how do I de-multiplex and sample signals that shares 1 input analog channel?

[student researcher]

Here is the deal: There are 2 LEDs(1 RED and the other Infrar Red) share 1 light sensor; I need to have a sampling sequence of RED on and IR off, IR on and RED off, and both OFF and then repeats.
Since they share 1 light sensor so there is only 1 analog input to my DAQ board.
How do I set up the I/O control in LabVIEW so I can sample in this sequence and de-multiplex the samples into 3 viewable graphs? And how do I synchronize the output turning signals to the 2 LEDs with the sampling?
I will be very appreciated with any insights,help, or suggestions. This has been bothering me for days and I need to figure it out in less a week.

[Dennis_Knutson]

Is there any difference in the sensor output depending on which LED is on? For example, do you have x volts for the red LED, y volts for the infared, and something else for both on? If there isn't, I don't see how you could differentiate between one or the other being on.

[student researcher]

There will be very small difference in the sensor output depending on which LED is on, and when they are both off, the output will have a distinctive smaller voltage range.
So if this is not doable in LabVIEW, I need to use an intermediate circuit that will control the LED turning on/off sequence, split the 1 output line into 3 analog input channels to the DAQ board?

[Dennis_Knutson]

Maybe you could explain exactly what you need to measure. If you have to verify that only one LED is on at a time, then I don't think you can do it with only one sensor or only one that can't differentiate. LabVIEW has nothing to do with it. If your aim is to simply turn the LEDs on and off in a specific sequence and don't care much about verifying exactly which one is on then its doeable.

[student researcher]

I do need to know exactly which one is on. Briefly, my project is a sensor that will detect blood oxygen saturation level. The sensor needs to detect the concentrations of hemoglobin(potential oxygen carrier) and oxyhemoglobin(oxygen carrier)in the blood. They have different light absorption patterns due to different wavelengths. That's why there are 2 LEDs, Red and Infrar Red (2 equations 2 unknonws)and it is easier to sample 1 LED at a time(a 3rd sampling when both are off is needed to discard the effect of ambient light).
So I want to control the LEDs turning on and off sequence and asscociate the ouput with its respective sampling in LabVIEW. Is this possible? If not I would have to build a intermediate circuit with a micro
controller.
Thanks for your quick responses. I am really appreciated.

[Dennis_Knutson]

So the sensor isn't for the detection of the LED itself but the light that passes through some sample? If that's the case, then LabVIEW and a DAQ board should be able to do the job. I'm not at a computer with LabVIEW right now so I can't post an example but you would have a while loop that would cycle through the three different control signals. In the loop would be a single measurement. The result of the measurement would be placed in one of three different arrays. Cycle one results would be placed in array 1, cycle 2 in array 2, cycle three in array 3, cycle 4 in array 1, etc. A little modulo arithmetic on the loop count and a case statement with three different numeric cases should be all that you need. If you need an e
xample, I'll see if I can post something when I'm back at work on Monday.

[srm@viewpointusa.com]

If you can sequence the LED/acquisition sequence, Dennis's approach will work fine and is the easiest. If you need faster or continuous acquisition, acquire data from your sensor on channel 0 and digitize the command signal for each LED on channels 1 and 2 using multi-channel acquire. you will be able to decode the data on channel 0 by looking at channel 1 and 2.