I think that would be a little difficult for me in the first place.
Maybe I should stick to your first solution, just plugging battery into barrel jack.
Can I please ask, could my modification with two bucks, one set to 9V for Arduino barrel jack and one set to 5V for relays, cause any problems?
Basically, what you are trying to do as shown in post #1 will eventually fail either due to heat, vibration, electrical overstress or EMI.
You can try it but don’t depend on it being very reliable.
Is it due to voltage or current or both?
I fully understand that. But at this point, starting the project I mostly want to see how the sensor reacts. I know nothing how ultrasound bounces off road surface, for example.
There are so many possible sources of error I can not prevent them all.
I will try to emulate the genius Elon Musk's tactics for his famed Starship "It exploded again. We learnt a lot from this explosion." I am sure his present setbacks are only temporary. I do not have time, knowledge or resources to use NASA tactics from the 60's: First shot at the moon - success.
But seriously I am very grateful for your help so far and this thread is solved. The project is limping on and next time I run into a hurdle I will ask for help here again.
Thanks a lot.
Only because you don't have the technical skill.
For the bucks don't buy the adjustable type. The little pot that adjusts the voltage can cause erratic voltage outputs.
Also don't go overboard on the currentrating. The Uno does not use a lot of current so try to find one with a 1A rating
That is correct.
Thank you and everyone for many very useful advice. I just have to walk on slowly and ask for help when I stumble.
Right now I will try to test the system on my desk. 9V from a grid transformer to Arduino barrel jack and also to a 7805 stabilizer and 5V further to the peripherals.
@jim-p
You know this field and the advice appear very good. Just to try out connecting outside USB I connected a grid transformer to barrel jack and also diverted it to a 7805 for Bluetooth and Ultrasound. Without load it gives a perfect 5V.
When I connect the three boards and gradually raise the current it works fine around 8 V for the grid transformer. 4V at US board. When I raise to 11V the US (JS SR 04T) stops flashing and BT can not connect until I lower voltage again. The voltage seems lower (!) at the US card when I raise input to 7805 from 8 to 11V.
Then put a fixed voltage 5V buck. Much better result, but if I raise grid transformer voltage slowly it can also fail.
All advice welcome. Getting late here. Reply will definitely come from me, but maybe tomorrow.
That can happen when you slowly change the voltage.
Just set it to 9V and don't adjust it while it is connected.
Quite a few new post but to answer your question in post #35.
Here is how it plays out. This is a partial of an Arduino Uno Rev 3 board:
Blockquote The Arduino Uno 5V pin acts as a regulated output (when powered by USB/VIN) or an input to power the board.
So we can power the board using the 5.0 volt pin. 
I would just use a buck converter to get my regulated 5.0 volts for my Arduino and any other 5.0 volt boards (sensors or whatever). Just make sure your buck converter is rated for the current demand you will have. I would do everything using 5.0 volt supply. The 7 to 12 volts is applied upstream of the regulator so why apply 7 to 12 volts? The schematic I posted shows how things are on the board.
Ron
Directly from the official Uno documentation:
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.
If I turn up the input voltage to 12-14V, like a bike, all seems to work. I keep a finger on the Arduino voltage regulator, definitely not hot, maybe a little warm. Now stable 5V at US card.
And the buck is fixed 12 to 5 V.
So I am slowly navigating forward and @jim-p :s advice to just run Arduino on battery and the relays on a fixed voltage buck may be the best.
Going to sleep now.
Thanks to all contributors to the thread.
Is that really true ?
Should that read :
Supplying excess voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board.
Yes
If the board is powered by USB then there will be 5V present at the 5V pin and 3.3V present at the 3.3V pin
Is that different when powering the board using the 5V or 3.3V pins ?
The advice from Arduino is quite clear, but what I am trying to understand is why it matters whether the 5V comes via the on board regulator or from an external source
Well Arduino does not specify the requirements of the 5V supply such as ripple, noise, transient response, voltage tolerance, etc., so you could easily damage your Uno with a bad supply or just experience erratic operation.
You could also damage the onboard regulator if you should accidentally or on purpose have something connected to Vin at the same time. You could also damage your computer and/or Uno if you should have a low resistance source connected to 5V output when connected to USB.
I seem to have fried one board. I was running the 5V as input via the 5V port. It came from a small grid transformer that I carefully turned up to 5V. Whether that fried it or it was damaged when I applied hot air to shrink large diameter shrink tube around the Arduino board itself, I don't know. The board was new but a manufacturing defect is less likely, I guess.
So I will use the Vin pin and barrel jack from now on. I take the inconvenience of having to supply 7-12V to Arduino and also 5 V to peripherals, instead of 5V to all.
And just "adding a 7805" to a 9-12 V supply did not turn out so well for me. Inconsistent behavior. But should probably have gone more in depth on that. Now I use a 12->5V buck that seems reliable if it gets 12V.
But the only experience I draw from is this that happened during this, my first project.
Don’t feel bad. As you can see from the many responses here, even experienced users are confused about the proper way to power an Uno.
Of course, all sorts of things could happen as they could with anything connected to any pin on the board if it is used incorrectly
As a matter of interest, what is the specification of the 5V output on the board with respect to ripple, noise, transient response, voltage tolerance, etc.
I ask because I don't want a sensor being powered from that pin to be damaged
