More complicated laser receiver
Here is a more complicated design. Jonathan did most of the coding and figured out how to make this work in the code. This receiver has a health bar. When you get hit, the health decreases. There is a heal button that restores health. When the health is depleted, the red LED lights showing that you are dead. Hits no longer register once you are dead. There is a re spawn button that allows you to come back to life (yes, you can push-button Jesus it).
This is the circuit diagram for the more complicated receiver. Below is the Arduino code written by Jonathan.
And a youtube video which hopefully explains the whole thing.
To put in an IR receiver, I changed the code below. The advantage of that is that a remote controller can be used as a gun and it has much wider ranger than a laser pointer, which is very difficult to use to hit a target from across the room. Of course builing a light gun with more focused optics will help to get to a middle ground between easy and difficulty of hitting the target.
There is the modified sketch. and below is another youtube video hopefully explaining how to build one for yourself.