Car Horns for Bicycles




About: Autistic person who's interests include utility cycling, recreational cycling, cycling safety, electronics, gardening, Arduino, and LEDs.

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.


  • 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

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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 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.

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    14 Discussions


    3 years ago

    Your parts list is excellent.

    I upgraded the weak little horn
    in my Toyota, and plan on using the original beeper on a city bike one
    of these days. I think this is a good approach (if you own a car). That
    way you get a LOUD horn in the car and a much smaller lighter one on the
    bicycle, that's still loud enough for a bicycle. I will dig around and
    find the horn later today and add a picture for you.

    The problem with super loud horns on a bike are (1) they are bulky and heavy (2) they are SO loud you will refrain from using them after watching the hair stand on end of the car driver in front of you a few times -- this can be satisfying sometimes but most of the time you want something loud but not painfully loud else it becomes a weapon instead of a signalling device.

    If you
    integrate a 12v lighting system then you can more easily justify
    carrying a 12v battery around for the horn. I am especially fond of the
    many options for 12v LED running lights available now. I have had good
    luck finding them at Look for "rectangular truck running lights". Use RED on the back and AMBER on the sides and possibly front. White belongs ONLY on the front. Blue is for emergency vehicles ONLY.

    or 5AH battery is plenty big enough for both lights and horns. In fact, you can
    add a 12V-USB adapter and power up a smartphone too. I wish I could go with lithium batteries. There are really nice 12v lithium iron phosphate batteries out now too. Lightweight but too expensive for me (did I mention I have a Toyota not a Tesla? :-)

    My favorite
    battery charger for this kind of set up is the Battery Tender Junior. The one you choose it probably the same for all practical purposes. Just another option.

    favorite connectors are Anderson Powerpoles. If you use powerpoles you can completely eliminate the terminal strip from your set up. Your approach is a little more flexible but harder to mount and possibly more prone to short circuits. You can eliminate the separate charging connector, the terminal strip, and the metal box. To charge you just unplug the battery and plug in the charger. Been doing this with ham radio set ups for years now. Works great.

    You said "put the fuse close to the battery". Great advice! I put ATC fuseholders ON the battery, I tape them down permanently to the top of the battery so they are always in the circuit. (I don't PLAN on blowing fuses so having them taped down is okay.) Then the output from the fuse goes to a powerpole connector, this makes swapping out or disconnecting the battery really easy. I usually buy powerpoles from West Mountain Radio.

    Fuse holder suggestion:

    All Electronics has a good price on the relay too:

    Using foam to pad the battery is a good idea.

    Solid state relays are more finicky generally. Building your own (say using a MOSFET transistor) is not worth it. Mechanical relays are very, very reliable and cheap as long as you don't let them fill up with rain water. (Yep. Happened under the dash in my '71 VW bus! Leaky windshield seal. And they will even work when filled with water for about a month... that's how reliable they are.)

    Cheers and keep riding!

    3 replies

    Reply 3 years ago

    I might use the battery for spare lights. If you're running bike lights with SLA, you would have to top it up every ride to prevent sulfation. Deep cycle batteries would be better. Battery backups always float charge their lead acid batteries. Solar charging might be more convenient because it charges while your bike is parked.


    Reply 3 years ago

    I use a bell when passing cyclists and pedestrians. An effective horn should cause a driver to brake. I find that I tend to pulse it because they are close to my ears and unmuffled. When the windows are up or the stereo is on, the horn needs to be loud. To test it, you can have someone blow the horn while you're in a car. The times that I use it include a car about to turn right in front of me and pull outs without slowing down. Usually they stop. When I'm near conflict zones, I slow down so that I can see if the cars have stopped or there's a safe gap. Ride safely.


    3 years ago

    Great idea for Vancouver, BC. In my experience they are the worse drivers I have ever seen. I am curious as to the weight of the lead acid battery. As well as my bike I would be very interested in putting this on my Motorino scooter. Motorino makes good scooters with crappy lights and an even worse horn. Thanks

    1 reply

    Reply 3 years ago

    Here are the details for a 12v 4ah SLA BATTERY.


    Reply 3 years ago

    Hey, camo is an important component of any city bike to reduce the risk of theft. Also parking next to fancy bikes when possible helps. Hmmm. horn could be part of antitheft system... hmmm....


    3 years ago

    This will defineteilly draw everyone's attention ;-)


    3 years ago

    This seems like a really great idea. Definitely will make the roads safer for the cyclists!