I ride with a brass bell and like it, but I find myself in a damned if I do, damned if I don't circumstances. Half of the time I use my bell, people give me a dirty look. When I don't use it, people also give me a dirty look or yell at me half the time. The other half of the time in both instances, most people are wearing headphones and are oblivious to anything going on around them.
I am lucky that I don't ride around cars too often, but I have in the past and I know my brass bell likely would not get the attention of someone piloting a motor vehicle. A horn on a bike is not a new idea, but reader George came up with a rather sophisticated DIY version. Take it away George . . .
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The key component is a 3 way 2 position manual valve that can be
found in specialized pneumatics & automation stores, choose
the smallest model. Now you can select the other components as
connectors, the "T" and the air hose diameter based on the size
of the valve and it's connections. Explain the schematics to the
owner, it helps to find the best solution available.
The schrader valve I took from an used inner tube, and the horn body from an used gas horn. To adapt those and the PET bottle cap to the connectors, I used bicomponent epoxy glue. It dries fast and seals tightly.
At last you'll have to improvise ways to fix the parts to the frame, since many things can vary. For instance, I used a rubber pad to put between the valve and the handlebar, tied with nylon wraps.
For safety avoid to put more than 80 PSI in the bottle, although I read it can hold more.