90 degree stepper or oscillator motor

Yu best choice depends on the load to be moved and how long the operating arm needs to be.

1. Stepper with proper control circuit
2. Servo (as in R/C car/plane) with proper control circuit
3. Reversible geared motor with limit switches
4. Geared motor driving eccentric/cam of proper design
5. push-pull solenoid (or pair of solenoids) moving one end of a pivoted arm

John

do it mechanically. mount a disc on the shaft of a continuos motor type and put a hole somewhere from the center off to one side. you place a rode in this hole that will connect to what ever to make it oscillate.

Any sin/cos motor or 'DC torquer' will step 90 deg if you energize the two coils in a gray code pattern 00 01 10 11. 0 means juice goes this way, 1 means the other way (like from an h-bridge on each coil)