>> Does anyone have any hints on laser rangefinder design? I'm looking

...
>> good/easy way of doing it? Is there a totally different approach 
>> I'm
>> overlooking?


> Radio altimeters.

Radio altimeters are very 'time honoured' and can be very simple.
Robert's description is enough to build one from :-)

I long ago had an "APN1" radio altimeter from a bristol Freighter. 
Alas I sold it - it would make a nice museum piece nowadays. It used 
two "acorn tubes" in a push pull oscillator to drive the aerial. A 
'voicecoil" drove a variable capacitor to modulate the transmitter 
frequency in a saw tooth fashion (ie semi linear). The reflected 
signal was heterodyned with the current signal and detected by a AFAIR 
single tube diode mixer. The resulstant audio frequency was 
proportional to altitude. Had a range of thousands of feet AFAIR - 
says much for the sensitivity of the simple receiver arrangement. 
Something like this should be easily replicable with modern 
components.

BUT the doppler radar units used in security systems a few decades ago 
before PIRs got cheap could probably be modulated and do the same 
task. I haven't seen these around of late but i imagine they still 
exist in some form or other.

Long ago a ?UK? electronics magazine did a doppler radar door / 
security alarm and used a discrete transistor oscillator as the 
source - probably at VHF. Trying this out should be quite easy. Any 
VHF band transistor radio, sawtooth (rather than triangle) modulate 
local oscillator somewhat, couple to an aerial, mix return signal with 
current local (a second receiver would make this easy when playing) 
and listen for heterodynes.


        RM


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