Is demultiplexing red and infrared signals from a single photodetector can be done in LabVIEW?

[Annj]

Hi!
Anyone can point me out is this possible ? I understand that there is an analog solution for this that is by using sample and hold circuit. But can it be done in the LabView environment?

[RavensFan]

How do you demultiplex two different colors with one device?  All colors that make up the light source are going to fall on the photodector simultaneously.

[Annj]

Sorry I don't make my question clear before-let me clarify here. The concept of the project is similar to pulse oximeter. The Red and IR LEDs are lit up alternately. In other words, when the red led on, the IR led will be off and vice versa. So, each time one of the Led on and shone through the tissue, it will carry information back into the photodetector.

Now the photodetector will acquire both signals in different time. So can I demultiplex the information carry by those LEDs? If possible, what method that I can use? I'm not sure if I can use phase locked loop..please shed some lights on this matter. I really appreciate.

[johnsold]

You need to have some means of knowing which LED is on at any specific time. Once you know that , then recommendations for separating the signals can be made.

Is one of the LEDs always on or is there a period when both are off? Are they ever both one simultaneously? Do you have access to one or more signals from the LED drivers?

Lynn

[altenbach]

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@Annj wrote:
Anyone can point me out is this possible ? I understand that there is an analog solution for this that is by using sample and hold circuit. But can it be done in the LabView environment?

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LabVIEW is a full featured programming environment, so everything is easily possible.

You need to tell us a little bit more about your instrument.  Is the LabVIEW program also driving the LEDs or are you just basically measuring a square wave at the frequency of the alternating LEDs? Is the frequency known? Is one of the values always much smaller than the other or do you have a sync signal to define the phase?

[Annj]

Hello altenbach!

Yes, the LabVIEW is used to drive the LEDs. Since I'm using time multiplexing, so the Red led will be lit up first. After 540us, the IR led will be lit up. The period for both LEDs is 2000us or 500Hz.

I am not exactly understand your last question on sync signal to define the phase. I hope the above explanation is adequate. Hope you can share your opinions with me.

Thanks 

[Annj]

Hi Lynn,

Sorry I did not notice your reply. The LEDs will lit up alternately. The frequency and duty cycle for both is 500Hz and 11%; respectively. The timing diagram is like this:

 ___                          ___

|     |______________|     |_____     Red

               ____                           ___

 ________|     |_______________|     |____  IR

I drive both LEDs using Labview. The IR will lit up after 540us has passed. Please share your view on how can I separate the signals. Thank you