Current source for pulse oximeter purpose

Thread Starter

Dever

Joined May 2, 2022
28
Hi,
I'm designing LED driver in my pulse oximeter. It would be two LED's, I would like to auto adjust current by DAC and be able to turn off one LED at time. I came up with an idea. In simulation, I got what I wanted. What about real circuit? The mosfet is actaully IRLML6244TR. 5V is for. ex. MAX619 from two AAA batteries. DAC has 3.3V logic and is powered by 3.3V DC-DC pump.

LED are IR and RED SMD, max 20mA.

1661628368488.png
IRLML6244TR
 

crutschow

Joined Mar 14, 2008
31,091
So are you building duplicate current drivers for each LED?

For 20mA LED current the DAC output will be 2V in your circuit.
 
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Thread Starter

Dever

Joined May 2, 2022
28
Yes, I'm goind to duplicate this circuit.
To turn it off, I thought about set 0V on DAC. I couldn't find opamp with enable pin.
 

DickCappels

Joined Aug 21, 2008
9,501
Operational Transconductance Amplifiers are like op amps with a gain control for the output but that is probably not an efficient way to turn LEDs on and off. What would you do with the enable pin? There is probably an easier way.
 

Jerry-Hat-Trick

Joined Aug 31, 2022
204
Yes, that will work if you have enough DACs.
I've controlled an SPO2 finger sensor using a Speeduino XIAO which only has only one genuine DAC - the other analog outputs are PWM. I just used an npn transistor emitter follower to drive the current - using the DAC output from 0.6 to 3.3V which gave enough resolution and selected which LED with pnp transistors above the LEDs as the XIAO has ample digital I/O
 
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