72 yrs old. Novice to Arduino. Able to upload BMP180 sensor sketch to Arduino Uno and get results. Disconnecting USB from laptop and using external power supply "freezes" Tx LED light(usually 1 per 10 second flash) and I assume that there is no cycling. Reconnecting USB supply from laptop unfreezes. I expected independence once sketch is uploaded to Uno irrespective of power supply. Eventually I want to connect a display and have a standalone barometer.
Yes, the Arduino normally runs stand-alone. The "regular" Arduino can be powered via USB, via the 5V pin, or with 7V or more into Vin with.
You have to be careful about Vin if you are powering "other things" through the on-board 5V regulator because it can't handle a lot of extra current. I don't know anything about that particular sensor but most sensors don't need much power.
Is there some kind of LED display or something possibly "loading" the on-board voltage regulator?
The makers of the Uno have finally corrected the way you should power the board:
5V. This pin outputs a regulated 5V from the regulator on the board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V). Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it.
Not healthy to supply regulated 5 volt to the 5 volt pin? That's against all forum advice so far.
Arduino are being cautious, assuming anyone reading that page is probably a beginner.
Powering Uno via the Vin pin provides a level of protection to the board, in case an inexperienced user accidentally or through ignorance connects a higher voltage than is needed.
When powering Uno through the 5V pin, there is almost no protection, anything over 5.5V could damage the chip.
What is the voltage and current rating of your external supply? How is it connected to the UNO? How many "things" are connected to UNO's I/O pins, 5V and 3.3V pins?
That statement is not technically correct. The act of "bypassing the regulator" cannot cause damage to the Arduino. This is just CYA in action.
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