Could sending 8v AC down twisted pair telephone wire


#1

Quick one: Could sending 8v AC down four core twisted pair telephone wire (the other two cores being used for a button input connected to a Photon using the internal pullup) be enough to cause ANOTHER input on the Photon to falsely trigger?
To be clear: the other input is not in any way connected to the telephone cable, it’s on its own, separate short length of cable. However, two cores of the telephone cable are in use by an input on the Photon. To repeat myself - it isn’t THAT input that’s falsely triggering! :wink:
Thanks!


#2

I don’t quite understand, are you putting 8V AC on a pin on the photon or are those 2 8V AC lines not used by the photons (but are in the telephone wire)?

If the former is the case you might have damaged your photon,it is only 5V tolerant.

If the latter is the case, crosstalk in twisted pair could be the case if used improperly although I have no experience with this myself.


#3

It’s the latter case. The 8v AC lines are not used by the Photons but seem to cause Crosstalk…but what’s odd is the crosstalk doesn’t appear on the pin I would expect (the other half of the telephone cable which IS connected to the Photon.


#4

What pin is the telephone cable connected to and what pin is reading (what appears to be) crosstalk?


#5

@netpex, can you share a sketch/schematic of your setup. Besides crosstalk, long wires will also act as antennas. BTW, internal pull-ups are quite weak (~40K ohms) so for long wires (over 6 inches), a strong external pull-up (2.2K to 10K) is recommended.


#6

Telephone wire is rated at 600 ohms impedence. It needs to be terminated at 600 ohms at the far end and the source needs to be 600 ohms. The pairs in the phone cable are not shielded between each other so without the proper terminations you will have crosstalk.