Hi,m
First post here. I hope I have the correct forum.
I have a project where I want to use Arduino to operate a car horn.
The Adruino is a 9v but the horn unit is 12v.
Is there a simple way to "trigger" the horn rather than power it from Arduino?
See wiring diagram. The project works with the 9v actvie buzzer but I want to replace the buzzer with a car horn. That ius to say, I want this arduino project to attach to an existing car horn.
Thanks.
Thank you, FEBaily
I haven't looked closely at that circuit, but the first thing I saw, and need to question, is why the Common on the relay is ignored.
As far as I know, and just tested with a relay of my own, the NO and NC never show continuity across them. I stopped trying to make sense of it then 
Can I trigger a 12v car horn using 9v arduino?
An Arduino is a 5 volt device, not a 9 volt device.
If you power it with a 9v power supply the onboard voltage regulator reduces the voltage to 5v for the rest of the circuit board.
...R
FEBaily:
I haven't looked closely at that circuit, but the first thing I saw, and need to question, is why the Common on the relay is ignored.As far as I know, and just tested with a relay of my own, the NO and NC never show continuity across them. I stopped trying to make sense of it then
Correct, the diag is wrong. I have moved the NC to C
Car horns can take very high currents, and are inductive, you need to establish what the load characteristics are
before selecting a switching device for it.
If you really have that exact 10A relay then it is probably sufficient for a horn. What happened when you tried it?
Don't use the Arduino grpund pins for the horn. Wire its ground back to the battery or to another connector. If this is installed in a car then chassis ground is good.
I'll ask a question nobody else has.
What's the purpose?
If it's just to blow the horn, as one would in any vehicle, the relay and Arduino are not necessary. A 12v compatible 2 wire button on the negative or positive side will do the trick.
MorganS:
If you really have that exact 10A relay then it is probably sufficient for a horn. What happened when you tried it?Don't use the Arduino grpund pins for the horn. Wire its ground back to the battery or to another connector. If this is installed in a car then chassis ground is good.
I tried, but nothing happened. I actually dont even know how to connect the horn as it has only 1 connector, not 2 like in this diag.
thanks
The horn's ground is it's mounting bracket, the one terminal is connection for 12V, those cheapy blue relays will probably start sticking after a few dozen blows and you will have to jerk the hood (bonnet) up and yank the wire off the horn to stop it blowing. Splurge on a REAL car horn relay (12V coil), or what @DangerToMyself said.
I guess what I am asking is what do I need to do to make htis work? I connected to the car horn as shown and nothing happened. I had assumed it was because I was trying to power the horn via the adruino, which doesnt supply enough power. so without going t buy more devices, is there any reasonable way I could make this device, work with any car horn, by connecting it differntly? Can I use MY device to trigger the horn, but the horn is powered by the car, as it normally is? If so, where would I my device.
Just to let everyone know also, I am a total electronics noob. Whilste I am quite mechanically minded, this is the first time I have tried to build anything electronic based.
Everyones help is very much appreciated.Thanks you.
Please tell us if the horn is still in the car and does it work, right now, as it should, when you try to use the car horn?
Is the horn in a newer car or is it in an old car, say 15 years old? The reason is the newer cars probably have the car computer actually honk the horn.
Paul
OffTheRicta:
I guess what I am asking is what do I need to do to make htis work? I connected to the car horn as shown and nothing happened. I had assumed it was because I was trying to power the horn via the adruino, which doesnt supply enough power.
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There is an error in your original diagram. As noted in post #3
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The Arduino is not supplying power. Power comes from the battery to the relay, which switches under the Arduino's control.
MorganS:
There is an error in your original diagram. As noted in post #3
The Arduino is not supplying power. Power comes from the battery to the relay, which switches under the Arduino's control.
Yes, this is as I thought it might be when it didnt work. What I am asking, I suppose, is where would I 'attach' the adruino? So the horn still uses the car battery for power, but is triggered by the arduino?
OffTheRicta:
Yes, this is as I thought it might be when it didnt work. What I am asking, I suppose, is where would I 'attach' the adruino? So the horn still uses the car battery for power, but is triggered by the arduino?
Then you must control the relay that is between the horn and the battery.
Paul