Russell McMahon wrote:
>I have an application requiring very short range low speed telemetry.
>Low cost is an overwhelmingly significant factor.
>The application is to transfer an occasional on/off signal over a distance
>of around 1 metre.
>Transmitter and receiver orientation cannot be guaranteed. There may be
>object(s) (possibly a person, metalwork etc) in the direct path although
>there will not be solid metal blockage (OK - I'll tell - it's part of an an
>exercise machine.).

Hi Russell,

[OK, last week I fixed your battery charger, this week it'll be
your exercise machine - maybe you oughta hire me as a full-time
consultant - he, he <:-)))].

Funny thing you should mention exercise machine here. You've
probably seen the Polar heart rate monitors. The h.w. inside the
chest belt will signal heart rate to about 200 bpm - and run for
months off a sealed-in lithium battery <-- presumably Li, since
nothing else could fit in that belt. The range is 3 feet or so
- that's about 1M to outlanders.

You can pick up the signal from the Polar belt using a simple coil
and see it on a scope. Each heart beat is signalled by a decaying
5khz tone that lasts for a few msec.

For a receiver, a simple coil-capacitor resonant ckt tuned to 5
Khz, followed by a 2-stage BJT amplifier and rectifier work fine.
Costs about 50 cents US.

The chest belt itself, uses a very simple ckt. BJT operating in
class C, for power savings, picks up/amplifys the heart signal,
which is in the mV range, and bangs a loop coil that resonates
at 5 khz. When not connected to the chest the BJT is off, and
no power is expended. Cost about 50 cents US.

Your constraints would probably be no where near as bad as the
Polar unit. Also, 5 Khz probably isn't even covered by the FCC,
so you won't have to worry about Harold H. jinxing your plans.

The basic amplifier described above will pick up misc garbage,
like emissions from your CRT [although I don't know which ones
specifically]. Also, nearby Polar units interfere with each
other. HOwever, you could send a little coded-pulsetrain to
reject unwanted garbage.

BTW, you can deal with the orientation thing by using 2 or
3 coils, mounted orthogonally w.r.t. each other, and wired
in series - or wired to separate amps for better response.

best regards,
- Dan Michaels
Oricom Technologies
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