IR transmitter You will need:
IR LED: Look for an LED with a narrow viewing angle (to ensure as much of the IR light makes it through the optics).
Higher power the better.
Pick LED’s with a wavelength that matches your IR receivers.
Get some spare LED’s as they are operated above their rated current so may burn out.
You can just attach an IR LED (with a current limiting resistor) to an output pin on the arduino and it will be able transmit, but the range of the duino tagger won’t be very impressive. In order to improve the range of the duino tagger amplification is needed.
The basic amplifier I made was a simple transistor circuit (circuit diagram provided), you may wish to consider a mosfet instead of the transistor.
Current through the LED: I aimed for about 300mA current through the IR LED. This is more than the rated current for the LED, but the LED seems to be able to cope because you are only pulsing the high current for a short time. If you are using the 5V supply from the arduino to power the LED it can only provide about 400/500mA so using 300mA for the IR transmitter Led does not leave too much for the other LED’s and speakers etc, so bare this in mind when designing building your duino tagger, it might be a good idea to add a voltage regulator capable of supplying higher currents. Optics
You will need to use a lense to focus the IR beam, you can use the lense that came with the light gun. You can use a red LED to work out where to put the IR led to get a good focused beam.
For more details see the miles tag site.