How to Build an Optical Rangefinder
From construction sites to superhighways, there are many places where an accurate measurement of distance can be handy. Although optical rangefinders are not quite at the price/performance point where they will be designed into smartphones, the technology is dropping in cost and you can expect to see them in more situations. There are dozens of different ways you can design an optical rangefinder. One common design is the time-of-flight laser rangefinder.
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Things You'll Need
- Diode laser
- Photodetector
- Telescope
- Timing circuit
- Bandpass filter
- Mounting platform
Instructions
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1
Mount the diode laser to the platform so that the output beam is normal to the plane of the mounting surface. That is, if the mounting platform is placed flat on a table, the laser should point straight up in the air.
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2
Mount the bandpass filter to the front of the photodetector. The bandpass filter will allow the reflected laser light to pass through while eliminating much of the background. This will extend the range of your system.
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3
Attach the photodetector to the telescope. The telescope is there to increase the range of the system by collecting more of the reflected laser light. You can choose something as fancy as an 8-inch diameter astronomical telescope to something as simple as a 1-inch diameter, 4-inch focal length lens followed 5 inches later by a 1/4-inch diameter, 1-inch focal length lens.
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4
Mount the photodetector assembly to the platform adjacent to the laser and pointing in the same direction as the laser. Ideally, the reflected laser spot will appear in the center of the photodetector. You can check this by removing the photodetector from the telescope, shining the laser at a reflective target and observing the position of the laser spot coming out of the final telescope lens.
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Connect the laser and photodetector to the timing circuit. The timing circuit will initiate a laser pulse and start a clock, then stop the clock when the photodetector signal rises above a threshold value.
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Calculate the distance by multiplying the clock reading by the speed of light and dividing by 2. That is, distance = 3 x 10^8 meters/second x clocktime (sec) / 2. The factor of 2 accounts for the fact that the laser pulse travels to and from the target in the measured time.
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Tips & Warnings
The type of laser and photodetector are options you’ll need to evaluate for your specific requirements, considering such things as weight and electrical power, maximum range and sensitivity, and — of course — cost.
You can skip the telescope if you're working with a short-range system.
There are many other styles of optical rangefinder, from rotating mirror systems where a user aligns two images to find the range, to systems where the external reflection is used to define an oscillating cavity whose frequency changes with distance. Evaluate your requirements to see what style fits your needs.
Lasers can temporarily or permanently damage your eyesight. You can minimize the danger by using what’s called an “eyesafe” wavelength, such as 1.55 micrometers. If you’re doing short-distance ranging, you can get by with a low-power visible laser, but otherwise you’ll need to go with an eyesafe laser. If so, you’ll need to get fluorescent cards or and infrared viewer to do the alignment of your system.
References
Resources
- Photo Credit Jupiterimages/Comstock/Getty Images
