, preferably in low-humidity conditions to prevent moisture absorption.
• ESD Precautions: Although not explicitly stated as sensitive, standard ESD (Electrostatic Discharge) handling procedures for semiconductor devices should be followed during assembly.

5. Application Suggestions

5.1 Typical Application Circuits

The most common configuration is to connect the IR LED in series with a current-limiting resistor to a voltage source (e.g., 5V). The phototransistor is typically connected in a common-emitter configuration: the collector is pulled up to a supply voltage (e.g., 5V) via a load resistor (R_L), and the emitter is connected to ground. The output signal is taken from the collector node.

• When the beam is unblocked, light falls on the phototransistor, causing it to conduct and pull the collector voltage low (near V_CE(SAT)).
• When an object blocks the beam, the phototransistor turns off, and the collector voltage is pulled high by the load resistor.
• The value of the load resistor (R_L) determines the switching speed and current consumption. A smaller R_L allows faster switching but draws more current when the transistor is on.

5.2 Design Considerations

• Alignment: Precise mechanical alignment of the object path with the sensor gap is critical for reliable operation.
• Ambient Light: As the sensor uses infrared light, it can be susceptible to interference from strong ambient IR sources (e.g., sunlight, incandescent bulbs). Using a modulated IR signal and a synchronized detector circuit can greatly improve immunity.
• Object Characteristics: The sensor's effectiveness depends on the object's ability to reflect or absorb the IR beam. Dark, non-reflective objects may not be detected as reliably as light-colored ones. Testing with the actual target material is recommended.
• Debouncing: While the sensor itself does not have contact bounce, the electrical output may still have noise. Software or hardware debouncing (e.g., a simple RC filter or a Schmitt trigger input) may be necessary for clean digital signals.

6. Operating Principle

The photointerrupter operates on the principle of optical beam interruption. Internally, an infrared LED emits light at a wavelength typically around 940nm, which is invisible to the human eye. Directly opposite, a silicon phototransistor is positioned to receive this light. The phototransistor acts as a light-controlled switch. When photons from the IR LED strike its base region, they generate electron-hole pairs, which in turn allow a much larger collector current to flow—this is the photoelectric effect. The magnitude of this collector current is proportional to the intensity of the incident light. When an opaque object enters the gap between the LED and the phototransistor, the light path is blocked. The intensity of light on the phototransistor drops dramatically, causing its collector current to fall to a very low value (essentially the dark current). This sharp change in current (or the corresponding voltage change across a load resistor) is detected by external circuitry and interpreted as a switching event.

7. Performance Curves & Analysis

The datasheet includes typical characteristic curves which provide valuable insights beyond the tabulated min/typ/max values.

• Transfer Characteristics (I_C vs. I_F): This curve shows how the phototransistor's output current (I_C) varies with the LED's input current (I_F) at a fixed collector-emitter voltage. It demonstrates the linear relationship between input drive and output response under specific conditions, helping to optimize the LED drive current for desired sensitivity.
• Output Characteristics (I_C vs. V_CE): These curves, plotted for different levels of incident light (or different I_F), show how the phototransistor behaves like a current source. The collector current remains relatively constant over a range of V_CE until it reaches saturation.
• Temperature Dependence: Curves showing the variation of parameters like forward voltage (V_F) or collector dark current (I_CEO) with temperature are crucial for designing systems that operate over the full specified temperature range. For example, V_F typically decreases with increasing temperature, which could slightly affect the LED's light output if driven by a constant voltage source.

8. Common Questions & Answers

Q: What is the typical response time of this sensor?
A: While not explicitly stated in the provided data, photointerrupters like this typically have response times in the microsecond range, making them suitable for high-speed counting. The actual speed is limited by the phototransistor's rise/fall time and the external circuit's RC time constant.

Q: Can I use this sensor outdoors?
A: With caution. Direct sunlight contains strong infrared components that can saturate the phototransistor, causing false triggering. A physical shield or housing to block ambient light, along with optical filtering or signal modulation techniques, is necessary for reliable outdoor use.

Q: How do I choose the value for the LED current-limiting resistor?
A: Use the formula: R = (V_CC - V_F) / I_F. For example, with a 5V supply (V_CC), a typical V_F of 1.6V, and a desired I_F of 20 mA: R = (5 - 1.6) / 0.02 = 170 Ω. A standard 180 Ω resistor would be appropriate, resulting in I_F ≈ 18.9 mA.

Q: What is the purpose of the Emitter-Collector Breakdown Voltage (V_(BR)ECO) rating?
A> This rating (5V) is relevant if the phototransistor is connected in an inverted configuration (emitter at a higher potential than collector), which is uncommon. It ensures the device can withstand a small reverse voltage across the C-E junction without damage.

9. Practical Use Case Example

Application: Paper Detection in a Printer
The LTH-301-27P1 can be used to detect the leading edge of paper in a printer or photocopier. The sensor is mounted so the paper passes through its gap. A reflective flag or the paper itself interrupts the beam. When the beam is unblocked (no paper), the phototransistor is on, outputting a low voltage. When paper enters the gap, the beam is blocked, the phototransistor turns off, and the output voltage goes high. This rising edge signal can be fed to a microcontroller to initiate a print sequence, confirm paper presence, or count pages. The non-contact nature ensures no wear on the paper or sensor, and the fast response allows detection even at high paper feed speeds. Design considerations would include ensuring the paper path is accurately aligned with the sensor gap and selecting a load resistor that provides a clean, fast voltage swing for the microcontroller's input pin.

10. Technology Trends

Photointerrupters remain a fundamental sensing technology due to their simplicity, reliability, and low cost. Current trends focus on miniaturization, leading to surface-mount device (SMD) packages that save board space in modern electronics. There is also integration of additional circuitry, such as built-in Schmitt triggers for hysteresis and clean digital output, or even fully integrated solutions with a modulated IR driver and a synchronized detector IC on a single chip for superior ambient light rejection. Furthermore, advancements in materials and packaging are extending the operating temperature ranges and improving long-term reliability for automotive and industrial applications. While newer technologies like time-of-flight (ToF) sensors offer distance measurement, the basic photointerrupter's role for simple, binary presence detection in cost-sensitive applications remains firmly established.