Anton:

Make both units ping; the stationary unit in response to the handheld unit,
at a different frequency. This shouldn't require too much support circuitry
(given the restricted operating conditions) and might be optimal in terms of
the difficulty of dealing with interference and complexity (in comparison
to, say, infra red or the original radio idea.

The ultrasonic chirp will bend around trees, so the distance measured is
going to be a little bit relative to the immediate situation. However, our
experience with a similar piece of equipment is that when a human being is
in the loop, there are a multitude of sins that can be overcome by the
observers judgement.

--Tom Rogers  VP-R&D  Time Tech Inc.

-----Original Message-----
From: Anton Schoultz <antons@GRINDROD.CO.ZA>
To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
Date: Friday, November 21, 1997 2:10 AM
Subject: Help ! - radio TX/rx for ultrasonic measurement


>A week or so ago, I posted my ranger problem, (7.5m in the forest)
expecting to flooded with ingenious solutions ;) Apart for a suggestion to
use a chain, I got zip. Am I to assume that no-one had a better solution to
offer?
>
>I need your help PLEASE !
>
>If you can help me with suggestions on a SIMPLE radio transmitter and/or
receiver please e-mail me (antons@grindrod.co.za) or post to the list.
>
>Transmitter.
>Should be around 27MHz for legal reasons (no license rqd for low power).
>Range required no more that 10 meters.
>Absolute minimum parts, perhaps a simple colpits crystal osc?
>Need to be able to turn the transmitter On and Off via PIC port pin.
>
>Receiver.
>27Mhz, 10meter - all I need is a carrier detection. Perhaps something
similar to the old 'crystal' radio? (aerial coil, tuning cap, diode
rectifier, low pass filter) followed by an op-amp comparitor. The output of
this will be used to gate the ultrasonic sender.
>
>The requirement was to measure 7.5meters in a wooded forest, to inscribe a
circle. (about 12 measurements per circle). My plan was to have two units,
one at the center (hub) and one roving unit for the perimeter (hand-held).
>The hand held's PIC would turn on a radio transmitter, and start a timer.
The hub would, upon detecting the radio signal, emit an ultrasonic ping.
When the hand held detects this ultrasonic ping, the PIC would stop the
timer. The time taken would be indicative of the distance.
>
>Any help welcome
>
>Thanks in advance
>
>Anton Schoultz
>antons@grindrod.co.za
>