[time-nuts] Laser oscillator distance measurement ckt
jimlux at earthlink.net
Fri Nov 19 17:41:05 UTC 2010
lstoskopf at cox.net wrote:
> I want to build a simple digital tape measure for the range from near zero to perhaps 10 ft with some remote output. The off the shelf units are accurate to perhaps 1/16 inch, but do not provide continuous outputs. The Bluetooth units seem to require pushing a button for each measurement.
> The AR4000 uses an open loop oscillator method: Laser on, beam to target, reflected to photodiode (the time delay we want), then detected, amplified to turn off the laser, photodiode (decides) no light and turns the laser on (time to be minimized). Measure the frequency and calculate the distance. The AR4000 has oscillation frequency of about 50 MHz at zero distance (the circuit delay) and about 4 MHz at 50 ft. Easily measured.
> Circuit looks pretty easy with modern devices. Anyone already have something or ideas for best devices? Thanks, N0UU
It doesn't use time of flight, but there are a variety of short range
distance measuring schemes that rely on parallax. you have a linear
sensor next to the laser, and you basically look for where the "spot"
is. The sensors vary all the way from simple segmented
photovoltaic/photoresistive (DC voltage proportional to position of spot
on sensor) to linear CCD/CMOS sensors (a fax sensor with 1800 pixels,
for instance) to inexpensive CMOS cameras (640 pixels with a suitable
lens can give you subpixel resolution by centroiding the spot... perhaps
1 part in 1000/2000 is possible.. out of 120 inches, that would get you
in the tenths of an inch, with better resolution close up)
These things are available as an off the shelf device for <$50, by the way.
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