Conrad,

My home brew PIC development board which only accommodates 18 pin devices
has an LED in series with a 330 ohm resistor for every I/O pin. There are
also dip switches in series with the LEDs so they may be disconnected.
However, it's never been necessary to disconnect an LED because of
insignificant drive to other devices. As an example, the development board
has a Seetron serial LCD display that operates flawlessly at 9,600 baud
with the attached LED in series with the 330 ohm resistor. Obviously,
disconnecting the LED would be viewed as preferable, however, it has not
been necessary. For driving most other output devices I use a darlington
transistor array IC such as the Toshiba ULN2803A which can provide up to
500 mA and is available from Jameco (www.jameco.com) for $0.69 as Jameco
P/N 34315.

Sincerely,
Ned Seith
Nedtronics
59 3rd Street
Gilroy, CA 95020
(408) 842-0858


At 12:05 PM 8/3/01 +0200, you wrote:
>Hi Everyone,
>
>I am stuck with a problem I just can't seem to find an answer to, maybe
>someone can shed some light.
>
>My project uses serial comms to connect to other devices: What would be the
>best way to implement little 3mm LED's to indicate the RX/TX status ?
>
>I can't decide on a solution, because from what I have seen in the past,
>driving an led from a outpin on a PIC, seems to consume all the current, so
>that there is none left to drive an input on another input pin of a TTL ic
>for example. So I will see my RX/TX status, but the signal will not get to
>to my driver device. I thought of using something like a buffer driver, to
>drive the led and to leave enough current to actually drive the rest of the
>circuit. Am I supposed to use some current limiting resistor on the led?
>
>I hope this will be clear. What will a common industrial solution be to this
>problem?
>
>Conrad
>
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