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Interfacing a switch with an end of line resistor

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AverageGuy

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I've been asked to come up with an interface to a switch with an end of line resistor. I'm planning on using a I2C parallel port IC (8 bit) communicating to a ARM micro processor and am considering using a couple of programmable Schmitt triggers to sense open circuit vs close circuit vs a normal unswitched condition. One trigger set to fire at or near VCC, one trigger set to fire at or near ground. Is there a better way to do this?

If this is a reasonable approach does anyone have any suggestions as to which part (IC) might be a reasonable choice for a Schmitt trigger?

Thanks,
Jim.
 

AverageGuy

New Member
I'll try again. For background, alarm switches used in the security business frequently have a resistor across the switch. The purpose of this resistor is to help detect a switch being disconnected. So you need to be able to sense 3 conditions with respect to the switch. Switch closed, alarm condition, switch open, someone has disconnected the switch and you need to alert someone and switch open, but resistance present, all's well.

Hence the question of how to detect those three conditions. I'm proposing connecting one side of the switch to VCC via a resistor, the other side connected to ground, so a normal operating condition would provide a voltage somewhere between VCC and ground. I suggested two Schmitt triggers one biased to sense a transition near ground when the switch is closed and one near VCC which would sense a disconnected switch. Normal operation would have the output from one trigger a logical 1 and the other a logical 0. Two logical 1s or two logical zeros would indicate either an alarm or a fault. Which is which depends on the inverting or non inverting property of the trigger.

That was my suggested design but I was wondering if anyone had a better design or if anyone saw any problems with this design.

Thanks,
Jim
 
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