Dr. Acula advice please


Senior Member
Hi Dr.,

How do the devices you see patients in IC wards etc. wearing on their fingers work? I think they measure pulse and body temp.

What are they called? Do you know off hand where I can buy them from?


Michael 2727

Senior Member
I have an old Heart Rate Monitor with Finger Clip
made by REPCO (bought it in a bin of junk $5.00 the lot)
It uses a 3mm incandescent lamp (grain of wheat Bulb)
mounted on a steep angle which shines light into the
finger, the pickup close by is an LDR.
The OP amps inside must look for a peak in the light
levels returned (blood pumping modulates the light).

I had thought about a PICAXE version some time back
but the sampling needs to be constant, better suited
to OP Amps etc. So I didn't bother much after that.

Not saying that it can't be done with a Picaxe, but
there may be better ways to go about it. ;)

Remember that Microchip and Rev-Ed don't endorse
their products to be used for any medical purposes at all.
(you need a special note from your Mother for that type of thing ) :(


Senior Member
I was hoping Dr. Acula would reply, (I was going to ask about my haemroids while I had his attention.) But thanks for the tips guys.


New Member
Ok, back now. Busy building op amp circuits for the 10-15V expanded scale thread.

The device is a pulse oximeter. Haemoglobin is a protein in red blood cells. When oxygen enters the blood it is picked up by the haemoglobin and carried around the body attached to the haemoglobin. Haemoglobin with oxygen attached to it is called Oxyhaemoglobin.

How it works A pulse oximeter probe consists of:
- 2 low power LEDs: Infra Red 940 nm & Red 660 nm
- One photodetector
The infrared and red light is shone alternatively through some tissue (finger, foot, toe, earlobe or nose). As the light is passed through the tissue some of it is absorbed. The amount of light absorbed changes every time the heart beats as the blood pulses past the sensor. The light is absorbed differently by haemoglobin and oxyhaemoglobin. The light intensity of the infrared and red light is measured by the photodetector after it has passed through the finger. The pulse oximeter calculates the percentage of haemoglobin which it oxygenated. SpO2 = Oxyhaemoglobin x 100 % Total Haemoglobin The time interval between 4 – 8 Heart Beats is measured and from that a Pulse (or Heart) Rate is calculated and displayed.

I guess a picaxe could do this if one had leds of the right colour.

As for haemorrhoids - sure. But maybe not on this forum. An 84 year old patient came in the other day to see me and told me how to get rid of cold-callers from India who always seem to call at dinner time: "Indian call centre: Good evening Mr J., are you having a good day?" Mr J: "Yes, I was having a great day until my haemorrhoids got caught in my bicycle chain!"

Hair loss - Get a #2 buzz cut. Do not, I repeat not, ever try to do a comb over!


Senior Member
Thanks for your advice Dr.

My girl friend is a speech pathologist at a large hospital. Maybe she can pull one off the hand of a unconscious patient in the IC ward and bring it home.......on second thoughts she does a lot for me but thats probably asking a bit too much.


Dr Acula's explaination is as good as it gets.

The scary thing is the cost of these items.
The component value, (2 leds and 1 photodiode) is less than a cheap mouse, but they vary in price from about $200 to $500 depending on brand. Bit of a rip off.

The ones used on pre-term babies are actually disposable and consist of the optics stuck to a backing strip and wrapped around the bubs heel. I think these are about $50.

The latest technology have a pic chip in them to provide cal factors, trimming the optics to improve the measuring algorythm, but this is just a look up table I think. The real cost is in the software of the monitor.

We once had a cheap generic copy monitor which always gave 100% O2 on a baby. A switched on nurse didn't trust it and brought it to our attention. Turned out the IR Led was faulty but the monitor didn't pick it up. The same probe on a quality monitor alarmed as it should. Consequently that brand was removed from service.

With all the high tech gear in hospitals, we still need skilled staff to interpret the results.


Senior Member
Yep I saw the disposable items along with the other types. I want to do some /experiments research using them on animals so I might try the disposables.


Senior Member
steirney: "The real cost is in the software of the monitor."

It's possible the real cost is the insurance the manufacturer has to pay. The equipment may rarely fail but our societies place a high value on a lost life.


Too true, covering one's backside is a high cost, but put "medical" in front of any generic device and the price goes up 10 times. Its a bit like NASA screwdrivers.


New Member
steirney: "The real cost is in the software of the monitor."

It's possible the real cost is the insurance the manufacturer has to pay. The equipment may rarely fail but our societies place a high value on a lost life.
The FDA also has very high standards for medical manufacturers, raising development costs. You have to double-check everything, lots of paperwork, frequent audits from outside auditors. All that extra effort adds to the cost.

Back on the topic of this thread, the most recent issue of Make has an article about building your own EKG. I can't find the specific article on their website, but several other EKG articles there.