Re: Converting Signal from Polar Heart Rate Monitor


petrus bitbyter wrote:
<mattjohnson36@xxxxxxxxx> schreef in bericht
news:1160449611.879954.280880@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hey All,
I'm trying to design a circuit to input a signal from a Polar heart
monitor (3V Square Wave, ~1Hz) to a serial port RS-232. The frequency
of the signal varies according to the users heart rate. I am having
trouble trying to find a F/V that will work for my application. My idea
was to convert the input with the F/V then use a ADC using this design
http://www.engj.ulst.ac.uk/sidk/resources/serial2.htm If anyone has a
better idea or any advice it would be greatly appreciated. Thanks for
the help.
Matt


Matt,

There's no use in converting a digital signal to analog only to convert it
to some other taste of digital again. I see two solutions that might help
you:
1 - Use some micro with a build-in UART and a MAX232 like 5V(logic) to RS232
level converter. The X-tal clock of the micro should be accurate enough for
your application but that can hardle be a problem. You can make the micro
count clockpulses between two rising edges of your heartbeat monitor,
convert the result to an appropriate ASCII and send it to the serial port.
No need to say the bulk of the work wil be programming the micro.
2 - Convert your 3V square wave to RS232 level and connect it to an input of
the serial port. No need to use RxD. DSR or CTS can also be used. Now write
an interupt routine that acts on a change of the input used. Increment a
counter and check the time. After 60s you process the counter contents and
reset it. You can vary in different ways. Use two interrupt routines, one
for the clock and one for the heartbeat. Use more counters to provide an
update on every heartbeat and so on. This solution places the burden on the
PC-programmer.

petrus bitbyter

Note most 232 receivers do not need the signal to go beneath ground. In
fact, they are designed to be somewhat TTL compatible. I never bother
with 232 converters for simple hacking purposes. Note that the 232
converters with charge pumps put jitter on the pulse. Not significant
in this application, but it can be around half a microsecond, maybe
more.

Also, as windows progressed, the interupt timing got much more poorly
controlled. You really need DOS to get good timing on the handshake
line. Again, probably not a problem in this application. There is a
commonly used program called slicer.exe that is used for detecting the
presense of signals on the handshake lines.
http://home.ica.net/~phoenix/wap/slicer.htm
It is a good diagnostic to see that the line is wiggling.

.

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