I have two Arduino-based projects located within 30 feet of each other ("A" and "B"). "A" controls the Air Conditioning but can not report the status of the AC. "B" has a FONA and can text messages. I want "A" to tell "B" when the AC goes on and when it goes off. My method of doing this is to connect the grounds of the two Arduinos and connect a pin on "A to a pin on "B". When "A" starts the AC, it performs a digitalWrite HIGH on the line. When the AC goes off, "A" performs a digitalWrite LOW on the line. "B" is monitoring the line every second.
"A" does a digitalWrite HIGH and "B" picks it up immediately. However when "A" turns off the AC and does a digitalWrite LOW, "B" does not receive a low (zero) but rather it continues to read a HIGH for quite some time after the LOW was written by "A" .
Why does "B" not immediately see a LOW?
NOTE: if I change "B" to do an analogRead it reliably returns a 1023 for a HIGH and something less than 1000 for a low.
Adding "PULLUP" to the pinMode seems to have no effect.
Thank you for your consideration.
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