The Pressure Switch: A Gauge That Turns the Motor On and Off

Stretch a piece of balloon rubber over the mouth of a PET bottle's neck and clamp it with a hose clip: that is the diaphragm. Glue a small plywood disc to its centre. Mount a lever microswitch above it on a bracket, with a machine screw through the bracket as the adjustable stop that sets how far the diaphragm must bulge before it presses the lever. Drill the bottle's cap for a tube to a small air pump and a pressure gauge on a tee. Wire the microswitch through a 12 V lamp or buzzer. Pump slowly and note the gauge reading when the buzzer sounds (cut-out), then let air out slowly through a pin-hole and note where it stops (cut-in). The gap between them is the switch's differential — here set by the microswitch's own snap. Turn the screw and watch both readings move together.

Pressure switches grew from the pressure gauge: the same diaphragm or Bourdon tube, made to move a contact. Snap-action contacts made them trip cleanly, and adjustable range and differential springs made one switch fit many duties. **Honest limits.** The notebook treats the air as staying at one temperature; in a real receiver it warms as it is compressed and cools after, which shifts the cycle. A pressure switch is a control, not a safety device: every receiver still needs its safety valve. The balloon model is for low pressures only — never pressurise a bottle beyond a gentle bulge, and wear eye protection.