I have an application which needs to detect the state of a relay contact "out there". I need to detect the following three conditions:
1. Relay is connected, and contact is open (relay not energized)
2. Relay is connected, and contact is closed (relay is energized)
3 Relay is not connected.
The output should be a 2-bit result showing the 3 possible states (there's room for a 4th state, obviously, which is not used.
Now, the way you detect the difference between cases 2 and 3 is that you have what's called and "end-of-line" resistor connected directly across the relay contacts: some value like 10k. And you drive the relay circuit from relatively high resistance source, maybe again 10k, driven by say, your 3.3v VCC. In this configuration, the above conditions would reduce to:
1. Relay connected, contact is open, voltage across lines should be 3.3 * (10k/(10k+10k) ~= 1.65V,
2. Relay is connected, contact is closed, voltage across lines should be ~= 0V,
3. Relay is not connected (and thus the resistor is not connected), voltage across lines should be ~3.3V
So, I need a 3-window aperture detector, and it seems like the TPS3700 is a good candidate for such. It seems like it could be configured to tell me which of three windows the input is in, and outputs an appropriate 2-bit result
Is the TPS3700 a good candidate to use in such an application?
Is there a better source (better suited, maybe as good but cheaper?)?
Some details: the drive for the end-of-line resistor originates within the system I'm designing itself, and it seems the VCC is a good source for such. Typically, the EOL resistor will be sitting at the end of 20-30 feet (at the most) of 16-18 gauge solid copper wire, so line losses are largely a non-issue. And, sitting between the real contact(s) and the window comparator is an 8-channel analog MUX, so I can sequentially monitor the state of up to 8 contacts (those lines not connected to relays will have an EOL resistor installed). "Sampling rate" will be low: on the order of 10/sec. All measurements of EOL state should not require the drive voltage to be accurately known: simply whether the relay is open, closed or (it and the EOL resistor) is disconnected is all that needs to be known.
The sage advise of the assembled experts would be appreciated
Also (maybe for another thread) any recommendations for the analog MUX in such an application?
Thanks.
