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> 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 have just designed a circuit where I needed to do exactly this,
schematic diagram attached.

I put the LEDs directly on the RS232 lines, with a 2k2 current limit
resistor and a small signal diode to ground. As the RS232 levels will
be about +/-10V the diode was needed because the LEDs can only
handle about 5V reverse bias.

The current will only be about 3.5mA per LED. A quick test on my
PC showed the TxD data was typically +/-12V no load and dropped
to about +9V / -12V with my LED circuit attached. I think this will
work quite well as there is still plenty of amplitude according to
RS232 voltage specs.

Ask me again in a week when I should be testing the prototype!

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
eMail:  brent.brown@clear.net.nz

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