On Wed, 6 Mar 2002, Les Otter wrote:

> I'm working on an auto-ranging sequential shift
> lights array for racecars / bikes.  The array of
> LEDS will sequentially light up as the engine
> nears it's optimum "change-up" rpm.

Cool...  commercial or hobby?  I'd like one for my car... 8)

> I want the LEDs to get doubly bright when all
> lights are on.
> PWM seems the usual way to adjust brightness on
> LEDS, but can I just double up on the number of
> pins driving each LED (I have enough), so 1 pin is
> normal, then switch to 2 pins at the panic point?
> Can they both drive the same LED - each pin
> through it's own (say 68R) resistor?  WIll I need
> series diodes on each pin?

Sounds like that would work, but waste a lot of pins.

> I don't fancy bit banging the RPM in whilst PWM
> encoding an output for 8 leds...could get a bit
> frantic - plate spinning comes to mind!

Naaah.  You can drive all your LEDs from the anode end thru a small
resistor, tie all the cathodes together and slap a 2N2222 or something
between them and ground, drive it with PWM.  PWM is pretty much "set and
forget" if you're using a PIC with CCP.  The RPM can be measured with
another CCP or just a loop checking the pulse input and calculating the
pulse interval from a timer.  When using a PIC wth no CCP module to do
PWM, I just set up a TIMER0 interrupt routine to handle it.  Each time
it's called it checks a bit variable to see if the output is ON or OFF.
If it's OFF, turn it ON and set TIMER0 to 255-<pulse width>.  If it's ON,
turn it OFF and set TIMER0 to <pulse width>.  Quick and easy, I just did
another project that way last night to control the speed of a small DC
motor.

Doesn't sound like that tough a job...  give it a try, you might be
surprised!

Dale

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