Reading N.C/N.O switch 28v output with Arduino

Using Arduino General Electronics
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Hi guys...
Firt apologize me if I say something stupid, my basic knowledge of electronics was lost long time ago.
I'm working in a pet project for creating a security system. I'm trying to connect to an external motion sensor like Optex QXI-ST. This device has 3 different outputs for alarm, tampering or problem:

image
image875×148 77.3 KB

My concern is they are NC switches of 28Vcc but only max 0.1amp. In order to read it using a digital input in Arduino, would be enough to build a voltage divider with 2 resitors to decrease the voltage to 5v before the Arduino input?. or what would be the most reliable connect it?.
Thanks in advance for your support.
Regards,

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Most/many contacts are usually dry.

Use a DMM to confirm there is no voltage on them.

The 28V @.1A is probably the maximum rating the contacts can handle.

1 Like
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Yes! Resister divider is a best thing. But, the problem is accuracy. Is your setup going to be outdoor or indoor? Will it be exposed to the hot temperature all the time?
Two options here:

  1. Use reguarl carbon resistors. They may not be the best accuracy but they do the best job as long as the temperature is under control.
  2. Use Caddock 1776-C68. This can work in high temperature environments as well but bit expensive. Caddock's Ultra-Precision Voltage Divider Networks
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Motion sensor will be located outside, but controller electronics will be located inside the house so won't be affected by big changes of temperature.

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I confess I need to look for what a dry contact is. Having clear a "bit better" the big picture, I don't have the motion detectors at hand at this moment, I was preparing the "environment" before that. I'll try to order at least 1 of them for doing your suggested test.
My concern now is cable length/size. Motion sensor would be located outside and microcontroller inside. Worst case scenario is 20m length would be needed, as average 10m (one Arduino reading 6 motion sensors). I'm not feeling confident this could have some impact in the accuracy/stability of the reads because of cables acting as a big antenna.
Regards,

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IR Swnsor Connect
My read here is you have the QXI-ST version. The connector is shown in the image. The ST version does not have a Trouble output. The Alarm out looks to be just a dry contact switch between COM and AL . There is no 28 Volts present, the 28 V 100 mA is the Max the dry contacts can switch. The Alarm Out is dip switch selectable for N/O or N/C. The Tamper output is normally closed and goes open when the unit cover is removed.

Ron

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Thanks for the feedback! I'll do some testing when my unit arrive. Do you think N/O - N/C contacts reads could be affected by cable length?

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Cable length? Not likely within reason. Reason being not running 1,000 feet of cable. Look at the gauge of the cable and resistivity for the length you plan to run. You can also consider the N/C actual resistance.

Ron

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