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[on:]

sorry if this has probably been asked before but my searching did not turn up with results i was looking for.
what is the max voltage input i can give without frying the arduino?
i want to put the chip on a perf board and connect it directly to vcc with 4 AA batteries(6v or <5.4v with rechargeable batteries) so i wont have all its on board voltage regulators. i do have a 7805 but ive read it only works consistently with above 6v.
so am i good to go or do i still have to drop it a bit with a resistor/diode?
additional info:  i also have 4 7 segment displays connected
and i noticed with the 4 regular batteries, the displays are bright and good, but with rechargeable batteries, its a bit dimmer as expected

[Reply #1 on:]

Alkaline cells start out at around 1.6v or a little higher, so 4 of them give about 6.5v. That's too much for the +5v pin of an Arduino. You can get a low dropout voltage regulator that drops much less voltage than the 7805. The voltage regulator on the Arduino is a low dropout one, so you could feed the 6.4v into the Vin pin (not the barrel jack, it has a series protection diode that drops around 0.7v).

NiMH cells can be as high as 1.4v briefly but soon go down to 1.2v. For 4 cells, that's 5.6v worst case, 4.8v typical. You can connect 4 of them direct to the +5v line, as long as you get the polarity right.

[Reply #2 on:]

NiMH are 1.3V, not 1.2, you are confusing them with NiCd

4 x NiMH = 5.1..5.4V or so, which is just OK for the Arduino.

[Reply #3 on:]

I've seen NiMH cells quoted as nominally 1.2v (Wikipedia) and 1.25v (Energizer data sheet). If you look at page 6 on http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf you can see that these particular cells start out at 1.4v, rapidly go down to just over 1.2v, and end up just under 1.2v before they run out of charge.