Personally I would use a small PicMicro and using a timer with an input interrupt from the photo detector.
The way an anemometer is often constructed.
The relay would be triggered from a pic output.
The DAQ route would not have been the path I would have taken.
Max.

Does what you described involve a NI6008 board? Because although there might be other 'better' ways, I have to follow the guidelines I was given.

Can you describe what you said in a little bit more detail please.

 

I'm confused.
If you use the NI6008 board , what is it that you need?

The optic device described should interface, more or less, directly to the NI6008 board.
The rest is programming.

If you just need to trigger a relay from optic interupter at a set speed, then the NI6008 board seems unnecessary.

 

Personally I would not have thought that the NI unit was the most practical approach unless you intend to interface to some PC S/W such as Labview etc?
I was thinking more in terms of a small Micro processor using a method similar to shown in the attached PDF, I recently lifted some of the code out to embed in another similar application similar to what you are doing, going a step further and using the capture feature, the code shown was a simple self contained pgm so no capture/interrupt was used as the program simply loops around, the count range can be extended easily.
Also you can eliminate the need for the 3 digit display for your usage.
It uses a 32mhz clock crystal for precise timing, and uses a one count/rev detection of anemometer etc.
The code is in assembler, but it should be simple to convert to C if needed.
If you require the code, I can either post it here or send via email.
Max.