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I have a project that needs power.  It has both a 328P and an ATtiny85 on it.  I have two choices of wall wart here.  I can use an old cell phone charger that has a good 5.3V output or I can use an old 12V and regulate it with a 7805.  I don't want to use up all that energy in a regulator if I don't have to.  Are there any tricks to dropping a little bit of that voltage from the 5.3V to get it under 5V?  Maybe a series diode or two?  Or would it be safe to just feed the project 5.3V?
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5.3V is within the normal operating specifications for both processors (5.5V is the high end).

You will have to check the datasheets for everything else in your project.
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5.3V is fine.  Make sure whatever else you connect to it can address the slightly higher voltages that will result,
lows of (0.3 x 5.3V) highs of (0.6 x 5.3V).
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Thanks a million guys!

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As told 5.3vdc is acceptable. If you did wish to knock it down a tad, wiring a series Schottky diode should help drop it closer to 5.0
http://gdansk.bradley.edu/wp_mapping/wp-content/uploads/2010/04/1N_1N5822.pdf
http://www.allelectronics.com/make-a-store/item/1N5822/SCHOTTKY-RECTIFIER-3A-40V/1.html
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Quote
I have a project that needs power.

Even better headline would be "I have a project that needs no power"... then you'll really be onto something  smiley-cool
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Its pretty common for 5V logic devices to work with the range 4.5 to 5.5V, and for 5V regulators to be at least as
accurate as 4.75 -- 5.25V.   For accurate analogRead() results the accuracy may be more critical than this, and for
devices that work at 3.3V as well as 5V the 4.5V lower limit is not an issue.

Its worth checking that the power from the wall wart is a clean, regulated 5.3V (check with a multimeter the AC volts
reading is small, 0.1V or less would indicate proper regulation, 0.5V or more and you'd risk exceeding 6V on the peaks)

Check the wall wart both unloaded and under load if possible.
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I have a project that needs power.  It has both a 328P and an ATtiny85 on it.  I have two choices of wall wart here.  I can use an old cell phone charger that has a good 5.3V output or I can use an old 12V and regulate it with a 7805.  I don't want to use up all that energy in a regulator if I don't have to.  Are there any tricks to dropping a little bit of that voltage from the 5.3V to get it under 5V?  Maybe a series diode or two?  Or would it be safe to just feed the project 5.3V?

That's fine as long as you don't draw more than 200 mA through the 328P and likely less through the Tiny.

What needs so much power? It ain't just those 2 chips.

BTW, regulated 5V USB chargers are pretty cheap if you know how to shop and can wait for delivery.

Otherwise, is the 5.3V regulated? There is a trick called "voltage divider" you can use if it is.
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