Arduino Uno Infra Red
If you have got one of these:
And an IR LED, then you can use it as an IR sender.
Using this Arduino sketch, and an IR LED, and 4K7 resistor
you can make your Arduino Uno send IR commands to your set-top-box.
This working sample code will work with a View21 set-top-box which are sold in Currys in the UK.
The IR output is on Pin 3.
Make sure your IR LED is connected the right way round - otherwise it won't work.
IR is invisible to the human eye - but if you point your digital camera at the IR LED
then you should be able to see the LED flashing through the camera display.
That will prove if IR is actually being sent out.