low frequency voltage

Using Arduino Microcontrollers
1

Hi guys, can I use 3.3v on my arduino pro mini 1 Mhz instead of 1.8 volt because I want use green LED bargraph?
Is it safe for my arduino?

2

Yes, pro mini is safe up to 5V.

Running at higher frequency, like 16MHz or 20MHz, needs higher voltage like 5V. At lower frequency you can use a lower voltage, but you do not have to. You can still use up to 5V.

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3.3V with 1MHz is inside the safe area (shown in grey).