[time-nuts] Laser oscillator distance measurement ckt

[lstoskopf at cox.net]

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