time measurement when voltage is below a certain amount

[ramone2285]

In a project for class I built a pendulum that swings through an infrared emitter to a detector. when somethings cuts the beam from the emitter, the voltage drops to virtually zero.  I need to be able to measure how long the voltage was at zero for my project, but can't for the life of me figure out how to do it.  I really would appreciate any help!

[Wayne.C]

ramone2285,

What kind of hardware to you have available to make the measurement?  If it is a DAQmx device, then look at the Meas Pulse Width.vi example that ships with LV.

[ramone2285]

i'm using a daq device (i'd have to be in class to tell the exact version).  I'm reading the voltage on a channel I made, and the voltage bounces from 0V to 1.6V when the beam is cut. 

one idea i came up with was to setup a comparison where a caution/stop light would light up whenever the voltage was below 1V, and then have something measure how long that light was on, but I could find a vi that could do that.  I'll definitely give that measure pulse width thing a try, cuz I need to hurry up and finish this project already.

[ramone2285]

ouni think I found a way to do it. I went through the help context and found an example of how to measure pulse width.  They say i have to create a channel on my Daq (NI Daq 6251) to do the readings, which ends up being on the PFI9 port. All I need to know now is how exactly do I get a reading from that port on the daq device? it only has one hole, so where is the negative leg of my wire supposed to go.

[Jared_B]

ramone2285,

What wires are you connecting up?  I'm assuming you'd have the signal connected to the digital input and ground.