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The one I built, which has been in use for years, is based on a rubber diaphragm. I think that the friction in a syringe would cause wide swings between on and off. I used a small,strong tin can from yeast, and bound the rubber drum-head on with windings of rubber strip. The vacuum hose fits on a soldered-in tube. The microswitch is bare, mounted on a hinge, and gets pushed into the drumhead by an adjustment screw. You could probably use a jar, and glue the vacuum hose into the drumhead.
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