5khz reciever

M

melazarus

Member
Hi,
I have bought one of the cheaper Polar Heart rate monitoring watches. The come in 2 parts, a transmitter and a watch/receiver.

I have an idea of building a Picaxe data logger that logs my heart rate every X time.
To be able to do this I need to add some electronics to the Picaxe that enables it to receive the wireless signal coming from the transmitter.

This is what I found on the net:

> Polar Non-Coded and T31 Transmitters :
>
> Transmission frequency: 5kHz
> Means of transmission: On/Off -modulated electromagnetic field with a
> 5-7ms burst for each heart beat

Can anyone point me to the right direction so I can build this 5khz receiver that outputs a TTL level signal for the picaxe?

I don't think this is very hard to make as they don't seem to have any data in the transmission except for on/off (on being a heart beat I assume) But I'm a noob if it comes to wireless communications.

thanks for the tips
 
E

eclectic

Moderator
VLF

Do a google search for VLF radio receiver.

There are lots on the net.

And, while you're Googling, find then download a program called

Spectrogram.

It could prove useful for signal analysis.

e.
 
Last edited:
G

gengis

New Member
> Transmission frequency: 5kHz
> Means of transmission: On/Off -modulated electromagnetic field with a
> 5-7ms burst for each heart beat

Any high gain audio amplifier can handle 5khz - you would need a pickup coil on the input to capture the energy. A tuned coil would work much better for distance - but up against the transmitter any coil with a lot of turns of wire should do it.

A 5-7 ms burst of 5 KHZ is about 100 cycles of tone and will sound more like a "click."

If you have a tape recorder with one of those pickup coils for telephone monitoring that should work or just plug the coil into an audio amp. Moving the coil around to find the best orientation . . .

From the click to something a picaxe can use - I'd use it to trigger a (10-20 ms) 555 monostable multivibrator, the idea is to set the pulse width long enough for the picaxe to see it AND to only put a single pulse (not 100) into the 'axe.

You might even get by with a diode and cap on the output of the amp to rectify and stretch the pulse a bit.

Sounds like fun - shouldn't be difficult at all.

For tuned circuits at that frequency - a resonant circuit of a cap/coil gets a little unwieldy because the inductor is relatively large, but it will pick up a stronger signal. Another technique used in SLF, ULF, VLF is to just amplify the piss out of the signal from an antenna then use an active bandpass filter.

But for now I'd start with a coil and audio amp and see what you have to work with.
 
M

melazarus

Member
Thanks

Thanks guys, I tried google but I didn't know what to look for realy. But now I do.
I have some 3 months leave coming up soon so I'll have plenty of time to play around with it and testing different things.

thanks
 
You must log in or register to reply here.
Top