Jinx wrote:

> > > And consider this Wouter - you would need 70A of DC to
> > > display worst-case "88:88", assuming 7 x 4 x 120 LEDs
> > > pulling 20mA each. A 25A supply for each digit ? Not trying
> > > to put you off (especially as LEDs are "in" these days), but I
> > > got a surprise when I did the calcs for my request
> >
> > The customer only wants two digits, and I guess I can put at least 5 in
> > series, so that would be 7 x 2 x 120 x 0.02 / 5 = 6.72 A. No problem for a
> > car race environment where accu's are found all over the place. But how
> > large were your digits?

Hi Wouter, the easiest and cheapest electronic
solution which gives enough brightness etc is
to use 240vac or 120vac bulbs, at 2 or 3 bulbs per
segment. They can be switched easily with any cheap
triac. You can get millions+ of on/off cycles from
normal mains bulbs if you do this:

* run 40w bulbs at about 32w (80%)
* drop 20% on a series resistor - soft starts them
* keep filaments warm and JUST glowing with a
simple parallel resistor across your triac.
* consider a 4" fan in your cabinet.
* also put a "hood" on the display like traffic
lights have to keep the sun off the digits?

My preferred solution, being cheap, reliable,
and has excellent visibility even in HUGE
sizes; Make each segment mechanical, like a
2" x 2" wooden length (or something lighter like
PVC pipe), painted black and fluoro yellow on
different sides. Use a cheap "toy" type dc motor
one end, acting as rotary solenoid and bearing and
and a simple greased pin as bearing on the other
end. They rotate to change appearance, and simply
"hit the stop" at each end of the rotation. You
apply enough power to the motor to turn it, but
low enough power so that the motor "stalls" at each
side without getting too hot. Saves limit switches
etc. :o)

Controlling them cheaply is a challenge, I would
use a 0v,+6v,+12v power supply, common for all the
segments. The +6v connects to one motor terminal,
The other needs a half-bridge ie a cheap NPN and PNP
transistor and a series resistor to set stall
current. You could PWM the half bridge also for no
extra cost, giving adjustable motor current in each
direction. Really they just need to be on or off,
with the resistor size chosen to give good motion
with minimum heat.

Those hobby motors in the larger sizes are about $2
each, and have a LOT of quite reliable power in a
"rotary solenoid" of that price range. I've done
similar things in robotics projects. :o)
-Roman

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