An SMA device in which an SMA wire actuating element is operated periodically at a low voltage by power derived from a current source current which is too low to heat the wire directly sufficiently for effective actuation. The low current source is connected to a capacitor which discharges periodically through the SMA wire actuating element in response to an upper threshold charge stored by the capacitor thereby to periodically supply the SMA wire actuating element with a current level sufficiently high for effective operation. The discharge is effected through a signal amplifier formed by first and second transistors coupled to provide positive feedback and connected to a switch formed by a third transistor biased by output from the second transistor to periodically conduct, thereby permitting discharge of the capacitor through the SMA actuating element.