Necessity is the mother of invention.
Dean's never been the philosophical type but he totally agrees with Plato about this and yeah, he knows who Plato is. Sam would be shocked.
He finds the schematics online. Not the schematics for this exactly, but a couple were close enough and combining their best features made his version even better. He'd had to scavenge the parts but hell, he keeps the Impala running. This is nothing.
Dean switches it on and grins as it emits a satisfying whine. He built an EMF detector out of a freakin' Walkman. That'll impress his brother.