25V is plenty :) For longevity when using a capacitor as an actual component, most people tend to double the system’s voltage to get the desired specified voltage. In this case, your Arduino will probably be running at 5V, so 25V is fine. ( The capacitor is only used in this case to prevent the Arduino from auto-resetting, so it’s not as critical, as long as it isn’t below the system’s voltage. )
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