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Independent Research

Infrared Detectors

Marcin Ochman Dr. P. Reddy 0306.654. 08.05.

TABLE OF CONTENTS

• INTRODUCTION..............................................................................................................
• INFRARED TECHNOLOGY.......................................................................................... - THERMAL IR DETECTORS................................................................................................ - QUANTUM IR DETECTORS................................................................................................
• COMPONENT DETAIL................................................................................................... - PIN DIODE........................................................................................................................ - PHOTO IC.........................................................................................................................
• COMPONENT APPLICATION...................................................................................... - INTERFACE........................................................................................................................ - MOUNT............................................................................................................................. - OPERATIONAL LIMITATIONS............................................................................................ - TESTING............................................................................................................................ - COSTS.............................................................................................................................
• ALTERNATE COMPONENTS.....................................................................................
• Figure 1 – Infrared Spectrum............................................................................................... TABLE OF FIGURES
• Figure 2 – Thermocouple.....................................................................................................
• Figure 3 – Bolometer...........................................................................................................
• Figure 4 – PIN Diode...........................................................................................................
• Figure 5 – Example Intrinsic Materials...............................................................................
• Figure 6 – PIN Diode Housing............................................................................................
• Figure 7 – PIN Diode Spectral Response Figure 8 – Example Reception Unit..................
• Figure 7 – PIN Diode Spectral Response Figure 8 – Example Reception Unit..................
• Figure 9 – PNA4612M Block Diagram...............................................................................
• Figure 10 – PNA4612M Frequency Response Characteristics............................................
• Figure 11 – PNA4612M Operational Characteristics..........................................................
• Figure 12 – PNA4612M Reception Characteristics.............................................................
• Figure 13 – IR Detector Interface........................................................................................
• Figure 14 – IR Detector Configuration................................................................................
  • Figure 15 – Component Costs...........................................................................................

Figure 1 – Infrared Spectrum Thermal IR Detectors Thermal detectors as infrared detectors use temperature variation as an indirect way of measuring levels on infrared radiation.  Thermocouple - Thermocouples utilize the thermoelectric properties of conductors. The device consists of two different types of metals that produce a varying voltage as temperature changes at the measure junction. Note that the voltage-temperature relationship is non-linear and the device measures the difference of temperature between the measure junction and the reference junction. In an infrared detection application the measure junction would have a large heat sink attached to amplify the heating effect of incident infrared radiation. Figure 2 – Thermocouple  Bolometer - A bolometer is a device that produces a varying resistance with a varying level of radiation. Its applications mostly include detection of the far infrared range like those in astronomy. In Figure 3 a simple diagram of a bolometer is depicted. A photon detector(C), which responds to a band of wavelengths of interest (infrared in this case), is thermally attached to a link labeled G. The change in temperature in C will case the resistance of G to change, which then can be measured. Figure 3 – Bolometer  Pyroelectric detector - Much like the devices mentioned above, a pyroelectric detector is constructed of materials that respond to a specific type of radiation. However the way the material’s response is measured is what differs. Here, the crystals of the sensitive experience polarization as a response to the temperate increase caused by the incident radiation. A charge builds up in a layer of this material and a voltage potential can be measured and interpreted as a response to the amount of infrared radiation captured.

Quantum IR Detectors These are built small-scale with semi-conductors and they feature great response and ease of detection. Unlike thermal IR detectors, these don’t use temperature response as an indirect way of measuring incident radiation. A simple intrinsic type PIN diode is shown in Figure 4. It is constructed of p and n type semiconductors with an intrinsic and undoped material in the middle. Besides the intrinsic material, it will act just like a regular PN junction diode. A detector of this type changes its conductivity with the amount of radiation present. The intrinsic material between the PN junction is what causes this change. The selection of material is what determines the wavelength of radiation that will cause the diode to become most conductive. See Figure 5 for a list of intrinsic materials and the wavelength that can be detected with the particular material. Figure 4 – PIN Diode Material Wavelength Range(m) Indium gallium arsenide 0.7-2. Germanium 0.8-1. Lead sulfide 1.0-3. Lead selenide 1.5-5. Indium arsenide 1.0-3. Platinum silicide Indium antimonide

Mercury cadminum telluride 1.0-6. Figure 5 – Example Intrinsic Materials The actual housing of a PIN diode is shown in Figure 6. Note how the intrinsic layer is exposed so it can absorb infrared radiation. Figure 6 – PIN Diode Housing

takes over the signal, which is then filtered by the band pass filter set to 38KHz. See Figure 10 for band pass filter characteristics. The signal is then processed and compared to see if it is a valid 38KHz signal. The output range of this IC is typically in the range of 0.35V to 5.0V. See Figure 11 for detailed tolerances. Figure 9 – PNA4612M Block Diagram Figure 10 – PNA4612M Frequency Response Characteristics Figure 11 – PNA4612M Operational Characteristics

The pulses received by the detector will be converted into low and high level logic, which then can be used to interpret whether the sensor is seeing noise or the actual beacon. The detection area of the sensor is depicted in Figure 12. The relative reception distance decreases with the angle off center. Figure 12 – PNA4612M Reception Characteristics

Component Application

Interface Since the output of the PNA4612M is digital, the sensor will be tied almost directly to the microcontroller with only a voltage clamping circuit to protect it. A standard digital port will be used. The microcontroller will interpret the incoming data as a means of finding the beacon. This should be easy to accomplish since the beacon will send out a uniform pulsed signal. Bearing IR Set IR Sensor IR Sensor MC Interface Microcontroller Overhead IR Set TTL TTL IR Sensor TTL Voltage Clamp TTL Voltage Clamp TTL Voltage TTL Clamp Figure 13 – IR Detector Interface Mount The three IR detectors will be mounted in a configuration as depicted in Figure 14. The two sensors that will be used as direction finders will be mounted in front

oscilloscope to measure the output of the detector. Once close range detection is established, long range testing will begin. Of course, the tests will take place under a variable of different levels of light and radiation pollution to test the boundaries of the system. The data collected here will help determine the best method for the microcontroller to interpret the signals the detectors will be transmitting. Costs Part Source Qty Individual Cost Total Cost PNA4612M - IR Sensor www.digikey.com 3 $1.43 $4. Schottky Diodes www.digikey.com 6 $0.70 $0. Total N/A N/A N/A $4. Figure 15 – Component Costs

Alternate Components

There is another method to accomplish what is going to be achieved with the IR detectors in this project. However, the different approach has different costs, operational limitations, and implementation difficulties. These factors were considered when this particular technology was turned down for the application. The alternate technology in question was radio frequency detection. When the project was first being brainstormed, a beacon that would send out radio signals was the original idea. Upon further investigation, it was realized that determining direction with RF detection was too much risk. Although the RF technology seems much more difficult to implement in this application, it does have an advantage of much greater range. However, since the project will be operating indoors, there wasn’t need for greater ranges than what IR technology provided. The RF direction detection would work on the principal of human ears. A human being can tell which direction a sound is coming from by sensing the difference between the amounts of time it took to reach each ear. With receivers a similar effect can be accomplished here. Again, it was determined that this approach would be much to error prone so IR technology was deemed much more appropriate.