Monitoring a contact closure

[Tony617]

Hi,

I can't find anything on how to set up a DAQ to detect a contact closure.  I have the M Series USB-6221.  I'm not familiar with DAQ's or Signal Express so please include the basics when explaining stuff.  Please help!

I have a fridge in the lab that outputs alarms via a contact closure.  I want to record the activity of the alarm (when and how long it's triggered).

I thought I could maybe connect the 5V output of the DAQ to the contact closure and measure voltage on the DAQ but I took a multimeter to the 5V output and I'm not reading any voltage.  Do I need to turn on the 5V output?  I had the DAQ measuring temp from a thermocouple when I tested the 5V so the device was operating, not just sitting there idle.  Do I even want to use the 5V output for this application?

The fridges alarm output has three terminals: Common, Normally Open and Normally Closed.

What do I need to connect to where?

Thanks,
Tony

[Dennis_Knutson]

You can use a digital input to detect the relay state. Connect the 5V through pullup resistors to the nc and no terminals. Connect gnd to the common. You'll read a logic 1 when a contact is open and a logic 0 when a contact is closed. Basic electronics.

If you can't measure the 5V output, you connected the meter incorrectly.

[SteveD123]

Dennis

You need to add that you connect the DIO pin or multimeter to either the NO or NC, but not the COMMON.

Tony

Relay terminals are notorious for briefly bouncing between states during a transition, because of the inherent mechanical vibration. If using any edge triggered logic, i.e. to detect a state change, you will need to debounce the contact. For instance you might want to count the number of separate times the alarm is raised, but not consider anything under 1 second duration a real alarm.   You can do this in software, or google "555 debounce circuit".