The goalkeeper signed a two-year contract at Meadow Lane in the summer of 2016.
It brought to an end a three-year stay at Rotherham United, where the stopper helped the Millers to win promotion to the Championship in 2013-14 through the League One play-offs.
Collin had previously spent four years with Carlisle United after being picked up from local non-league outfit Workington.
Home kit: S & P Steel Services Ltd
Away kit: Available
Third kit: Available
Boots & socks: Luie Everitt