Update: All information available at http://redmine.hcpl.be/projects/tag-combat/wiki
One of these projects I always wanted to get started on and it looks like I finally did. Imagine those laser games where you can shoot each other, much like paintball but without the paint and balls (ouch!). Well these systems use lasers or infrared signals. That's what I'm going to make for my RC combat planes to have some fun with friends.
IR is all around us. Best example is the remote control of your tv. In the television an IR receiver is watching for signals at a specific frequency. Using modulation we can send encoded information. The remote control has an IR led that sends the desired command. Remote controls want a wide range so you don't have to really point the remote towards your television.
For our IR tagging combat system we do want a more bundled beam so we can use it like a gun and need to point our airplanes towards each other to register a hit. The signal can have information like who is performing the hit and much more commands. For now we'll keep things simple (KISS).
The idea is to have an arduino with up front an IR led with a lens to bundle the beam. Much like a flashlight works. Those cheap small keychain led flashlights might be a good source for the IR led housing and lens. In the back an IR receiver is watching for incoming hits.
A buzzer can be added to make some noise on impact and on shooting. More important is to have some bright leds we can see from the ground indicating the hits we took so far. For instance 6 red leds that indicate the number of hits taken.
So far nothing too hard to get this working. A good lens and range will probably be the hardest to find out. I already found a tutorial on ibm developerworks for an arduino laser tag game having some nice ideas and working code. You might need to register for a free account in order to view the article. It's 3 parts long, working code example can be downloaded from part 3 resources.
It's inspired by some classic computer game and has some basic rules. Each player has 6 safe shots. If you fire more than 6 times the chance to self destruction increases by each shot. What I don't really need so far is the referee option. In a first iteration of this project I'll bundling all this into an easy to handle brick to add to your plane. I'll probably be using the arduino pro mini to keep things small. I'll be adding a series of leds to indicate the amount of shots taken.
Another iteration of this project could have the throttle channel as input and on impact make throttle stutter or stop for a few seconds. Since we are up in the air we don't want to stop throttle for too long. On heli's and multirotors throttle cut is no option, maybe you could make the craft circle around a few times.
As far as I can see the only commercially available product like this comes from HobbyZone: The sonic combat module HBZ4020. If I don't get any good results with the IR beams I'll be using I might still get one of these and intercept the signal with an arduino. Only the IR part will then be replaced by this system (not sure if it's even IR). The nice thing about making it yourself is off course the fun but also being able to make your own rules. As longs as every player uses the same code and parameters this is a fair game.