Car Horn for Bicycle

Riding a bike is fun as hell, good for the enviroment, good for your health and good for your wallet. But as any other street vehicle, using it leads to obvious risks (particularly if the drivers on your city rarely follow the traffic rules).

That’s why I can’t understand why bicycles are equipped with pussy-style horns. Pedestrians and drivers may never know about your presence, until is too late. Time to fix that!

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Ingredients

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The first thing it’s the power: Its a SLA (sealed lead acid) 12 volt, 7,2 Ah battery. Very common for UPS, home alarms and emergency backup lightning. Next to it as a pair of AA cells, so you can get an idea of size.

SLA batteries are rechargeable, so you need a charger Get one

To fasten the battery to the bicycle, I used a polypropylene strap (the same used for sealing boxes)

That pink color doesn’t help, but at least is cheap and very strong (except under cutting forces i.e. “sharp things”)

Below is an example of how to close it. The loop on the lower part of the photo is where the box, the battery, or the thing you want to fasten must be placed.

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Construction

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Attach two wires to the battery. Then isolate the terminals, to avoid short circuit if the bicycle metal frame touches them. Once mounted, accessing the battery or changing the connections (without breaking the polypropylene strap) will be very hard.

Fasten the battery to your bicycle. I took advantage of a little rear rack, previously used to carry friends, beer, tools etc.

Another view of the mounting. If you don’t have a back rack, be creative finding any other place. No matter where you install the battery, check that bike handling, brakes, gears etc. don’t get interfered. If you fall, the battery should never touch the ground (Acid spilled everywhere = not good)

Now the car horn. 48 watts of pure noise. Again, our friend the AA cell next to it

I could not get the OEM car connector, so I just welded two wires in place. Keep in mind the polarity (i.e. horn + and – must be connected to the corresponding + and – on the battery, not backwards)

I found an old metal tab with a bolt. By pure coincidence, the bolt matched the horn screw. Get a similar plate, or find any other method that works for you

Metal plate in place. I fastened it with plastic wire straps. Never drill your bike tubes: Interior may get corroded and fail, impaling you

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Wiring

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Now the electrical part. 18 AWG wire is enough for the work. The schematic is extremely easy (double-click on the picture to enlarge).

Locate the fuse as close as you can to the battery. It may save your ass (literally) because 7,2 Ah may easily start fires.

Now you need a simple push-switch. Some of them only have two leads

But some models have three leads, marked with COM – NC – NO. This is how to use them:

COM – connect the battery lead here

NO – Normaly open: Circuit is open all the time, except when you push the switch. That’s what we need for the horn.

NC – Normaly closed: Circuit is closed all the time, except when you press the switch. Don’t use it, for obvious reasons.

I ran out of crimp connectors, so I just soldered the wire

Place the swich next to your fingers

Make sure that all the connections are correct and properly isolated. Not like the picture.

Horn ready!

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Battery Charging

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I wanted an elegant way to connect the battery charger to the circuit. I used an old computer plug, and extracted the corresponding jack from and old CD-ROM drive. Invent your prefered method

Almost finished product – two options to choose from: Original pussy-style horn, and decent horn

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Converting the rear rack into a seat

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Last step: As I used the rear rack to carry girls, beer and other goods, I made a little seat with foam. If you do this use waterproof materials, to avoid nasty forms of life growing in your bike.

Done!

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Bonus – Bicycle lights

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Now, if you want to go PRO, you can use the same SLA battery to feed the bicycle lights (instead of buying AA alkaline cells)

Well, this is the problem: If your little LED’s are powered by two cells (each one 1,5V=3V) and you inject 12V on them, they will fry. We need a voltage regulator, like LM317. Is not very energy efficient, but is cheap as dirt.

We don’t have space for elegant PCB’s, so do the weldings like old school. I know you already know this, but it’s just for the liabilities  thing:

1) Place the components in a way they can’t move, to avoid short circuits

2) Check with a multimeter the output voltage before connecting the light