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Modified double-acting Hempel compressor
The compressor used on HanoiSolver is a version of the improved double-acting Hempel design. The motor's stub-axle carries a 24t gear which engages two, subsidiary 24t gears. The subsidiary gears share an axle with Technic cams which drive the small pneumatic pistons round. (You can also use Technic medium pulleys rather than the cams -- the distance provided is the same).

The original Hempel double-acting design connected the pumps directly to the subsidiary 24ts, but this gave them a slightly short stroke. My own conviction is that a LEGO pneumatic pump must end its stroke bottomed out in the fully closed position to get decent pressure. The cams (or medium pulleys) fully open and close the pump to give a bigger turning radius and better efficiency.

Switch on reverse of compressor
To activate the pneumatic valve, the axle of one of the subsidiary 24t gears runs to the rear of the compressor and terminates in another 24t gear. This gear engages yet another 24t attached to the arm of the pneumatic switch (adapted from a design by Mindstorms forums user Blaze2).

What happens is that the 24t on the switch arm is just in contact with the 24t on the drive train. The 24t on the drive train spins fairly freely (there is a very slight clicking noise of the tips of the teeth of 24t-drive knocking against the tips of the teeth on the 24t-switch).

When the motor reverses direction, there's enough contact for the 24t-drive to catch the 24t on the switch arm and throw the switch lever the other way. I've used this pump-switch combination a lot in the last few months and I've found that it works very well so long as the two 24t gears are absolutely parallel; it doesn't work well if the switch is not securely anchored.

Large pneumatic cylinder used as pressure detector The air pressure is monitored by a large pneumatic cylinder which is connected between the pumps and switch. The cylinder is held normally closed by four yellow rubber bands. As air pressure increases, the cylinder arm extends until it hits a touch sensor (SENSOR_1) connected to the RCX. The photo opposite shows the system fully charged (piston extended). The beauty part is that, for the small quantity of air used by the small cylinder on the claw, the piston also provides enough reserve air that I didn't need one of blue air tanks.

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