Detecting analogue signal change?

[D_Raz]

I have a LDR monitoring an LED that comes on for 2 sec and off for 1sec. (see the attached waveform)

At the moment I can detect (via a comparator) if the peak has changed to detect if the LED is on.

However the tricky part is detecting if the pattern of the on/off period has changed, hence if the LED is on for 5sec and off 3sec, the change of rate should be detected.

So if the signal doesn't match the waveform (both x axis and y axis ) then the user must be alerted.

I'm lost on how to approach this issue

Any suggestions will be appreciated.

I'm using labview 2011 and DAQ 6009

[crossrulz]

Duplicate post.  Please keep it in 1 thread.

[odessy27]

If this is a periodic process, then there are couple of ways to go at it. One way is you store one cycle (2+1 = 3 Seconds) in an array and then every 3 seconds compare (Is equal or difference is zero) and if they are not equal add an alarm indicator. This however relies on the response being exactly the same everytime.

  The other crude way of going at it is, if 2 sec data >0 and 1sec data=0 then you are good, otherwise indicate that cycle is broken.  

[D_Raz]

Thank you for the tip, as I’m new to lab view I’m not very knowledgeable in arrays, have you got an example where you can store cycles of a signal and compare.

 I will much appreciate your help.