I have had a rough time trying to understand why this isn't working correctly. Quick background: I set up a system to pop open car doors using touch sensors. I heard that powering the Arduino Uno R3 straight off the car battery is not a good idea, so I bought a buck converter. I wired it up and calibrated the output on the converter for 5V. When I hooked it up to the Arduino (via the barrel jack), I experienced some odd effects. The output pins and the 5v power pins were only getting 3.5V. I had to adjust the buck converter up to about over 7V in order to get 5v on the Uno.

Then the sensors started acting oddly. Auto triggering and alternately not working. Anyone know what I am doing wrong?

Converter I bought: https://www.amazon.com/gp/product/B0159W0UL2/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1

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    Connect the output of the buck converter directly to the 5V (and GND) pin, thereby bypassing the voltage regulator. Or do as Michel said. I'd suggest the latter, if you are running from a car, as the battery voltage in a running car can have a lot of spikes. – Gerben Apr 23 '18 at 15:41

The barrel jack needs 9-12 V, the internal voltage regulator of the Arduino will reduce this to 5 V.

Since you only supply 5 V, there is a loss in the internal voltage regulator why you probably get only 3.5 V.

And after applying 7 V, probably some (sub) system gets less than the necessary voltage, or there are occasional 'downspikes' lower than 5 V.

The default adapter that you use for supplying voltage to the Uno, also is 9V-12V. Also, check the polarity.

See the following link for more info:

What adapter

  • I did not know about the 9V. I just assumed that since the board ran on 5v, it would need 5v in. Makes sense now, but I still don't know what the converter should output to maintain the Uno's optimal 5V bus, while maintaining minimal drain on the car battery (efficiency). Is there an efficient ideal voltage to output? – Matthew Paulin Apr 23 '18 at 12:33
  • The Arduino can handle anything between 9 and 12V. Normally it is used if you have another device using 9 or 12V and you can use the same power source for both the other device and the Arduino. If you use 12V, there will be more heat generated in the Arduino, if you use 9V, there will be more heat generated in the buck converter. – Michel Keijzers Apr 23 '18 at 12:52

If you are getting a clean 5v from the buck converter, you can/should run that directly to the 5v pin on the Arduino. Using the barrel jack(at 7-12V) still uses the on board linear(inefficient) Regulator to produce the 5v the board needs. You will want to ensure you are outputing a clean 5v during all phases of the car being on/off/startup.

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