Heart rate monitor receiver

[upand_at_them]

I have a Polar heart rate monitor and found this online: **broken link removed**

But I don't have a MAX4169. I do, however, have a bunch of 741 Op-Amps.

I'd first like to try the author's approach of using a loop of wire as an antenna. The author used an oscilloscope, which I don't own, but I was using my Parallax Opta-scope and I think it wasn't sensitive enough. Is there any way I can improve this?

It would be cool if I could see the signal on my Opta-scope and then try to work through using an Op-amp to boost and clean up the signal without just copying that circuit and not knowing how it all works.

Mike

 

[williB]

What were you measuring that you say it wasnt sensitive enough??
the max4169 is just four op-amps on a single chip as i'm sure you know..
i hav'nt looked at the specs for it , but you might be able to use your 741's..

**broken link removed**

 

[audioguru]

Hi Mike,
Old 741 opamps won't work in that circuit:
1) They won't work with a supply of only 5V.
2) Their output can't swing anywhere near +5V and ground like the rail-to-rail MAX opamp.
3) They are very noisy so with the gain of 101,000 in this circuit before the peak detector, their noise will be detected instead of the signal.

There are lots of other low voltage, rail-to-rail and fairly low noise opamps in the world.

I would make it with single TL071, dual TL072 or a quad TL074 low noise opamps, and use +12V or two 9V batteries to power the circuit. Because these low noise opamps are so popular for audio circuits, they are actually cheaper than 741's. :lol:
With the higher supply voltage, the output swing will exceed the safe level for a microcontroller.

 

[upand_at_them]

Thanks for the replies.

Is there any way I can see the signal on my Opta-scope using some loops of wire as the antenna? Or could I make an inductor? I have some fine gauge magnet wire, but no cores. I did try a single loop of 22-gauge wire, but all the Opta-scope picked up was noise.

Mike

 

[audioguru]

Hi Mike,
Since the interface circuit has a gain before its detector of 101 thousand, your scope doesn't have nearly enough sensitivity to see an extremely low-level signal from a tuned antenna.
You need the interface circuit as a preamp with its extremely high gain.

 

[upand_at_them]

Therein lies my problem. How do I receive the signal so that I can amplify it? Can I build a simple resonator circuit with some magnet wire and capacitors?

Mike

 

[audioguru]

Why not buy or make a 6.8mH inductor and use the caps shown in the project?
You have one chance in 10million that your randomly made inductor will have 6.8mH. Pretty slim odds.