I have a screen that needs 3V wired to one breadboard, and the rest of the sensors need 5V, connected to another breadboard. How do I turn the arduino on/off by using one switch?
You realize that all the wires going to the screen probably need to be levelshifted to avoid damaging the screen, right? Unless specified otherwise by the applicable datasheet, applying voltages higher than Vcc or lower than ground should be expected to damage the part in question.
Why can't you just put the power switch before the Arduino, and turn the whole thing on and off at once?
One might be tempted to use a two pole switch to turn power to the two breadboards off, that's not a solution - consider what would happen; Vcc would essentially be zero (once the caps discharge and everything), but you've got other wires going to the Arduino. If the Arduino is trying to set one of those wires high, that would be holding a pin higher than Vcc on that device, which you shouldn't do (this will result in current flowing through the protection diode to the Vcc line, so the device can actually end up being powered in this way - but the protection diodes weren't meant to carry much current, so damage to the part will often result in such a case).
Power the Arduino from the external power jack using an external 9V or 12V supply. Switch THAT supply on and off and all arduino power is switched.
DON'T turn the external power off until you have uploaded code and UNplugged the USB.
An Arduino-compatible with higher 3,3V and 5V current like THIS: may be a help also.
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