# Putting a Visible LED on an IR Port for Testing

Posts: 54
Morning,

I want to make up a cable to plug into an IR Port with a visible LED at one end.

This is for testing as opposed to using my iphone to test if i'm receiving an IR signal.

Has anyone done this? What spec of LED did you use? I can't find any current, voltage rating online for the AMX IR Port.

Any help would be great.

Cheers,
Ant

• Posts: 4,368
• Posts: 186
Ant wrote: »
Morning,

I want to make up a cable to plug into an IR Port with a visible LED at one end.

This is for testing as opposed to using my iphone to test if i'm receiving an IR signal.

Has anyone done this? What spec of LED did you use? I can't find any current, voltage rating online for the AMX IR Port.

Any help would be great.

Cheers,
Ant

I suggest a plain vanilla red LED. I don't know if the IR emitters have a built-in resistor or not (see below).

If it were me, I'd do this: set the port to CAROFF, load a driver, connect the IR emitter and turn on a channel. Then measure the voltage across the Phoenix block. If it's in the neighborhood of 1.2V, you're probably safe to use an LED directly across the IR output. If it's more than that - say 5V or so - you'll need a current-limiting resistor in series with your visible light LED. What value? Glad you asked.

A "normal" red LED drops about 1.2V at around 10 to 20mA (but check the data sheet). You need a resistor to drop the remainder of the voltage while allowing that amount of current to flow. For example, say you measured 5V above. The remaining voltage is 5 - 1.2 = 3.8. To get (for example) 15 mA while dropping 3.8V, you need a resistor of 3.8 / 0.015 = 253.33 ohms. The nearest standard value is 240 ohms; the power dissipated would be (roughly) 3.8 * 0.015^2 * 240 or about 0.21W, so a 1/4 watt resistor should be fine for open air and a short duty cycle.

HTH.
.
• Posts: 506
I can confirm that this works with a bog standard red LED. No current limiting resistor required.

Edit: No need to turn the carrier off if all you want is a visual indication.
• Posts: 1,718
Seems like you could just look at the LED on the NetLinx for that port. It's bridged on the same output, right? It can't light if there's no current, and if it lights, there's current.

If what you mean is you want to test the house wiring, again as said above, the most-favored Xantec emitter has a visible flash. You need nothing else.
• Posts: 549
I use the Xantech Blink-IR emitters for testing too.
• Posts: 4,584
John Nagy wrote: »
Seems like you could just look at the LED on the NetLinx for that port. It's bridged on the same output, right? It can't light if there's no current, and if it lights, there's current.

If what you mean is you want to test the house wiring, again as said above, the most-favored Xantec emitter has a visible flash. You need nothing else.

Not necessarily. I have had bad ports where the front LED still flashed. Those LEDs must be on the input side of the IR chip, not the output.
• Posts: 506
DHawthorne wrote: »
Those LEDs must be on the input side of the IR chip, not the output.

Completely separate circuit as evidenced by the fact that they changed firmware awhile back so that the front panel LEDs light for a minimum of 0.5 seconds even when the IR output is just a single send_command 'SP'.
• Posts: 54
Thanks guys - used a standard red led. It's for testing IR being received when using cat 5 extenders and the NI isn't in my line of sight