Pulse/heart rate counter project.

[JPU]

I started last night on my own Pulse oximetry circuit. I managed to get a red led to dim very slightly in response to my heart beat measured through my finger. The LED dipped very slightly in brightness. Quite an emotional moment!

I used a KIA358p, an infra-red photo diode from a mouse and an infra-red LED I had bought for another project plus also the red LED. The OP amp was set to make a gain of 20. I had a 3*AA supply to the infra red LED and another 3*AA supply to the OP Amp. It worked..

I am a new bee to electronics and I wonder if you guys could help.

1. Why did I need to use 2 power supplies. If I tried to power the OP amp using the same supply that lit the infra red LED, it would not work.
2. How can I get a better reaction in the red LED. On Off to each beat, rather than a slight Dimming.

I can post the schematic after later, after work if that would help? Once I get this to work, I intend to connect the circuit to a Picaxe to then count the beats and produce a BMP rate.

Thanks

JPU

 

[Harald Kapp]

Add 1)
Most OpAmps need a dual supply in order to be able to work with input and output signals near 0V. If you can offset the input by e.g. Vcc/2 then a single supply can be sufficient.
There are OpAmps called rail-to-rail. One has to carefully read the datasheet as rail-to-rail can signify:
- only the output signal can swing from the negative rail to the positive rail (0V to Vcc in the case of a single supply)
- only the input accepts signals from rail to rail
- the input accepts and the output can drive signals rail to rail
But careful: rail-to-rail often means only to within a few 10s or 100s millivolts. Read here for more information.

Add 2)
Increase the gain. To make the LED cleary turn on and off instead of having the LED shine more or less brightly, use a Schmitt-trigger circuit. You will need this anyway if you connect the output of your circuit to the Picaxe since the µC normally requires clear High or Low signals, no analog inputs (at least not for this application).

 

[GreenGiant]

A word of advice to begin with try using your thumb, there is a larger vein in that than your fingers, this way you can get it working then dial up the sensitivity for use on your fingers

 

[Harald Kapp]

Here is a design. You'll have to klick the links in the text for the figures to display,
Or Google pulse sensor circuit and you'll find a lot of hits.

 

[CDRIVE]

Justin, are you gonna be checking the horse's heart rate while you mill their teeth?

Chris

 

[JPU]

Hi Chris

I was hoping you would take a look at this one. I have some photos for you for the old thread,,,,you may enjoy....I just haven`t had time yet to put them on..

I'll be milling their teeth with the motor you helped develop..LOL!!


I have just come in,,,as you might have guessed, its a Friday,,,,end of week,,,you know where I have been!!!!

Chris,,keep an eye on this thread, as I will need your help,,,,,,again!

JPU

 

[CDRIVE]

Yes, I know were you were and I'll be there tomorrow too. It's incumbent upon all able bodied men and women to do their part in supporting the world's Hops and Barley growers.

I'll keep my eyes open for your schematics. .. I hope they're Tina.

Chris