LM358 as a comparator (Pulse-oximeter)

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dhananjaya

Joined: 22 May 2009
Posts: 2

02 Jun 2009 10:09 lm358 comparator

I designed a pulse oximeter and it's working fine with LM 358 as the aplifier. But the output I get is only 3.68v. I tied this to an LED through a resistor so I can visually see the heart beats.

But I can't read this output from PIC 16f877. So I used another LM 358 as a basic comparator and tried to get a 5V pulse. But still the output I get is 3.6V.

What can be the reason? Can someone help?

These are my circuit pins.

LM 358
V+ (Pin 8 )= 5v
GND(Pin 4)= GnD
VA+ (Pin 3) = Input (3.6v)
VA-(Pin 2) = V reference 1.5V
Vout (Pin 1)

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banjo

Joined: 24 Dec 2005
Posts: 644
Helped: 118

02 Jun 2009 13:04 lm358 as comparator

The LM358 is not a rail to rail opamp. What this means is that it cannot swing its output pin to the power supply rail. Basically the upper transistor can not be saturated, so it always has some voltage drop.

The options are either switch to an opamp that does have rail to rail output, or try adding a pullup resistor to the Vout pin of the comparator. Place and 10K resistor from pin 1 to +5V. When the comparator is driving high, this should increase the max output voltage. When the comparator is supposed to drive low, the opamp can easily sink the extra current provided by the pullup resistor.

If the pullup resistor does not work for you, look for a rail to rail opamp in the same pin configuration and package. When reading the spec sheets, look at the maximum voltage swing parameter to determine how close to the rails you can swing.

Freddybaby

Joined: 27 Sep 2007
Posts: 27
Helped: 2

02 Jun 2009 23:40 lm358

The PIC can read both HIGH and LOW so why not make things simple and use a 2N3904 or similar and modify the software ?

dhananjaya

Joined: 22 May 2009
Posts: 2

04 Jun 2009 8:08 pulse oximeter circuit diagram

Hi guys, I managed to solve the problem. Thanks for the advices and help.

ArslanNihat

Joined: 23 Jun 2009
Posts: 3

07 Jul 2009 11:31 lm358 voltage comparator

can you give me the circuit diagram you are using ?

millwood

Joined: 02 Jul 2009
Posts: 121
Helped: 11

07 Jul 2009 12:37 comparator lm358

a 877 will have sufficient current capabilities to drive most leds. and if that doesn't work (for example you have a super duper HB led), you can use a small transistor (2n55551/5401) to buffer it. no reason to resource to something like an opamp.

edit: on 2nd read, you are trying to read the output, digitally?, via the pic. 3.6v should be enough to trigger a logic 1 for a pic.

gideon.levitt

Joined: 06 Oct 2009
Posts: 1
Location: Washington, DC

06 Oct 2009 1:56 Re: LM358 as a comparator (Pulse-oximeter)

Hey,
I am trying to design a pulse oximeter for a project i am working on, i saw one of your posts, and i think you could maybe help me..

you mind helping me out?

Thanks