At some point I needed a motorised air compressor for my pneumatic LEGO projects. One that apply for all kinds of purposes. It should automatically turn off when a particular high pressure level was reached and start again when it reached a low pressure watermark.
I viewed many example of other compressor on the web. I was particular interested in the pressure switch made by The Hempel Design Group.
I use this compressor now for more then a year. It is right here at my desk and it is always (electric) powered on (battery switches is on), but when no air is needed it automatically switch off.
Check the video at Google Video:
I used the NXT (part no: 53788) and a Mindsensors pneumatic pressure sensor to measure the pressure. The pressure is rising from 0 to 33 PSI (2275 mBar) and then the motor is turn off by the pole reverse switch. When I release air with the pneumatic switch the pressure falls back to 0. At about 15 PSI (1034 mBar) the motor starts again.