A PA28 alternator switch physically failed on a night flight resulting in a battery drain from lights so after securing all power except radio the pilot could hear ATC but not transmit and was cleared to return to the airport.

Narrative

In flight the alternator switch physically broke in half and would not operate. I was still on radar and in communication with Tower. The power drain on the battery because of the lights and strobes was causing the radio not to transmit at an audible level to the Tower. Incoming transmissions for the ATC were loud and clear. I turned back in for landing when the problem was evident. I had to turn off lights to maintain communication. The Controller could not hear my transmission and had me turn to a heading of 270 if I could establish that I could hear him. The turn was done about 6 miles northeast of the airport while back inbound. At that point the Controller established that I could hear him and cleared me to the airport with a right traffic to 31R. He gave me the clearance twice. I could hear all ATC Communication but they could not hear me. I landed as per his instructions and moved off Runway 31L at Taxiway N as instructed and parked in the N triangle area. I could hear all ATC Communication but ATC could not hear me. As soon as I had the airplane parked safely I called Tower on my cell phone and had an escort vehicle escort me to the parking area.