[Soekris] Extra LED (Design/Circuit/Software suggestions)

Poul-Henning Kamp phk at phk.freebsd.dk
Thu Mar 31 17:50:18 UTC 2005

In message <8A1DA40786CE4741850C0AF80F2D525602F39DE7 at idealab.printperpage.com>, "Robert Woodcock" writes
>On Thursday, March 31, 2005, Jonas Tärnström wrote:
>> I guess the GPIO pins on JP3 probably is the way to go.
>> Can they power a LED without a power circuit?
>I've only done this on a net4501 but I'd expect the limitations to be
>The net4501's SC520 supplies up to 6mA on the GPIO pins. Some LED's are
>under that, many are over. To be safe, I would hook the GPIO pin to the
>base of a PNP transistor through a 10k ohm resistor. This draws more
>like 0.25mA.

You can drive LEDS directly through a small resistor but you need to be
aware that the built-in pull-up/downs may make the led glow faintly
all the time.  Putting a TTL like 74ls04 or 74ls373 between the GPIO
pins and the LEDS is a good idea for many good reasons.

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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