[chbot] PWM control of William's LED display

[Mark Atherton]

William,

I have some thoughts about how you could control the brightness of 
your LED display unit using PWM.

Brielfy, an 8 bit shift register (74x595 etc) is loaded with a 
brightness value (0..255) this is fed into the A side of an 8 bit 
magnitude comparator (2 x cascaded 74x85 etc.). The B side of the 
comparator is fed with a continuously running 8 bit binary counter. 
This counter needs to wrap around at about 100Hz. The ">" output of 
the cascaded 74x85 then becomes the PWM brightness control signal and 
feeds the enable signal of each of the LED drivers.

South Island Components's (how do you pluralize that ?) web site 
claims to have everything in stock...

datasheets: 
http://www.nxp.com/documents/data_sheet/74HC_HCT85_CNV.pdf and 
http://www.fairchildsemi.com/ds/MM/MM74HC595.pdf

Anyway, just a thought after too much coffee.

-Mark