Multiplexing is only acceptable if you can attain the desired brightness with each segment energized only a small portion of the available time, relying on a combination of LED and retinal persitence to conceal the multiplexing's effects. Outdoor displays seldom allow for multiplexing.

The guy to ask about outdoor displays, since that's his domain, is Erik Malund.

You MUST know what the relationship between current and brightness for your LED's is, and you MUST know how many LED's you have to have energized at any given time in order to obtain the necessary light output from your LED's in order to obtain the required light output. These effects are not linear. There is what amounts to a "point of diminishing returns" abeyond with more current will yield an insignificant amount of additional illumination. Likewise, there is such a point for ON time, too. You can examine the current-to-light-output ratio for the LED's in order to ascertain the desired "on" time. You can do some arithmetic in order to ascertain the optimal on time and current.

You get NOTHING for "free." If you multiplex, you will be able to use less hardware and you will run less current through each LED per unit of time. However, illumination is a fuction of current and time. The longer you leave each LED in its ON state, the brighter it will appear to the eye.

You have a lot of work to do before you can make these determinations.

One thing you can consider, however, if you choose to multiplex your display, is that if you discharge a capacitor quickly through a semiconductor, you can charge it through the LED, which will give you a longer perceived ON time.

RE