LTH-301-19 Photointerrupter Data Sheet - Non-Contact Switching - English Technical Document

1. Product Overview

The LTH-301-19 is a compact, non-contact switching device designed for applications requiring reliable object detection or position sensing. It operates on the principle of an infrared light-emitting diode (IR LED) paired with a phototransistor. When an object interrupts the infrared beam between the emitter and detector, the phototransistor's output state changes, providing a switching signal. This device is suitable for direct PCB mounting or use with dual-in-line sockets, offering a fast and reliable solution for various industrial and consumer electronics applications.

2. Technical Parameters Deep Objective Interpretation

2.1 Absolute Maximum Ratings

These ratings define the limits beyond which permanent damage to the device may occur. The IR diode can handle a continuous forward current of 60 mA and a reverse voltage of 5 V. The phototransistor's collector current is limited to 20 mA with a power dissipation of 100 mW. For the IR diode, a peak forward current of 1 A is permissible under pulsed conditions (10 μs pulse width, 300 pps). The device is rated for an operating temperature range of -25°C to +85°C and a storage range of -40°C to +100°C. Lead soldering temperature must not exceed 260°C for 5 seconds when measured 1.6mm from the body.

2.2 Electrical & Optical Characteristics

This section details the device's performance under typical operating conditions at 25°C ambient temperature.

2.2.1 Input LED Characteristics

The forward voltage (VF) of the IR LED is typically 1.6V at a forward current (IF) of 20mA, with a maximum of 1.6V. The reverse current (IR) is a maximum of 100 μA at a reverse voltage (VR) of 5V.

2.2.2 Output Phototransistor Characteristics

The collector-emitter breakdown voltage (V(BR)CEO) is 30V minimum. The emitter-collector breakdown voltage (V(BR)ECO) is 5V minimum. The collector-emitter dark current (ICEO) is a maximum of 100 nA at VCE=10V, indicating the leakage current when the LED is off.

2.2.3 Coupler Characteristics

The collector-emitter saturation voltage (VCE(SAT)) is a maximum of 0.4V when the phototransistor is driven into saturation (IC=70μA, IF=1.4mA). The on-state collector current (IC(ON)) is typically 70 μA at VCE=3.3V and IF=1.4mA, and can reach 10 mA at VCE=5V and IF=20mA, showing the device's sensitivity and output capability under different drive conditions.

2.2.4 Response Time

The switching speed is characterized by rise time (tr) and fall time (tf). The typical rise time is 3 μs (max 15 μs), and the typical fall time is 4 μs (max 20 μs), measured under specific test conditions (VCE=5V, Ic=2mA, RL=100Ω). This defines the device's capability for high-speed detection.

3. Mechanical & Packaging Information

3.1 Package Dimensions

The device features a standard through-hole package. All dimensions are specified in millimeters with a default tolerance of ±0.25mm unless otherwise noted. The exact dimensional drawing is provided in the datasheet, detailing the body size, lead spacing, and overall footprint for PCB layout.

3.2 Polarity Identification

Correct orientation is critical. The datasheet includes a diagram clearly marking the anode and cathode of the IR LED and the collector and emitter of the phototransistor. Mounting the device incorrectly can lead to malfunction or damage.

4. Performance Curve Analysis

The datasheet includes typical electrical and optical characteristic curves, plotted at 25°C ambient temperature unless otherwise specified. These graphs are essential for understanding device behavior beyond the tabulated minimum, typical, and maximum values.

4.1 Transfer Characteristics

Curves likely show the relationship between the input LED forward current (IF) and the output phototransistor collector current (IC) at various collector-emitter voltages (VCE). This illustrates the current transfer ratio (CTR), a key parameter for gain.

4.2 Output Saturation Characteristics

Graphs depicting VCE(SAT) versus IC for different IF levels help designers understand the output voltage levels when the phototransistor is fully on, which is important for interfacing with logic circuits.

4.3 Temperature Dependence

While the primary data is at 25°C, characteristic curves may show how parameters like dark current (ICEO) and output current vary with temperature, which is crucial for designing stable systems over the specified operating range.

5. Soldering & Assembly Guidelines

5.1 Soldering Parameters

The absolute maximum rating specifies that leads can be soldered at 260°C for a maximum of 5 seconds, with the temperature measured 1.6mm (0.063") from the plastic housing. This is critical to prevent thermal damage to the internal components and the plastic package.

5.2 Handling & Storage

The device should be stored within the specified temperature range of -40°C to +100°C. Standard ESD (electrostatic discharge) precautions should be observed during handling and assembly to prevent damage to the semiconductor junctions.

6. Application Suggestions

6.1 Typical Application Scenarios

The LTH-301-19 is ideal for non-contact sensing in printers (paper jam detection, toner level), copiers, vending machines (coin/object detection), industrial automation (position sensing, limit switches), and consumer electronics. Its fast switching speed makes it suitable for counting or speed measurement applications.

6.2 Design Considerations

Current Limiting Resistor: An external resistor must be used in series with the IR LED to limit its forward current (IF) to a safe value, typically between the test condition of 1.4mA and the absolute maximum of 60mA, balancing brightness and longevity.
Load Resistor: The value of the load resistor (RL) connected to the phototransistor collector affects both the output voltage swing and the response time. A smaller RL provides faster switching but a smaller output voltage swing.
Ambient Light: As an infrared device, it is less susceptible to visible ambient light interference. However, for critical applications, mechanical shielding or modulation/demodulation techniques can be employed to enhance noise immunity.
Alignment: Precise mechanical alignment between the emitter and detector slots is necessary for optimal performance and maximum sensing distance.

7. Technical Comparison & Differentiation

Compared to mechanical switches, the LTH-301-19 offers the key advantage of non-contact operation, resulting in no wear, longer lifespan, silent operation, and higher potential switching speeds. Compared to other optical sensors, its integrated slotted package provides a built-in optical path, simplifying mechanical design and improving alignment reliability versus separate emitter and detector components. The specified saturation voltage (VCE(SAT) < 0.4V) ensures good compatibility with low-voltage logic circuits.

8. Frequently Asked Questions (Based on Technical Parameters)

Q: What is the purpose of the dark current (ICEO) parameter?
A: Dark current is the small leakage current that flows through the phototransistor when no light from the IR LED is incident (i.e., the beam is blocked or the LED is off). A low dark current (max 100 nA) is desirable as it minimizes the "off-state" current, leading to a clearer distinction between the on and off states of the switch.

Q: How do I choose the value for the LED current-limiting resistor?
A: Use Ohm's Law: R = (Vcc - VF) / IF. Vcc is your supply voltage, VF is the LED forward voltage (use 1.6V for design margin), and IF is your desired operating current (e.g., 20mA for full output). Ensure the calculated power dissipation in the resistor is within its rating.

Q: Can this sensor be used outdoors?
A: The operating temperature range is -25°C to +85°C, which covers many environments. However, direct sunlight contains strong infrared radiation which could saturate the sensor. Environmental sealing against dust and moisture is not part of the package specification and would need to be considered separately.

Q: What affects the sensing distance or gap?
A> The sensing gap is influenced by the LED drive current (IF), the sensitivity of the phototransistor, the alignment, and the opacity of the object interrupting the beam. The datasheet does not specify a maximum gap; it must be determined empirically for a specific object and required signal margin.

9. Practical Use Case

Case: Paper Detection in a Desktop Printer. The LTH-301-19 can be mounted so that the paper path runs through its slot. A microcontroller GPIO pin, configured with a pull-up resistor, monitors the phototransistor's collector. When no paper is present, the IR beam reaches the detector, turning the phototransistor on and pulling the collector voltage low (near VCE(SAT)). When paper enters the slot, it blocks the beam, turning the phototransistor off, allowing the pull-up resistor to pull the collector voltage high to Vcc. The microcontroller detects this voltage transition to confirm paper presence or trigger a paper-out alert. The fast response time ensures detection even for rapidly moving paper.

10. Operating Principle Introduction

The LTH-301-19 is a transmission-type optical sensor housed in a U-shaped plastic package. On one side, an infrared light-emitting diode (IR LED) emits light at a wavelength typically around 940nm. Directly opposite, on the other side of the slot, a silicon NPN phototransistor acts as the receiver. The phototransistor is designed so that incident light on its base region generates electron-hole pairs, which act as base current, thereby controlling a much larger collector-emitter current. When an object is not present in the slot, light from the IR LED strikes the phototransistor, causing it to conduct (ON state). When an object enters the slot, it obstructs the light path, drastically reducing the light on the phototransistor, causing it to stop conducting (OFF state). This change in output current/voltage is used as a switching signal.

11. Technology Trends

Photointerrupters like the LTH-301-19 represent a mature and reliable technology. Current trends in the field include miniaturization of the package for higher density PCB mounting, the development of surface-mount device (SMD) versions to facilitate automated assembly, and the integration of additional circuitry such as Schmitt triggers or amplifiers within the package to provide a clean digital output signal and improve noise immunity. There is also a focus on reducing power consumption, especially for battery-powered applications, by optimizing the LED efficiency and phototransistor sensitivity. Furthermore, some advanced variants incorporate multiple emitters or detectors in a single package for encoded position sensing or to provide redundancy.