Check car's ground continuity with arduino

Thread Starter

shachar85

Joined Aug 31, 2017
7
Hi guys
I'm new here, hope it's the right place to ask.

I'm in the middle of a small project for my car. A part of the project is to connect to the car's horn.
The horn is fixed connected to the positive of the battery. I use arduino and sparksfun's relay kit to switch the ground to the horn.
When the arduino output HIGH to the relay's control wire, the relay closes, the horn gets ground and the horn beeps. simple.

Now, I want to add a fuse, to be sure I don't blow up my car.
But the tricky part is that I don't have any way to know if the fuse burnt or something.
I mean - the horn doesn't sound usually. Just when something is wrong. Then - if the fuse is burnt - I don't know it and the horn will not alert me when something is wrong.

So what I thought to do is to check if the point after the fuse (between the fuse and one side of the relay, as can be seen in the diagram) is grounded.
That way - if it is grounded, I know the fuse is good. If it is not - then the fuse blew.
How can I check with arduino if the point is grounded?
 

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crutschow

Joined Mar 14, 2008
25,664
Welcome to AAC. :)
This is a good place to ask.

But first off, a fuse normally goes between the battery and the horn.
Otherwise a short to ground on the battery wire to the horn is not protected, (which is a reasonably likely failure).
(The connection you show for the fuse doesn't really do anything to protect the wiring except if the horn shorts.)
With the fuse in the battery lead you can readily determine if the fuse is good or not just by monitoring the voltage at the relay contact going to the horn.
12V it's good, 0V it's not.
 

Thread Starter

shachar85

Joined Aug 31, 2017
7
Welcome to AAC. :)
This is a good place to ask.

But first off, a fuse normally goes between the battery and the horn.
Otherwise a short to ground on the battery wire to the horn is not protected, (which is a reasonably likely failure).
(The connection you show for the fuse doesn't really do anything to protect the wiring except if the horn shorts.)
With the fuse in the battery lead you can readily determine if the fuse is good or not just by monitoring the voltage at the relay contact going to the horn.
12V it's good, 0V it's not.
 

Thread Starter

shachar85

Joined Aug 31, 2017
7
What you say make a lot of sense
The thing is that the horn is 12v fixed and I'm not going to touch the positive wire.

But from what you say, I can be pretty sure that the original fuse of the car's horn is on the positive wire.
If that is really the case, I can dismiss my fuse and the system is still protected.
Did I get it right?
 

Thread Starter

shachar85

Joined Aug 31, 2017
7
ok I looked up in the car's wiring and it seems to be ok
The horn is connected to he battery through a fuse.
I bypass the the car's switch with switch of my own. What it does is just to close the car's relay to activate the horn.

Now, the relay come in a very expensive component and I wouldn't want to risk it.
Though, I just connect it to the ground, so it is probably safe.

Should I take another measure of precaution? or is it safe enough? I mean...Honda didn't seem to put there another fuse, so it might be ok
 

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shortbus

Joined Sep 30, 2009
8,048
I'm in the middle of a small project for my car. A part of the project is to connect to the car's horn.
The horn is fixed connected to the positive of the battery. I use arduino and sparksfun's relay kit to switch the ground to the horn.
When the arduino output HIGH to the relay's control wire, the relay closes, the horn gets ground and the horn beeps. simple.
What make/brand of car is this? Don't think I've ever seen a car horn wired like that. Every car horn I've ever seen has the mounting brackets of the horn as it's ground. And the positive wire coming from the battery through a fuse, switch and relay. Just the opposite of what you are saying.
 

crutschow

Joined Mar 14, 2008
25,664
Older cars used the car horn button on the steering wheel to ground the wire from the horn to complete the circuit, no relay involved.
 

Thread Starter

shachar85

Joined Aug 31, 2017
7
Actually shortbus is very right.
The horn is fixed to the ground wire. I took another look at the circuit I posted before. I didn't realize that I didn't even connected to the horn's circuit. I connected to the circuit of the relay that closes the horn's circuit.

So all I do is enable a relay. Which is great because much lower currents are involved. I don't even need another relay. Though I will use it because I already have it and it is safer that way.

Anyway I'm pretty sure I can build the project without that fuse I talked about.

Thanks for the answers guys!
 

shortbus

Joined Sep 30, 2009
8,048
Older cars used the car horn button on the steering wheel to ground the wire from the horn to complete the circuit, no relay involved.
Maybe in Model T days. but any car I've worked on from the 1950's up the horn button grounds one side of a relay pull in coil and that sends + voltage to and through the horns to ground. Even the old 6V cars used a relay. And at least up until the 2000's they still work that way.
 
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