How to Build a Bicycle Car Horn Using 16 Standard Rechargable AA Batteries




Introduction: How to Build a Bicycle Car Horn Using 16 Standard Rechargable AA Batteries


Hello bicycle car horn fans!
After extensive testing and using the method of try and error I finally came up with this pretty reliable setup. I do not mean to say it is the best and ultimate solution but this one turns out to be the cheapest one for me - drill or notebook battery is still pretty expensive. I therefore decided to use rechargeable AA batteries.

 I am sharing  this project because I found out that  8 AA nimh batteries were not enough to power my car horn. Believe me I tried it and  it did not  work.  With only 8 rechargeable batteries (12v and 2,5 Amps current)  all I got was a slight beep. It doesnt supply enough Amps to make it work. So you have to double the current to make it work and this instructable will show you how.


1) With this power setup you can get about one to two months of honking. Or you will get about 35 minutes of continuous  loud honking before it dies out. I recharge every two months.It is also relatively light, the power source weighs only 300 grams. The complete setup with the horn switches and wires weighs about 450g.

 2) You´ll be having lots of fun. Before I installed the horn I cycled 1 hour a day now I cycle about 3 hours a day. It is especially beautiful under the bridges and tunnels But do not do it when you have pedestrians walking there as they would be damn scared. You will soon learn that every tunnel has different sound some of them have beautiful loooooong echoes  

3) The advantage of using AA Nimh battery power source is  that if you leave the  project you can still use them to power digital camera etc.   I know this is very subjective but I still find them very cheap since they can manage 1000 rechargeable cycles. I am using 16 rechargeable batteries, the cheapest  I found here on the European  market.  A pack of four cost me 3.6 €, one wold cost about 0.9€  and that equals about 1.24 USD for one rechargeable  battery.  That multiplied by 16 equals 19.84   USD  for the power source.
Remember to use Nimh batteries as they can supply high discharge current. Alkaline with its relatively low discharge current would not work here.

1) You will be doing lots of soldering! I recommend soldering the complete power circuit with the horn  in the comfort of your home and when you are finished start mounting the complete setup on the bike. That way you avoid long hours of slouching in front of your bike.  At least thats the way I did it.

2) I would also  like to stress that this setup works   only with one 12v car horn (see picture above what it looks like) and  I can assure you that it is as loud as regular car horn.
-    It did not work with dual horns as dual horns require more amperage -  6 to 8 Amps.
-    It also did not work with a single snail car horn. I bought one tried it and it did not work - not enough amps

I am planning to do instructable on the dual horns in the future. I have seen people running dual horns on drill batteries and it works great.That is one option.  But I would personally try here very light lipo batteries used in RC modeling with high discharge rates. If I had more money I would go for Turnigy nano-tech 2200mah 4S 45~90C  4S1P / 4 Cell / 14.8V or something like that.  The C there stands for discharge rate  45C x 2200 mah equals up to 99 Amps discharge. I haven't try the battery yet so I cannot recommend it but I am planning to buy one and  I ll let you know the results.

 3) Beware! Honking is quite addictive. 

Step 1: Step ONE Listing the Items Needed

Step 2: Step TWO - Building a Reliable Power Source

 Power source was the greatest challenge! You will need 16 AA batteries rating  2100 – 2500 mAh.  (I am using 16 rechargeable tronic AA 1,2V 2100 mAh.  I would strongly recommend rechargeable batteries.   It is also relatively light weighs only 300 grams. The complete setup with the horn weighs about 450g.

From these batteries you create two power sources.  Each of the power sources  using 8 AA batteries connected in series This can be achieved by buying two 8AA Battery Holders which I bought at the local store. You can google the term "8AA Battery Holder" to see what it looks like.

  The bettery holder was cheap plus please remember to buy connectors for the battery holder. So now we have two power sources each giving you 12v and 2,5 Amps current. Now you want the voltage remain the same  but  you want to double the amperage to make the horn work . Believe me  it doesnt  work with 12v and 2,5 Amps current. It is not enough amperage all I got was a slight beep..

Connect these two power sources in parallel. Parallel connections will double your current rating, but the voltage will stay the same. To join batteries in parallel, use a set of cables to connect both the positive terminals and another set of cables to connect both the negative terminals of both batteries to each other. Negative to negative and positive to positive.

With this power setup you can get about one to two months of honking. Or you will get about 35 minutes of continuous  loud honking before it dies out. I recharge every two months.

Step 3: Step THREE - Soldering a Power Circuit

Test the horn before soldering. Cover the horn with a blanket, take your power source and  put positive and negative cables to the connectors on the horn. REMEMBER TO COVER THE HORN WITH THE BLANKET OR A PIECE OF CLOTH otherwise you might go deaf for a while.  If it honks, create the power circuit using your power  source, switch, safety switch and the horn.  Follow the diagram. Remember to isolate all the exposed wire after soldering to prevent a short circuit.

The safety switch is optional. It is on and off button. I opted for it because  it happened to me two or three times that I hit the horn button accidentally when I was in my garage while manipulating the bike.

Step 4: Step FOUR - Mounting the Circuit on Your Bike

 Mount the circuit on the bike. I have it mounted on my handlebars and it is hardly visible. 
The batteries are strapped to the handlebars from bellow by strong rubber bands. You can make high quality rubber bands by cutting an old bicycle tube into strips.

Step 5: Mounting the Car Horn

Things you need:

-          1) a cap screw with a hexagonal head and two nuts - see picture above

-          2) one rear bicycle light holder - see picture above

Put the screw through the hole on the car horn, then put it through the whole on the bicycle light holder and tighten the nuts. That way you can put it wherever you like on the handlebars.  It also ensures that the car horn is fastened tightly to the bike.

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    3 years ago

    Sorry to resurrect an old thread but I would really like to know what make and model that horn is. It sounds great in the video. Also, I would like to wire up e-bike lights and a motorbike light switch to the same (twin) battery pack. How would I do that? Sorry, all I know about electricity is that it´s blue.


    Reply 3 years ago

    Great to hear from you!, thank you very much for you question. This horn comes from Czech car ... Škoda Fabia, it costs about 6 usd here in the Czech Republic. Very durable, for it still works on my bicycle even today after all those years.

    Yes, it is possible to wire up e-bike lights and a motorbike light switch to the same (twin) battery pack. For example I have on my bicycle ws2812 led strip lights which are powered from the same power source as the horn. All you have to do is to add one more circuit to the battery, I am attaching one image bellow .... the blue lines mark the way the e-bike lights and a motorbike light switch should be installed.
    One update on this project. Last year I switched from twin aa battery packs to lipo battery, it is much lighter and easier to charge and the results are the same, currently I am using lipo 7,4v 4000mah battery and it works great.


    Reply 3 years ago

    Thank you very much, that is so helpful.

    I don´t suppose you would have a part number for that horn?

    At the moment I am veering toward ordering one of these to power the lights and the horn:

    Users reckon that pack is good for about 60 Wh in the real world, so maybe I´ll get two.

    As you said easy of charging is the issue. For touring I´d still be carrying twin 8-cell AA battery packs in case I cannot charge anywhere. AA cells can be bought anywhere as you well know.


    Reply 3 years ago

    Great to hear from you again. I do not have a part number for that horn for I have done some paint job on it so it is no longer visible. However, if I go to that shop where I bought it again I will ask for that piece of information.

    Yes I saw the DC-12V-8000mAh, it should have enough amperage to run both the horn and lights at the same time. I have similar setup on my bike, i have my horn and led strips powered from the same power source. 3 images are attached bellow. These are ws 2812 waterproof strips running on 5v so I included a convertor to my circuit. These are remote controlled with many color patterns.

    I do not know if I mentioned it in the instructable but for the horn buy what is known as momentary switch == a push button that is only engaged while it is being depressed. And remeber to give me some updates on your project, or ask some questions.And when I go to that shop again I will ask about the horn.


    Reply 3 years ago

    Those strips look great. I´ve been meaning to get some yellow ones to serve as running lights for the cargobike as it´s a very long machine and it would add to the lorryish/trucking feel of the thing. For now I´m focused on this roadbike build.

    In case you´re interested it´s a BMC Roadmachine 02 2018. I did toy with the idea of building a battery pack inside the clip-on (aero) handlebar extensions as they´re the percect diameter and length for eight 18650 cells but then decided to order a pack from Banggood instead as some establishments like cafes and restaurants would probably be unwilling to accept a whole bike inside to recharge whereas the blue battery pack and the small charger are less conspicuous carry-on items.

    In the end I went for this:

    Users report that the real-world capacity tends to be somewhere between 9000 anad 12000 mAh, which is plenty for me.

    It´s going to have a Busch & Mueller IQ-X E headlight, a Spanninga Elips Xe taillight, a motorbike light switch as mentioned, an automotive horn, a motorbike push button as you mentioned and possibly a motorbike USB step-down converter and socket.

    For touring I´m going to bring an eight-cell AA battery holder just in case I cannot charge anywhere. I often sleep in the woods. As you know AA cells are sold in every supermarket and kiosk in most places in the world. As you said alkaline cells lack the discharge current to power the horn but it´s not exactly essential equipment and the bike will have a bell as well.

    I´ll post some photos of as soon as make progress with the build. For now I´m waiting for mudguards from the UK. There´s no point in starting before the mudguards arrive as I´ll probably mount the battery pack on the front mudguard (they´re aluminium) at the base of the headtube and then tuck in the wiring for the rear light inside the rolled edges of the rear guard.

    You don´t need to go out of your way to visit that shop but it would be nice to have that part number. Your horn has such a nice tone.


    7 years ago

    In most motorbike shops you can get horns which run off 6 v - half of th batteries!


    7 years ago

    I've been thinking of ways to reduce the battery cost and size and have the horn properly functioning. What we really need is a battery that can supply high current and has a low internal resistance. Two horns can draw 12 amps. AA's have too much internal resistance which was why you needed to have them in parallel. Two horns mean the load is around 1 ohm which means the battery needs to have an even lower internal resistance. My goal is to use two tones because it's supposed to be easier to hear.

    SLA batteries can be used for dual tone. It's cheaper than 40 rechargeable AA's. With a 4Ah 12V SLA, it sounded louder than using 2P4S with Li-ion cells from old laptop batteries. If it's 4S only, the horns would make a clicking sound unless you tighten its screws which doesn't solve the problem because the volume would be too quiet. Using LiFePO4's from Deal Extreme had the same problem too. You might be able to use a 1.3Ah SLA battery to cut down on the weight. I haven't tried it yet.


    8 years ago on Step 2

    I've used a motorcycle battery and two snail horns


    9 years ago on Introduction

    when i had my set up i used a 9 v cordless drill battery held under the seat. it worked well but it was really wirey and ugly but worked well, maybe u can dress it up by putting the batteries in a tool bag