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Problem in Pulse Oximeter circuit

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Alfred_27

New Member
Hello everyone,

This is my first post in this forum, I hope that you can help me figure out the problem in the attached circuit.

The attached circuit is my attempt to make a Pulse Oximeter after modifying the original circuit from online. But it does not work. I have checked all the wiring but nothing.

How I can know if the Op-Amp or the photodiode are working correctly?

The power source of the circuit 3~3.3V.

Thank you all in advance
 

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jpanhalt

Well-Known Member
Most Helpful Member
Are your LED's lighting?
upload_2017-10-5_9-12-44.png

They appear to be connected backward.

As for your photodiode, can you simply check its output with a known light source?

The datasheet I looked at had a different pinout than you show:

upload_2017-10-5_9-19-26.png

Whereas, you show pin3 as ground, and pin4 as output.

Perhaps post a link to the datasheet you are using.
 

Alfred_27

New Member
jpanhalt, thank you for your comment,

Yeah the LED is lighting without problem, because the I/O drive mode of microcontroller I use is fine in this design.

The connection and footprint of the photodiode is ok. I got response from it but still no good signal at the output. I just get noisy signal which is sometime high close to Vdd and sometimes low near to zero. There is signal from Vin-, it's alomost zero.

Attached is the datasheet of the photodiode.
 

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alec_t

Well-Known Member
Most Helpful Member
If the LEDs are lighting, then the circuit you actually have isn't the same as the one in your post #1 attachment (where you show the LED anodes grounded).
I agree with Jpanhalt about the pinout of the photosensor. The datasheet pinout differs radically from what is in post #1. Pin 3 should be a No-Connect.
Also, shorting the outputs of the two sensors together might not be advisable.
 

jpanhalt

Well-Known Member
Most Helpful Member
Can you give a link to the original schematic that you modified. I appears to be a common design, but having that available would avoid guessing.

If this is it (Source: http://www.instructables.com/file/F57X6S2IRRLFFN8/?size=ORIGINAL ):
upload_2017-10-6_6-42-31.png

The source is the problem.

Here is the same section from an Elecrow marketed device (https://www.elecrow.com/wiki/index.php?title=File:Crowtail-_Pulse_Sensor_eagle_file.zip ):

upload_2017-10-6_6-46-53.png

Other than the pin numbering, the designs are essentially identical. VCC is Pin 1 and the numbering is conventional. If your connections are as you show, it simply will not work. Pity that Instructable doesn't have editing. It appears the numbering in that source refers only to the connected pins and arbitrarily adds a number for a non-connected pin.
 

Alfred_27

New Member
alec_t, thank you for your comment,
I have the same circuit attached. The connection is correct, I just changed the output drive mode of the chip. I got LEDs lighting no problem.
Below is the link of the footprints of the photodiod, its similar to footprints of the circuit. You may confuse with the pin numbers of the component, but it is only in the schematic, but PCB footprints are same.

https://rs.componentsearchengine.com/part.php?partID=326324

I will try to disconnect the output of the two PD, but I do not think that is the main problem.
 

jpanhalt

Well-Known Member
Most Helpful Member
What happens when you disconnect one of the photodiodes that are in parallel? Say, disconnect the left one and see if the one on the right works when it is alone.
 
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