Auto Horn Selectioin

[ChrisG]

I have very limited knowledge here and am looking for some advise from you guys (and gals)!

I have a car with a horn. What I would like is to have a selector switch on the dash to select the original horn or a new air horn when I press the momentary contact switch on the steering wheel button. In addition, I would like to have a momentary contact swicth on the dash to activate the air horn independent of the new in-dash selector switch used for the traditional steering wheel horn button.

Couple of notes:
Currently, there are no relays in this vehicle (It's an older car). I have added relays for the headlights so I am comfortable in adding them. Also, the air horn draws 18 Amps.

Attached is a schematic I have drawn up. Does that look logical? If so, I am confused on what type/size diodes I should have (if any)? I have the diodes for protection in case the traditional horn switch and the dash switch is pushed at the same time. I can't imaging that happening, but who knows! Am I using those diodes correctly?

Or, if I am totally off base, and you have another idea that a rookie could tackle, feel fee to let me know.

Thanks in advance for your help.

- Chris

edit:
Dang! My first post and a misspelling in the title!!!

 

[KeepItSimpleStupid]

In older cars, I remember the horn switch to switch ground. Check that.

You are correct in using two relays, one to drive each horn and the Bosch automotive relays would be fine. You can get them with a plug harness. The relays should have a diode across the coil in the reverse direction, The arrow of the diode pointing to positive.

Your selector switch just has to select, one relay or the other to the horn contact.

What I want you to do is to put a diode (1n4002 would be fine) in series with each of the coils of the relays before it hits the SPDT switch.

Now, for the trick. Add another diode to the side with the new horn and connect to a switch. The diodes should connect together at the relay coill, such that one goes to the SPDT switch and one goes to the momentary switch. The diode will act as a "wire OR" and isolate the two mechanisms of actions.

Yes, both could be on at the same time if both switches are pressed and the OEM horn is selected, but that should be OK.

 

[ChrisG]

Your are 100% correct. The original horn is wired to switch ground. I am going to change that, but I appreciate (and impressed) that you mentioned it. I have Bosch relays, pretty certain they have the diode internally. I will check. Or try to.

I understand about using a switch to select which horn to choose as we are on the low current side. And I think I see the diodes on the coils before the new SPDT horn selector switch. [edit: Although I don't see the purpose of diode 3 in my schematic.]

I am a visual guy. Is the attached it? I think I got the diodes going in the right direction.

Thanks for your fast response.

- Chris

 

[KeepItSimpleStupid]

Yep. you can remove D3. Sorry. You pretty much got it. I owned a few old cars (1965) and it had a horn/steering wheel problem. My 1982 had a horn problem too. It' just rusted out because of where it was mounted. All I did was asked you to check.

The FORD I had, switched positive for the interior lights and most of my other cars or my parents switched ground.

 

[dr pepper]

I dont think you need the diodes, if you had the air horn selected, and pressed the dash air horn switch there wouldnt be a short or back feed of any kind so the diodes are not going to do much.
Other than that looks as though it'll work.
I've seen this done with a high current 3 terminal toggle switch, but they are ugly and theres not a great deal of choice.
If you thought about it I think you might be able to do that with just 2 relays.

 

[gary350]

I have a K3 locomotive train horn on my SUV. It is best to leave the original factory horn along. Add the new air horn then mount a push button switch on the dash. Push the button and it honks. No need for a selector switch.

I also have a K3 locomotive train horn on my bicycle. I built a 2 wheel trailer to pull behind my bike with an 11 gallon air tank. I ran a 3/8" air hose from the trailer to a diaphram valve on the bike very near the air horn that is in front of the handle bars. I have a push button air switch at the handle grip. There is nothing electric. The push button valve activates the diaphram valve that honks the 135 db air horn. I use my home air compressor to air up the tank before I ride my bike.

You can buy K3 horns on Ebay for about $600 each. You can buy Plans to build your own K3 and K5 horn for $10. You can build your own K3 or K5 horn from PVC pipe for $20.

When I was in high school 1968, in this days car horns were loud and pickup trucks were even LOUDER. I got 14 truck horns from the junk yard and put them all on my 1965 Chevy. I had to use a starter selonoid as a relay and #12 wire to connect all the horns to the selonoid. The selonoid was connected directly to the battery with battery cable wire. Wow it was LOUD, if I honked the horn after dark the car head lights got so dim I could not see to drive. We had a lot of fun with those horns, it was fun to park in the dark near the railroad tracks then honk at the vehicles on the street. Traffic stopped to look but no train. LOL. That was good old fashion fun.

 

[Tony Stewart]

gary, that must cause skid marks in the drivers you beep at.

ChrisG, keep in mind an electric horn is not a resistive load and contacts are significantly derated for motors or solenoids or horns.

I am not even sure if this 40A relay is enough. ( rated for 1000 events)
As 14.2V@18A is 255W inductive load, which will arc on the contacts.

Below is a TE brand 40A contact spec. and 18A is rated for 22Vmax, so 20V MOV or Transorb across contacts is required.

 

[Tony Stewart]

MOV like this may work
**broken link removed**

 

[dr pepper]

I just realised you posted 3 schems, the schem in post 3 should work just as well without the diodes, if you have air horn selected and you press the vehicles original horn button and the dash air horn button together so what, your not going to get a short and you'll get both horns sounding either way.

 

[Rich D.]

Yea, I don't see that the diodes are doing anything but reducing the voltage (and therefore the total power) delivered to the relays. Just more parts that are more likely to fail.

 

[ChrisG]

Thanks for all the comments and suggestions.

Gary350:
First, a $600 horn would double the purchase price of the vehicle. I can't imagine this is going to happen!
You obviously had fun as a kid. That sounds like something I might do now, but never had the courage as a youngster!

Tony Stewart:
Note my earlier comment, as being a rookie. That is WAY over my head.

All:
Diodes removed. Got the parts needed. Final Schematic is attached. I wired it as such and it works. Right now it is using a 15 amp fuse, and we will see what happens. So far so good. Using a varistor is a good idea.

Thanks!

 

[gary350]

My SUV has 1 common wire that operates the horn, AC unit, rear wiper, and radio. If you do anything to that wire it could effect the other 3 items on that wire. You need to check see if your horn wire is a common wire for other things.

 

[Tony Stewart]

Tony Stewart:
Note my earlier comment, as being a rookie. That is WAY over my head.

Thanks!

Just remember if the load is a motor or solenoid or magnetic horn, the arc rating on contacts is 8x rated current of load, so 5A load needs 40A contact rating for a resistive rating (for long life >50k flips), otherwise Motor HP ratings are used.

For a fuse, 1.5x rated current in slow blow may be adequate . Fast blow will be short life.

PTC Polyfuses are used in car window and wiper motors to reduce fuse size to 1.3 fast blow with PTC protection added. These are cheap.

In the old days cars used 6V batteries and the horns when used on 12V sounded like air horns, when used in short spurts.

Nice bike BTW, no side flags?

 

[Rich D.]

Now that schematic looks fine! Simple is most often the best solution.

(Only sorry I didn't get to write this before you posted "...and it works.", 'cause now this comment just seems silly.)

 

[dr pepper]

That looks good Chris, glad you sussed it.