how an optical rangefinder works and how to build one
Optical rangefinders use the principle of parallax. Two lenses focus on the same object from different ends of the rangefinder, then both images are superimposed on each other by means of a focusing knob. The knob is calibrated with the range.
Since parallax is greatest (most measurable) at short ranges, optical rangefinders are most accurate at short ranges. Expect an accuracy of +,- 5 yds at ranges of less than 100 yds, but on the order of 50 yds at 500 yds. Much of the performance of these devices will depend on how precisely the operator can focus.
The best known optical rangefinders are from Ranging. Their website is http://www.ranging.net/
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Laser range finders pulse laser light at an object and then time how long it takes the light to reflect off of the object and return to the rangefinder. Since the speed of light is fairly constant, the disance can be measured.
I'd like to see you build one with wood, glass, and nails.
Im not sure that time of flight can be accurately used for short distance measurements as light travels at 3e8 m/s. For a sensor with a precision of 1m a clock of 300 Mhz would be required. A sensor with precision of 10 cm would therefore require a 3 Ghz clock. Oscillators at these frequencies are difficult to produce with any accuracy, thus time of flight rangefinders are expensive. If Im not mistaken you're looking for a short distance rangefinder. These are cheap and readily available from a number of manufacturers. I reccomend the Sharp GPD12 or similar device. These devices operate in a manner very similar to that described by the first post. They emit an infrared beam, which then reflects off some nearby object at say 20 cm away. The returning beam will then hit a PSD; essentially 2 photodiodes with a common anode (maybe cathode, I always confuse the two). The beam causes a current to flow thru each diode. The ratio of these currents can then be used to determine the displacement of the beam on the PSD. The displacement of the beam can then be used to determine the distance assuming the all other critical distances of the device are known.
Sharp makes several different varities of these sensors in both digital and analog versions ranging in price from ~$10-20. These devices are both cheaper and more accurate than anything you could make yourself with wood, nails and the like (unless of course you have access to an optoelectronics lab and materials). Hope this helps.
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