Step 1: Build the cage
Step 2: Make bars
Step 5: Install pneumatic cylinder and switch
The 6 inch throw, 3 inch diameter double-acting pneumatic cylinder was a backup from another project that acutally needed such a thing. The valve is a 4-way, 2-position lever-operated control valve (part #4493k34 from McMaster-Carr). Both items are kind of pricey, so since being purchased originally for a consulting project they have been reused in a number of other projects. Compressed air is generated by a portable compressor used to power the air flow on the Squid Labs laser cutter when we take it to conferences or events.
The 4-way, 2-position valve is pretty neat. When you switch the position, it vents one side of the cylinder and pressurizes the other.