Flexy Hand 2 with an automated finger, made for an educational demonstration. Printed in PLA+. I didn't have any flexible filament available, so the joints were fashioned from sections cut from an old spring-steel tape measure and glued in place using some custom printed inserts. A wrist plate was created to hold the stepper motor, and a cyclical mechanism to pull the finger control tendon printed. Stepper motor is driven by an Arduino, with an off-the-shelf stepper driver to act as a buffer between the low current pulses generated by the Arduino and the high(ish) current pulses needed to drive the motor.
The simple cyclical drive removes the need to add additional sensors for proprioception. It doesn't matter where in the cycle the hand is powered down - driving the stepper forward will always pick up from where the movement was last left.
A push button and potentiometer was added to the arduino control circuit to allow the user to pause/alter the speed of finger movement. The push button grounded a pin that was normally pulled high, and a falling-edge interrupt was used to listen for button presses (the button was debounced with a non-blocking software routine). The potentiometer essentially formed a adjustable voltage divider connected to an analog pin to provide a means for the user to vary the speed of the stepper motor.