I wanted to see if I m overthinking this project, I m using a car horn and it uses 12v 7amps, I h...

[G+_Donald Kloss]

I wanted to see if I’m overthinking this project, I’m using a car horn and it uses 12v 7amps, I have the following limit switch, will it be easier to hard wire it directly or should I utilize a relay module with an arduino nano? This will be sitting for a while with power going to it constantly until the lever gets tripped

[G+_John Sullivan]

 

Your switch is rated 0.3 Amps at 250 VDC, 0.6 a at 125 VDC, so if we follow that math, at 12 VDC it should be rated at 6 amps, which is slightly less than the 7 amps you want to put through it.

You can try it if you like and see what happens. You haven't told us how long you would expect the horn to blow ... for a few seconds probably would not burn out the switch.

If you're concerned that the switch might fail, then use an automotive horn relay in between. If you decide to go that route, let us know

[G+_Donald Kloss]

cosmic Ray oh ok so the bottom portion of the switch is the max input, the horn will only go off for a second or so, would I be able to use my 5v relays modules that I just got in?

[G+_Todd Barnard]

Donald Kloss As long and the relays are rated to handle minimum 12v @ 7amps across the contacts, then you can run 5v thru the micro switch to energize the coil.

[G+_Donald Kloss]

Thank you all again for the answers I appreciate it

[G+_Dave Schmarder]

A .6A @ 125VDC contact is still .6A @ 12VDC.

[G+_Jason Marsh]

Back in the day the courtesy light switches in the door jamb of a car were commonly repurposed to sound the horn when the hood was raised. Think of it as a ghetto fabulous alarm. They were wired directly, no relays needed. The switch you have may work fine, or it may not break contact when opened. Test it out. Just wear earplugs if you're going to be right next to the horn.

[G+_Donald Kloss]

Jason Marsh I will test it I wrapped it up and put it in a box I found the horn somewhere a while back

[G+_Arcturus Lancaster]

The switch will work for a while. Eventually will burn off the contacts. Hardest part in it is opening the circuit. It will eventually either stop closing or fuse closed

[G+_Donald Kloss]

Arcturus Lancaster would it be easier to just utilize the 5v relays modules I have and use an arduino with a separate 5v battery

[G+_John Sullivan]

Dave Schmarder So why is it rated at 0.3 a at 250 VDC? But it does not show a 12 v DC rating, so putting 7 amps through it probably won't last long!

[G+_John Sullivan]

Jason Marsh Not true .. the door jamb switches were wired to the horn relay, not the horn itself. (Although I have not yet lived in a ghetto, so possible that they do things different)

The door jamb switches are on the ground side of the relay, (provide a ground path) therefore only require one wire run to them. The horn requires 12 v Pos because it also is grounded to the frams.

[G+_Arcturus Lancaster]

You could use the switch to control the relay so long as the relay is rated for that current load. Breaking a DC circuit is hard . When the circuit opens the voltage willl spike as it attempts to keep current flowing

[G+_John Sullivan]

Donald Kloss Now you are over thinking it ... just use an automotive horn relay, just the same as your car does. You use the switch to energize the relay, and when the relay contacts close, it blows the horn. The higher current only goes through the horn relay contacts.

[G+_Donald Kloss]

cosmic Ray ? I’ll look into that

[G+_John Sullivan]

Arcturus Lancaster True, It's common to use a diode reverse-biased across the relay coil leads, to provide a path for the discharge.

[G+_John Sullivan]

Donald Kloss I just looked at the horn relay for my old 1989 Ford van, and it's got multiple connections that you won't need. You might take a look at a three connection one, maybe from something like a 1960's model, to keep it simple.

Here"s a diagram for a 1966 Ford F100 pickup.

http://www.fordification.info/tech/images/wiring/66extlight2s.jpg

If you enlarge it, you can trace the path from the starter relay, where it picks up the 12v, through the horn relay and out to the horns and to the horn button. It even shows an interior diagram of the relay.

[G+_Dave Schmarder]

cosmic Ray That's true, but the current rating doesn't keep going up as the voltage goes below 125VAC. I sold these kinds of switches (plus a lot of others) for 34 years. There have been way more than a few customers that thought these switches would do as you claim, only to have been disappointed.

They do make those kind of switches with a high DC current rating, but they aren't that common.

[G+_Telford Dorr]

X2 on using a horn relay. As previously said, put a diode across the coil backwards (e.g. so that it doesn't conduct when the relay is activated) to absorb the coil's back-EMF on turn-off. Drive the relay coil with a suitable power mosfet from the Arduino output pin. If not using an Arduino (or other logic), then skip the mosfet and drive the relay coil directly with the switch. Leave the diode - it'll make the switch live longer...

[G+_Paul Hutchinson]

Data sheet for the switch is here:

omronfs.omron.com - omronfs.omron.com/en_US/ecb/products/pdf/en-v.pdf

 

Note there are minimum voltage and current specs for the switch (5VDC and 0.16A) so make sure the 12VDC relay draws at least that much current for reliable operation. Too little voltage/current will eventually stop breaking through the natural oxidation of the silver contacts greatly increasing the contact resistance.

 

Also as Dave Schmarder points out, you can't modify the specs like they are a power (wattage) rating. The voltage/current rating for the contacts is all about preventing a sustained, or quickly extinguishing the, arc as the contacts open/close, not about the power handling (it's a UL fire safety rating). The arcing in a switch carbonizes and/or pits the contacts and will make the switch fail eventually.

 

So while it looks like you could use a much higher current with lower DC voltages the reality is that arc sustain/extinguish is a very non-linear function. So without the manufacturer explicitly stating the current at 12VDC you can't be sure how reliable it would be. It's almost certainly higher at 12VDC than at 125VDC but how much is virtually impossible to reliably determine without access to confidential engineering details of the switch's construction.

[G+_Jason Marsh]

cosmic Ray Depends. I've seen horns with two leads; one to supply power and one for ground. Could be switched on either lead. Such horns are probably outliers, though. I've honestly never had to repair a vehicle with an inoperable horn, so haven't looked at how the OEMs wire them. I have seen them with doorjamb switches on the core support so they'd blow the horn when the hood is opened. Either way, I'm thinking his project might not require a lot of on/off cycles under load, so selecting a switch that'll take 10,000 cycles may not be necessary. Might make a nice alarm for the liquor cabinet ;)

 

Oh, and any non-standard repair or shady mod can reach ghetto-fabulous status. It's a mark of distinction, not to be confused as a derogatory remark. I borrowed the term from an associate's MX5 with a SBC under through the hood.

[G+_John Sullivan]

Paul Hutchinson Thanks Paul (and Dave) for clarifying, and thus reinforcing my suggestion to use a horn relay. ; )