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How to Build an Infrared Proximity Detector That Works at 20 Feet

Infrared is a convenient region of the spectrum for many applications. It̵7;s invisible to the human eye, but infrared radiation works by the same rules as visible light. Infrared uses the same general kind of lenses, mirrors, sources and detectors as visible optics. There are some differences in materials, but the principles are the same. So if you want to do something like sense the distance from an object, you can use infrared to create an optical system, but have it be completely unobtrusive to humans.

Things You'll Need

1.55-micrometer laser and drive circuit
InGaAs avalanche photodiode and amplifier circuit
1.55-µm bandpass filter
Mounting platform
2-inch diameter, 100-mm focal length infrared lens

Instructions

- 1
  Calculate the amount of energy necessary to achieve a signal-to-noise ratio of 10 at your detector. For a rough approximation, assume the noise is all in dark current, so look on the data sheet for your detector and find the responsivity and the dark current for your detector.
  For purposes of illustration, assume you have a detector with a responsivity of 0.95 amps/watt, and a dark current of 1 nA, which is 10^-9 amps. To get 10 nA of current, you will need 10nA/0.95 watts of infrared power, which equals 1.1 x 10^-8 W.
- 2
  Calculate the amount of infrared energy needed from the object. Since you̵7;re using a 2-inch diameter lens, the lens appears to subtend a solid angle of (pi x (1/12)^2)/20^2 = 5.5 x 10^-5 sr (steradian). This is essentially a measure of how big the detector looks from the object that̵7;s doing the reflecting. For the example system, your detector needs to collect 1.1 x 10^-8 watts in 5.5 x 10^-5 sr, which is 2.0 x 10^-4 W/sr.
- 3
  Figure out how much energy needs to hit the target. Assume the object reflects light evenly back in the direction in which it̵7;s being illuminated. That kind of an object is called a Lambertian scatterer, and it reflects the light that hits it into a solid angle of 2 x pi steradian.
  The example system, then, needs to put out 2.0 x 10^-4 W/sr over the entire 2 x pi angle, which means it reflects 2 x pi x 2.0 x 10^-4 W = 1.3 x 10^-3 W, or 1.3 mW.
- 4
  Find a source that will deliver the right amount of power to the object. For the example system, find a 1.55-µm laser that will supply a power of 1.3 mW to the object. Attach the laser to the mounting platform so that it points straight out.
- 5
  Put the bandpass filter in front of the photodetector and attach the assembly to the lens. Mount that on the mounting platform, lined up in the direction to collect the laser light and put it on the detector.
- 6
  Operate the system with an object at various distances, recording the voltage as a function of distance. Use that data to create a curve that will give you distance as a function of voltage.

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