Heart rate counter - polar and similar

[nelsonmf]

Hi,

I have an old heart beat monitor opened and there is no intrumentation opamp, it does not even have a opamp. I just see transistors. It seems to be very very simple but it is very difficult to get the schematic out.
I also found on the net a picture from a polar circuit and it is the same. No expensive intrumentation opamps.
https://www.timetriallingforum.co.uk/index.php?showtopic=10082

Anyone knows how they do that? I think they use the gates of the transistors to sense the electric pulses from the heart, but that's only the basic idea.

 

[keith1200rs]

What's the IC in the picture? A custom chip with instrumentation amp?

Keith.

 

[nelsonmf]

It is a not gate chip. It seems to act as buffers or just to drive the transmitter coil has an oscillator?

 

[keith1200rs]

You would have to trace out the circuit to see what connects to where, but my guess would be the discrete transistors drive the coil and form an oscillator and the IC is a custom one with an instrumentation amplifier. Trace the heart pickup pads and see where they go. Also, trace the coil connections and see where that goes.

Keith.

 

[nelsonmf]

I don't have the circuit board with me at this moment, but I'm pretty sure it is a not gate chip because I searched the part number and I remember it is a common logic gate part number.

Here is what I managed to get from the circuit board.

 

[keith1200rs]

I don't see the chip on your diagram. It is possible that they just make a differential amplifier out of a couple of transistors. I cannot work out where the electrodes are on your diagram. I think one may be on the top left, but I cannot find the other. It could even use a single ended amplifier, relying on the other electrode being "ground".

Keith.

 

[nelsonmf]

Hello,

Yes, you'r right, the other electrode is GND.
The chip is the NOT gates on the diagram. I know this diagram is a bit difficult to follow

"differential amplifier out of a couple of transistors" it is possible, but what is the advantage? More sensitivity?

 

[keith1200rs]

"differential amplifier out of a couple of transistors" it is possible, but what is the advantage? More sensitivity?

Cheap. Far better to use an instrumentation amplifier but a couple of transistors are far cheaper. In the early years I guess Polar didn't have the money to make a custom IC and maybe instrumentation amplifiers were too expensive.

Keith.

 

[nelsonmf]

Don't know how they can do it. On my experiments, with an instrumentation opamp it is already difficult to have a good signal. If they do with regular transistors, how do they manage to get the ECG?

 

[keith1200rs]

It may be a JFET for high impedance. What is the marking on the transistor which is connected to the electrode through 56k? Bear in mind they just want pulses for counting, not a real shape, so signal integrity isn't as critical as an ECG. Also, as far as I know they have quite a bit of logic in the watches to ignore missing/spurious pulses.

Keith.

 

[nelsonmf]

Hi,

I'll try to get the circuit board again to see the smd markings. I just wanted to make a simple and cheap heart beat real time counter. I can always do with opamps, but even with them I can't get a clear signal from the noise and the circuit gets a bit big.

Thanks for the help. I'll make some tests with the transistors.