When you're cycling, it's important to have some kind of sounding device. A bell can be heard by pedestrians and cyclists but to be heard by drivers, you need something louder such as a horn. Car horns can be heard by drivers when their windows are rolled up. In this setup, the horns were attached underneath a bike basket and the battery and relay were stored in the basket.

Features:

112 dB @ 1 metre
Combined high and low note horns for better perception
Handlebar mounted horn button
On/off switch to prevent accidental horn trigger when stored in public places such as public transit

Step 1: Materials

The parts can be found at hardware stores, electronic component stores, and battery stores. Canadian Tire and Lee's Electronics carry the materials.

Bicycle with a fixed front basket (The basket should be made of thick metal wires.)
6 - position terminal block
Horn button
Twin car horns (The FIAMM El Grande horns were used. Select a high and a low note horns. Horns with grill covers are easier to install as they have more areas to tie to.)
12V sealed lead acid battery (The battery should be at least 1.2Ah. I used a 4Ah battery and it worked well. If you're not sure that the battery can be used, read the data sheet or test it by connecting it with the two horns. It has to be able to supply at least 12A to the horns.)
Pair of two-wire connectors
FIAMM 12V 30A relay
20A fuse
In-line fuse holder
18 AWG wires
22 AWG wires
On/off switch
Crimp connectors
Screws
Nut
Washers
Box
SLA battery charger (The 12BC0500D charger was used. A smart charger is recommended for battery longevity because it stops charging when fully charged.)
Cable ties
Tape
Plastic film

Step 2: Warnings

Wash hands after handling. Sealed lead acid batteries contain lead.
Wear goggles when soldering, cutting, and crimping.
Use hearing protection when testing the horn.
The fuse should be wired as close to the battery as possible.
Make sure that loose wires are secured.

Step 3: Wiring

Follow the wiring diagram. The on/off switch, relay, and terminal block were attached to a box. Crimp connectors were used for connecting to the battery's and relay's contacts. Cable ties can be used to strain relief the wires. Make sure that when the box opens and closes the wires won't over-bend.

Step 4: Mount Horn Button to Handlebar

Loop the wire connecting the horn button around the handlebar twice. If possible, mount the button where you can cover the brake lever and button at the same time. The reason for this is that when you honk, you need to be able to brake.

For weatherproofing, the button should be loosely wrapped in plastic and taped.

The button may be cable tied to the handlebar. To protect the handlebar or grip from being scratched by the metallic part of the button, wrap the bar or grip with duct tape.

Step 5: Mount Horns Under Basket

To mount the horns, use cable ties. You can attach the horns through their grill covers and mounting plates. The horns should be forward facing.

Step 6: Wrap Battery With Foam

Step 7: Attach Battery and Wiring Box to the Basket

When attaching to the basket, the battery and wiring box should be in the basket. They both require two velcro straps perpendicular to each other. That way, they won't slip when your bike vibrates.

To weatherproof them, you can cover the screws and contacts with duct tape.

Step 8: Secure Any Loose Wires

Use cable ties or Velcro to do this.

Step 9: When to Use the Horn?

The horn should be used to promote safety when road users make a mistake or alert them of your presence. Use the horn with caution when other cyclists are nearby. If you're passing cyclists, you should use your bell instead. You can reduce the horn's volume by tapping the button.

Bike safely. Always be prepared to brake near intersections and driveways and obey the rules of the road. The following article teaches you how to cycle safely http://bicyclesafe.com/. Using the horn applies to some of those safety tips.

Step 10: Charge the Battery Every Few Months

Every few months, top up the battery's charge. Even if the battery is not used, it will lose charge. Avoid prolonged storage at partial charge because it causes sulfation which shortens the battery's lifespan. Charging needs to be less frequent in cooler weather and more frequent in warmer weather. If you use the horn more often, the battery needs to be topped up more often.

Step 11: Possible Improvements

Substitute the SLA battery with a Li-ion or Lipo battery pack to reduce the weight.
Use a smaller battery to reduce the weight.
Mount the horns and batteries into a box.
Substitute the horn relay with a solid state relay or a transistor so that the horn turns on faster.
Use a horn switch designed for handlebars such as those used for electric bikes, scooters, and motorcycles.
Use louder horns such as the FIAMM Freeway Blaster horns.

Step 12: Update: Horns Moved Into the Basket

It may be better to move the horns into the basket to avoid interfering with bike parking. With no horns under it, you can park your bike in the middle of bike racks. Before that, the bike could be locked only to the sides of the bike racks and sign posts. I used the Abus Bordo folding lock.