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From the emonTX firmware library I found lines of code that blink the LED (indicates radio transmissions in this firmware)

https://github.com/openenergymonitor/emonTxFirmware

const int LEDpin = 9;                                                                                    // On-board emonTx LED

digitalWrite(LEDpin, HIGH); delay(2); digitalWrite(LEDpin, LOW);      // flash LED

Hope this gives you a starting point.

Andrew

The quickest way to find out that sort of thing is to look at the example code.

The LED for emonTx is digital pin 9

You might find this useful...

http://openenergymonitor.org/emon/emontx/reference

Thanks guys. 

Robin

With an emonTx, it's not immediately obvious how to connect to the various digital pins that it supports.  Seven of these pins are taken up by the Serial interface and the RF chip, but that leads seven which are available for the user to use.  But where exactly are they on the board?

Here's a little demo to show these pins connected up and working as intended.  The sketch just drives each of them on in turn to light an LED.

Yesterday, being in need of a couple of LEDs and connector strips, I went to Maplin and B&Q  and returned with industrial quantities of each.  This seemed like a good use for some of them!

(Sorry, I forgot to attach the sketch yesterday)

Nice work Robin, I've added a link to the emontx design reference page to your comment here http://openenergymonitor.org/emon/emontx/reference

My latest router is based on an emonTx and I needed to gain access to several of the digital IO lines.  There are three switches, three LEDs and two control outputs for hard-wired dump loads.

Sections of in-line header strips provide a convenient way of accessing these lines.  These are soldered into holes in the board as required.  Robert's recently updated diagram in Section 8 of my PV Router article shows all the of connection points.  By populating only the areas that are needed, it's very easy to plug the corresponding cables onto the right pins.  Multi-coloured cable helps too.  All of these items are available from Spiratronix who have recently published their first printed catalog, which is free. 

The attached sketch uses the on-board switches to turn the LEDs on/off.  The same approach could be easily extended for different hardware configurations.  In my case, I'm simply using the digital pins that can be most easily accessed. and will then allocate the pin numbers as necessary within the Router's sketch.  

An emonTx PCB (£3) plus a Atmega chip (£3.50) from the OEM shop, is a very cheap way to build a basic system.