We have a second-hand air conditioner in our guest bedroom. Unfortunately, this summer the "Master Control" knob broke:
The thermostat knob still works, and you can use a needle-nosed pliers to turn the "Master Control" peg which lies under the plastic panel, but it's annoying (and will probably eventually break the peg). So, I set the AC "Master Control" (which sounds like a term over-used in bad sci-fi movies) to medium cool (it's a small room) and devised a new on/off switch:
There, I fixed it!