I am posting here because someone I know needs help designing a circuit for an airsoft project. I have taken some high school electronics classes but I cant seem to wrap my brain around this one. Basically Airsoft involves replica models of weapons that fire plastic BBs in a game similar to paintball. These replicas have 3 fire modes, Safe- The gun cannot fire, Semi Auto- The gun is limited to firing one BB per trigger pull based on a mechanical restriction and Full Auto in which it fires as long as the trigger is held down. It does this by having the trigger complete a circuit which turns a small electric motor which operates the firing mechanism. The problem is that he wants to add a 4th firing mode which will cause the gun to fire a burst of 3 BB's per one trigger pull. I can see that a circuit will be needed to cause an output to go HIGH for about 3 seconds and then go low, no matter how long the trigger is held, until it is released and repulled. The other problem is figuring out what combination of resistors and capacitors is needed to attain the desired length for the HIGH signal. Any help regarding schematics and parts would be most appreciated.