We did a unit at my last job called 'velocity of light'.

It was pretty simple and you cud accurately measure distance down to 1cm.

Infrared led emitter switched at 50MHz, focused using a lens to a 2 inch beam,
reflected back by a mirror to an infrared detector (5 inches away from the
emitter).

The tx 50MHz was generated using a 50MHz xtal which drove the tx led.

Their was a 50.025MHz second xtal osc which was used to mix down both the tx
signal and the rx signal to 25KHz IF (we now have 2 25KHz waveforms), these two
25KHz carriers were then phase compared - so easy to see the smallest of
movements in the mirror on a basic scope.

So all the hard work is done at 25KHz (phase measuring) - one of todays little
mcpu's will do this easily (ATmega16 for example).

You could just as easily use laser or maybe rf in place of the IR led's, though
directing RF at such low freq's would be somewhat difficult.

Obviously at 50MHz, the phase difference would cycle every 6 meters (total
reflected path), but if your a bit cleverer (though not hard to do) you could
get the freq to sweep from a low freq (say 5MHz) upto wot ever you like - using
a pair of single xtal referenced PLL's to generate the two oscillator freq's
(whilst maintaining the 25KHz difference) and then calculate an exact distance
in the cpu.

This method is simple and relatively cheap to judge distances very accurately
with no need for very short pulses (high bandwidths) and very fast logic.

Clive

Forgot to say.

A 90deg phase change in the reflected beam (at 50MHz) also results in a 90deg
phase change in the rx'ed 25KHz IF) - this is why no fast logic is required.

I think this will be easier than using RF. However, even though the IF is
25 kHz, you are still measuring small time differences. I just think this
hardware will be easier than RF. Mostly because you can control where it
goes better. This, of course, assumes you can do that in the target system
The OP has to decide.

That being said.... oneof those 10 GHz door opener or motion detector units
(does the GunnPlexer have a DC output from its detector?) would be another
interesting method. You can easily see small motions in the reflected
signal phase difference and this is at the 10GHz freq. so the resolution is
high. You do need to insure that the desired reflection is the only
one...similar to the LED type of system.

An aiming problem.

Steve N.

wrote in message
...

We did a unit at my last job called 'velocity of light'.

It was pretty simple and you cud accurately measure distance down to 1cm.

Infrared led emitter switched at 50MHz, focused using a lens to a 2 inch
beam,
reflected back by a mirror to an infrared detector (5 inches away from the
emitter).

The tx 50MHz was generated using a 50MHz xtal which drove the tx led.

Their was a 50.025MHz second xtal osc which was used to mix down both the
tx
signal and the rx signal to 25KHz IF (we now have 2 25KHz waveforms),
these two
25KHz carriers were then phase compared - so easy to see the smallest of
movements in the mirror on a basic scope.

So all the hard work is done at 25KHz (phase measuring) - one of todays
little
mcpu's will do this easily (ATmega16 for example).

You could just as easily use laser or maybe rf in place of the IR led's,
though
directing RF at such low freq's would be somewhat difficult.

Obviously at 50MHz, the phase difference would cycle every 6 meters (total
reflected path), but if your a bit cleverer (though not hard to do) you
could
get the freq to sweep from a low freq (say 5MHz) upto wot ever you like -
using
a pair of single xtal referenced PLL's to generate the two oscillator
freq's
(whilst maintaining the 25KHz difference) and then calculate an exact
distance
in the cpu.

This method is simple and relatively cheap to judge distances very
accurately
with no need for very short pulses (high bandwidths) and very fast logic.

Clive


Forgot to say.

A 90deg phase change in the reflected beam (at 50MHz) also results in a
90deg
phase change in the rx'ed 25KHz IF) - this is why no fast logic is required.

literally I am talking about a transmitter within a cuboid shaped
enclosure around 10m maximum and being able to pinpoint that transmitter
within that enclosure accurately, to around 1cm, perhaps 2cm.

This is a typical application of an acoustic positioning system. With a
speed of sound of about 340 m/s, this is feasible. I have made such a system
with acoustic tags on 40 kHz using 8 receiver nodes in the room. That's more
than required, but it gives enough redundancy to be robust against shadowing
as things move around in the room. Accuracy is in the 1-2 cm range.

--
Sverre Holm, LA3ZA
---------------------------------
www.qsl.net/la3za

Yep! acoustic is one. another is infrared

Over this short distance RF travel time is in the 30 nano second range.
This requires some pretty good timing measurements. Sound, on the other
hand, has a speed of around 30 mili seconds for 10M (30 ft).

Something I wanted to do for a long time is a model rocket altitude system.
Rocket has a one transistor 10M Rx ( have the circuit around here from some
1960's Pop Electronics for a 1 tranny FM broadcast Rx) and a one or two
transistor 2M Tx (FM) -- rocket antenna easier.. Ground station on 10M
(lots of power available for the wooden rocket Rx) transmits a TONE (not a
pulse). Rocket transponds (retransmits it on 2M). Measure the zero
crossing time delay and subtract the overhead time. With proper (and
simple) digital design, you can have a digital readout in feet, inches,
whatever. With a tone, common ham radios are just fine.
With audio, pulses are easy except how do you keep the target from hearing
it's own echo and "oscillating by itself...?


or use subcarriers - this is a little harder. Transmit an FM modulated tone
(carrier, say 10KHz, tone 1KHz) and transponding it back on another
carrier, say 15 KHz.

Who was it that used to sell the sonar modules from the cameras for
experimentation

acoustic


"Sverre Holm" wrote in message
...
literally I am talking about a transmitter within a cuboid shaped
enclosure around 10m maximum and being able to pinpoint that transmitter
within that enclosure accurately, to around 1cm, perhaps 2cm.

This is a typical application of an acoustic positioning system. With a
speed of sound of about 340 m/s, this is feasible. I have made such a
system
with acoustic tags on 40 kHz using 8 receiver nodes in the room. That's
more
than required, but it gives enough redundancy to be robust against
shadowing
as things move around in the room. Accuracy is in the 1-2 cm range.

--
Sverre Holm, LA3ZA
---------------------------------
www.qsl.net/la3za

Steve Nosko wrote:

Something I wanted to do for a long time is a model rocket altitude system.
Rocket has a one transistor 10M Rx ( have the circuit around here from some
1960's Pop Electronics for a 1 tranny FM broadcast Rx) and a one or two
transistor 2M Tx (FM) -- rocket antenna easier.. Ground station on 10M
(lots of power available for the wooden rocket Rx) transmits a TONE (not a
pulse). Rocket transponds (retransmits it on 2M). Measure the zero
crossing time delay and subtract the overhead time. With proper (and
simple) digital design, you can have a digital readout in feet, inches,
whatever. With a tone, common ham radios are just fine.
With audio, pulses are easy except how do you keep the target from hearing
it's own echo and "oscillating by itself...?

Block the RX input for a few ohnoseconds[1] longer

or use subcarriers - this is a little harder. Transmit an FM modulated tone
(carrier, say 10KHz, tone 1KHz) and transponding it back on another
carrier, say 15 KHz.

Might be a useful alternative. Have to have sharp filters.

Who was it that used to sell the sonar modules from the cameras for
experimentation

Polaroid, IIRC.

[1] The shortest possible unit of time: the interval between hitting
the "do it" key and the realization that you shouldn't have.

--
I swear to god, if people treated their cars they way they treat their
computers, half the cars on the road would be covered in bumper stickers
advertising porno, and their trunks would be filled with rotting garbage.
-- Christian Wagner

Yea. Easy.Steve N.

"Mike Andrews" wrote in message
...
Steve Nosko wrote:

...With audio, pulses are easy except how do you keep the target from
hearing
it's own echo and "oscillating by itself...?

Block the RX input for a few ohnoseconds[1] longer

or use subcarriers - this is a little harder. Transmit an FM modulated
tone
(carrier, say 10KHz, tone 1KHz) and transponding it back on another
carrier, say 15 KHz.

Might be a useful alternative. Have to have sharp filters.

Who was it that used to sell the sonar modules from the cameras for
experimentation

Polaroid, IIRC.

[1] The shortest possible unit of time: the interval between hitting
the "do it" key and the realization that you shouldn't have.

--
I swear to god, if people treated their cars they way they treat their
computers, half the cars on the road would be covered in bumper stickers
advertising porno, and their trunks would be filled with rotting garbage.
-- Christian Wagner

Steve Nosko wrote:
. . .
Who was it that used to sell the sonar modules from the cameras for
experimentation


It was Polaroid.

You can get sonar measuring devices at home handyman stores for a few
bucks. They might be useful with a little hacking.

Roy Lewallen, W7EL