How-To: Forrest Mims Puff Switch

How-To: Forrest Mims Puff Switch
Forrest Mims classic puff-switch hack, from
Forrest Mims’ puff-switch hack, from the 1996 edition of his Sensor Projects mini-notebook, now available as a compilation with two other classic Mims books in Electronic Sensor Circuits & Projects.

Component of the Month: SwitchesA puff switch is operated by blowing into a tube. They are useful in applications in which the user, for whatever reason, cannot be expected to activate a switch by more conventional means. The use of the term “puff switch” implies a switch intended to be used in this manner, but of course the more general name “pressure switch” (in this case, “positive pressure switch”) also applies. The opposite type—a mouth operated switch operated by negative air pressure—is usually called a sip switch.

Sip / Puff switch, with gooseneck mount, by Origin Systems.
Sip / Puff switch, with gooseneck mount, by Origin Systems.

Combination puff/sip switches, marketed primarily to the disabled, are commercially available, but can be expensive for experimenters. Forrest Mims presented a classic DIY puff switch hack in his 1996 RadioShack book Engineer’s Mini-Notebook: Sensor Projects, and the version I made is a very close variation on his design.

Step by step

To build the basic model, you just need a medium binder clip, a balloon, a lever switch (the longer the lever, the better), a foot or two of 3/8″ OD vinyl tubing, and a small o-ring (or rubber band). Construction is simple: first, slip the balloon about 3/8″ over the end of the tubing, and secure with o-ring or rubber band. Then open the binder clip and clamp it over the joint, positioning the tubing as deep in the clip as possible. Finally, secure the lever switch body in the clip, at the front edge, positioning the lever arm as necessary to obtain a reliable switching action when the balloon begins to inflate.

Puff switch animated

Once you’ve got it working, you can build a semi-permanent version by screwing the binder clip and the lever switch body to a small board, as shown, and a “professional” version by mounting everything inside a small enclosure, with terminals for attaching the switch leads and a threaded hose barb fitting to connect tubing through the enclosure wall.

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I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I am a long-time contributor to MAKE magazine and My work has also appeared in ReadyMade, c't – Magazin für Computertechnik, and The Wall Street Journal.

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