Can I connect an LED to an RS232 port?

I don't know how much current it can supply but it should be able to light a

*high efficiency - low current * led ok.

You'll need a series current limiting resistor and maybe a series diode too. Some 'RS-232' ports actually run off only 5V or maybe even 3V btw particularly on small battery operated kit, so be aware of that.

Graham

An LED on a serial port works fine. RS232 is specified to drive an impedance >3k ohm, so use a resistor of at least 3k ohm to avoid overloading the port. Also, be aware that signalling is +/- 15V, not

+15V/0V (actually you usually get a bit less than +/-15V, sometimes as low as +/-5V). The -15V could result in excessive reverse voltage across your LED, which could make it go pop. Add a second diode in parallel with your LED with opposite polarity to protect against this. Any diode is fine; a 1N4148 will do nicely. The second diode can be a different coloured LED, which would let you switch between two colours rather than just on and off. Finally, be aware that logic levels are reversed from the norm on RS232 ports - a logic 1 is a negative voltage, logic 0 a positive voltage.

Tim

I second that, i've done it with a bog standard LED and a 5k resistor connected between the signal ground and the pin(s) in question.

Mark

Good idea. That'll make for less volts drop so it would even work on a non-standard 3.3/0V implementation ( they do exist )

Graham

30 years ago when I was designing RS-232 break out boxes on transistor driven +/-12/15V signals, I always used a 1 K resistor and two LED's back to back to do the monitoring.

Today, just use a tri-color LED and a resistor, much the same. As modern LED's are of greater intensity, and voltage swing could be a lot less, I'm sure the resistor value of 3 to 5K would be more suitable.

Don...

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it works here, I measure closer to 12V on the wires.

I used a 3.3K resistor so as not to load the line too much. LED's are oftem rated for only 5V in te reverse direction but I've not had any fail with only a series resistor

The CD line is an input on your serial port, outputs are TD, RTS, DTR

Ethernet ports have thousands of volts of isolation. wi-fi is another off-the-shelf solution.

Bye. Jasen

Gentlemen,

THANK YOU ALL very much for all your help and advice.

I've selected some LEDs to try from Digi-Key. I also selected a phototransistor to use on the detector side. I'm hoping that the phototransistor is sensitive enough for the red LED to trigger it. It is really designed for IR, but the sensitivity versus wavelength graph shows what I believe to be sufficient sensitivity.

I REALLY REALLY appreciate all the advice. It has helped a me a lot!

If you're interested, the parts I've selected to test are:

Manufacturer Part Number Description Digi-Key Page Digi-Key Part Sunbrite SSP-LXC128624 Poly LED Super Red 32.50 mm 1823 441-1067-ND Sunbrite SSP-LXC1282S24 Poly LED Super Green 32.50 mm 1823 441-1065-ND Lumex SSL-LX5093SRC/E High Brightness LED T-1 5mm Super Red 1814 67-1612-ND Lumex SSL-LX5093SOC High Brightness LED T-1 5mm Super Orange 1814 67-1113-ND Lumex SSL-LX5093SYC High Brightness LED T-1 5mm Super Yellow 1814 67-1115-ND Avago HLMP-CB15-R0000 LED 5MM 470NM BLUE WATER CLEAR 1858 516-1360-ND Avago HLMP-CW16-R0000 LED 5MM WHITE WTR CLR W/STANDOFF 516-1392-ND Avago HLMP-EG08-Y2000 LED 5MM 635NM RED WATER CLEAR 516-1377-ND Optek OP295B LED IR 5MM 890NM PLASTIC 1767 365-1062-ND

Sharp PT-550F Photo Transistor 425-1021-5-ND

John

jasen wrote:

Why aren't you looking at optoisolators, which put the LED and the photodiode or phototransistor in one package, ensuring no external light and making a perfect match?

Michael

The two systems must be a specific distance apart for security reasons. An optoisolator would only yield a few millimeters of separation. An optoisolator would be the perfect solution if it weren't for the security requirements.

Thanks! John