In my case the draw per segment is only 20mA (and I have resistors on each segment), so as long as I'm only turning one digit on at a time, I shouldn't have a problem?
But one digit = 7 segments = 140mA...??
The arduino doesn't supply a maximum of 40mA, that is the maximum amount of current you can draw before damaging it.
Keeping it down to 20mA is sensible. So yes 7 segments at 20mA = 140mA. This is close to the limit of directly sourcing current through an arduino (200mA total through all the pins).
You MUST put current limiting resistors on each of the cathodes:-http://www.thebox.myzen.co.uk/Tutorial/LEDs.htmlIf you want more digits you have to source the current from either the 5V rail or an external supply and control it with an arduino.
if I want more segments, then I'd be close to pushing the 200mA total.
Quoteif I want more segments, then I'd be close to pushing the 200mA total.Only if the all the segments were on at the same time. You can multiplex them, (rapidly turn them on an off) not only to save pins but also to save current. However, multiplexing will make them dimmer and you can have exactly the same effect by dropping the current through the segment. Have you tried seeing how bright they are at 10mA? It won't seem half as bright because the eye has a log response.