[sdiy] VCA controls

[Steve Ridley]

> I was wondering the same thing (in slightly less acerbic terms) --
> have some of you come up with ways of including mass LEDs without
> the associated monster current drain?

A technique used in some broadcast mixing desks is to have
all the LEDs connected in series between the positive and
negative supply, fed with a constant current source.  A switch 
across each LED shorts it out when it needs to be off.  This
arrangement only takes one LED worth of current, and the 
current doesn't change much as LEDs are switched in and
out.  

Bad points - if one LED fails O/C, it can take all your
indication out.  Also, the actual voltage at each end of the LEDs
varies with the state of the other LEDs, so if you're using 
electronic switching you need to bear this in mind.

Two more alternatives for indication on a VCA - 

Real (moving coil with pointer) VU meters.  They look
very retro.  If space is a problem, edgewise meters are
small and I've even seem miniature circular ones that 
are only 1.5cm diameter.  Easier to drive too.

LED and microcontroller.  Use the ADC on a PIC, and
have a single LED flashing out the level in Morse code.
You could have it flash out the levels of the two channels
alternately, and save another LED.  That would minimise 
the drilling and panel space needed. 



Steve Ridley