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I have a micro board attached to two separate buttons on a heat press. One wire on each of the buttons connect to A0 and A5, the other connects to a separate ground pin for each button. The buttons give off a constant 5v until pressed, they then drop to 0v. Heat press example:http://hotronix.com/heat-press/air-fusion-heat-press

My sketch is checking for voltage on the buttons, when there is no voltage, it sends a serial signal to the connected PC.

Everything works exactly as expected when the computer that the Arduino is connected to is turned on. But, If you unplug the Arduino from the computer, or you turn off the computer. The wires that connect to the Arduino to the heat press ground out to each other and cause the heat press to activate.  Any ideas on how to keep the Arduino from grounding out these wires when not powered?
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Read up on "pull-up resistors" - it may be a solution to your problem.

-br
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unless you added a resistor 1-10k to gnd.. what do you think happens when + and - touch? it's called a short.... if power by the 5v rail, your pc should protect it but you could damage something...
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You need to find out the value of the current pull-up resistors in the heat press (measure the current across
the switch terminals, R = 5.0 / I.

Then choose resistor value say 20 times that, and place between the switches and the A0 / A5 pins - that
way when the Arduino is powered down it won't clobber the pull-up current.

Whenever a CMOS chip is powered down its inputs are diode-clamped to ground by the input protection
diodes.
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