Abstract: A novel shape memory alloy (SMA) actuator which responds to changes in ambient temperature. The actuator is capable of operating bidirectionally over a smaller temperature range than conventional SMA actuators by taking advantage of the R-phase characteristics of the SMA material. A coiled SMA spring 14 is provided with an enabled R-phase by limiting recoverable strain in the SMA material to less than about 1 percent. The force of the SMA spring 14 is counteracted by a non-SMA spring 15. The stronger of the two springs 14, 15 controls the position of an actuating element 13, with SMA spring 14 being the stronger spring while in its austenitic phase and being the weaker spring while in its R-phase. Triggering the actuator with entry into the R-phase rather than the martensitic phase during cooling reduces the hysteresis normally associated with an SMA actuator, allowing the actuator to react bidirectionally to smaller changes in ambient temperature.