Once players have explored the Vale and brought their resources (5kg sandbags) back to camp the bags are 'poured' into the 'Infernal Machine' which is a game mechanic where a referee switches a heavy sandbag full of ore for a small metal ingot of the appropriate metal. At the first event players were left to waft bags in the general direction of the machine so I thought this would make a really nice addition to the existing machine.
The principle of operation is simple, open the chute at the bottom and pretend to pour resources into the machine. The gauge should slowly rise to 100% and it should ding a bell when it gets there to let you know the machine is full. Stay at 100% until the chute is closed, at which point the gauge should return back to 0%.
I started prototyping the 'ding' mechanism, if I couldn't get that to work then there was no point continue. A screw on a spring is pulled back by an arm on a gear, once the spring slips off the arm it swings back against the bell and makes a single ding.
The second essential mechanism is the gauge. After a quick search of possible options I settled on a quick return mechanism, this has the desirable feature that 0->100% takes two thirds of the rotation but 100%->0% only needs the remaining third, so the machine is faster to return to zero.
Finally the start/stop cam. Because I wanted the gauge to stop at 100% I added a cam that presses a microswitch at 100%. There is a second switch on the chute to determine if the drawer is open or closed, when the chute is closed the cam starts to rotate again until the cam microswitch is released again, the gauge will be back at 0%.