I am using a 7.5 volt external power source to power my UNO and I can't get the program to start. It starts when using the usb cable. I've tried connecting a 10K resistor between GND (next to D13) and RX but it still doesn't start. Any other ideas?
Also: you can use only 3V and it still work. The level of the Vcc depends only on the speed of the processor. Naturally, I am assuming that no other external component which demands a higher voltage level is involved in this test. Try to do the simple test with Blink application.
do I need to comment that out when running not connected via the usb cable?
You do NOT have a too low voltage problem it is, I am sure a connection one.
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