Place a piston facing 2 slime blocks.

Put a slime block on the side of the piston and place a sticky piston facing that then place an observer block facing the sticky piston.

Put a redstone block on top of the piston and it will go.

Put an obsidian block in front of it to make it stop. To start it, break the obsidian block and update the observer.

If you want it to dispense things put 2 observer blocks in the front.

Now put 2 dispensers in front of the observer blocks.

Fill the dispensers.

If you want it to go both ways, mirror it on the back but with one redstone block on the other side.

Build something like this to make it automatically change direction. The right piston is sticky.

Build another one on the other side.