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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?
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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?

7.5 into the external power connector may be marginal as there is a on-board series polarity protection diode that will drop .6vdc. If I recall correctly the auto voltage switching circuit needs to see => 7.5 volts at the opamp comparator before it will switch to external power. Don't have a Uno board so maybe this info just applies to the prior Arduino board.

Lefty
« Last Edit: December 30, 2010, 05:09:39 pm by retrolefty » Logged

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It was working with only the external source after programming it to alternate power to two motors.  Since then I have changed the pin outputs from the analog outputs to the digital outputs, thinking it wouldn't matter.  This must have something to do with it since this is my only changes from when it worked.
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Oh...and the external power source is 5 AA batteries, so it's probably a little closer to 8 Volts.
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When everything is connected up, what does your voltage read from the batteries?

A no-load measurement of a battery is pretty much meaningless.
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Assuming that USB connector is open, if you plug the battery at +5V pin and GND pin it must work. I do not have a Nano, but I saw at the schematics and I cannot see why it will not work.

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.

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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.
The main risk in this is if you then do plug in the USB to the PC and forget to disconnect the battery, you will have The PC's +5vdc hardwired to your 3volt battery (or other voltage source), and that is not a good situation for several reasons.

Lefty
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It measures only about 6.9 volts with the load applied.  Maybe I need to add the 6th battery.  Another difference between when it was working and now is adding the Serial.begin(9600); line in the setup loops, do I need to comment that out when running not connected via the usb cable?  Thanks for the responses.
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do I need to comment that out when running not connected via the usb cable?

No, that is not a requirement for running a sketch without being wired to a PC.

Lefty
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When you connect the batteries do any leds come on? If not one of two things is wrong. The + and - is backwards OR the connector you are using isn't really the right power connector but one that has a larger inside barrel and the center connector isn't well.. connecting.

You do NOT have a too low voltage problem it is, I am sure a connection one.

Kevin
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You do NOT have a too low voltage problem it is, I am sure a connection one.

I think that might be an opinion rather then a fact, but that's just my opinion. smiley-wink

 I think 6.9vdc is too low for the auto-voltage switching circuit on an arduino to function correctly.

Lefty

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thanks
the same problem i was having
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