[sdiy] Driving LEDs

[Brock Russell]

Hi John:

You've got lots of options.

1 - Use three 74HC138s. They have 3 enable inputs as well as
the 3 decode lines so you can easily control 24 LEDs with 5 signals.
This is probably the cheapest option with the LEDs configured
common anode with a single current limiting resistor.

2 - Go serial. Use another micro as the LED driver. A 28 pin PIC
such as a 16F873 will control 21 LEDs with one serial data signal.
You can also have more than one LED on although you will be limited
by package power dissipation and you would need current limiting
resistors on all the LEDs. This would be the lowest interconnect
option.

3 - Use LED driver chips that have serial inputs. Allegro makes
some nice ones. The UCN5833 will give you 32 outputs with only
3 or 4 control signals. You can also gang them for more outputs
without adding more control signals. Once again, if you only want
one LED on at a time, a single current limiting resistor can be used
for minimal parts count.
Other good Allegro parts are the A6275 (8-bit) and A6276 (16-bit)
constant current drivers. These eliminate current limiting resistors
and allow you to have multiple LEDs on at a time. I use these in a
number of systems.

4 - You can use HC logic rather than LED drivers. Three 74HC299s or
74HC4094s serial shift registers with output enables will allow you to
control as many LEDs as you want with three signals.

Brock



>OK, here's the problem:
>
>I need to drive 20 LEDs from a PIC type uC. The LEDs are on a separate
>board so I want to keep the number of interconnects small. I only need
>one LED at a time to be on. So, I seem to need a decoder with 5 lines in
>to 20 out. Most of the stuff I see is 3 to 8. Any ideas?
>
>--
>Regards,
>--/////--
>John Blacet
>Blacet Research
>http://www.blacet.com
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