Thursday, April 19, 2012

Bike horn

I’ve been riding with an AirZound air horn on my commuter bike for a couple of years now, and it’s pretty good but it is not durable.  It’s now almost totally dead, only working when the pressure is over 110 PSI.  Also they are known for not working in cold temperatures, below about 35*F.  This makes them useless for months out of the year.
I bought a car horn to put on my bike probably 7 years ago but it’s just been sitting around.  I finally decided to do something about it.  Here are photos of the (nearly) complete project.
This is the horn mounted on the rear rack deck with an L bracket using two #10 screws and nuts with lockwashers.  You can also see the saddle bag that holds the battery and 12v relay.
Here is the 12v lead acid cell and the relay.  The relay might be able to be skipped, but it’s really a good idea to use it.  The horn button really isn’t supposed to take this kind of load, and also you’d have to use heavier wire to go to the button.
I bought one of the standard type horn buttons available at any auto parts store:
I bent the tabs down so that they fit nicely alongside the stem on my handlebars, connected the wires to it, wrapped the stem with some electrical tape to insulate it, slid the tabs alongside the stem, secured through the tab holes with a tie wrap, then covered the whole mess with some old inner tube scraps and secured that with two tie wraps, like so:
The parts list goes something like this:
Car horn – $15 at auto parts store
Button – $4 at auto parts store
Relay – $7 at auto parts store
12v 1.8A lead acid battery – $14 on eBay
L bracket, nuts, bolts, wire, connectors, zip ties, inner tubes – Lying around or maybe $10 if you need to buy them.
You’ll also need a battery charger.  I have several lying around, but if you needed to buy one, for a battery this size something like a Harbor Freight float charger, which often goes on sale for $6, would be fine.
So this is probably a $40 to $50 project.  An AirZound is $25 and works really well if you only ride in warm weather.  It’s possible that an AirZound might last a long time if you took it inside in the winter. After two years on the bike mine is basically useless.
One note if you’re not so hot on electronics.  You need a battery capable of delivering at least 6 amps of current.  A battery the size of your pinky won’t run a horn even though it may be a 12v battery.  A batch of AA cells probably wouldn’t do it either.  Lead acid cells are really good at delivering high currents, and this tiny one only weighs a bit over one pound.  I think the whole thing probably weighs under 3 pounds.

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