How do I make a break beam sensor that triggers a relay?

I want to make a break beam sensor that will trigger a 12v relay. I am trying to add a second stage to a coilgun and I want the second stage coil to fire when the projectile passes through the break beam sensor. I want to power the relay and the sensor with a 9v battery. Also the more simple the circuit the... show more I want to make a break beam sensor that will trigger a 12v relay. I am trying to add a second stage to a coilgun and I want the second stage coil to fire when the projectile passes through the break beam sensor. I want to power the relay and the sensor with a 9v battery. Also the more simple the circuit the better. Thanks
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