You guess right!

This is more or less what I've ended up doing:



Prior to that I just had the switch :rolleyes:

Wow! I didn't think it would be that complicated, I don't understand. Wouldn't the buzzer just buzz all the time when the fog light is on? Or does the 1 amp diode some how stop it buzzing! Ehh I feel thick! Why is the relay at the bottom not doing anything? No I'm confused! lol!

Oh I see now the electric current passes through the diode when the ignition is switched off and the fog light switch is on because it then becomes the path of least resistance? Am I right??:D I'm not, am I? ....Doh!

When the ignition is on the buzzer has 12v at either side, when the ignition is off (0v) it will buzz as that's the earth.

Mmmm I still don't get it but I started looking at relay stuff and ended up coming up with this....



Would that work? Other than the fact I'm probably drawing too much curent through the 1st relays coil to power the fog but another relay would solve that. Or am I way off track?

In a word, no :D
You're right - the fog won't light up because it won't get enough current through the coil of the relay.
You could add a third relay, but you'd be over complicating it a bit!

You can completely ignore the second relay on my circuit. I only needed that because I'm using a switch with a built in LED.
If I was doing it from scratch I'd use a seperate warning lamp wired in parallel with the fog light.
So without that my circuit looks like this:



So... if you ignore the buzzer and diode first. When the switch is on it turns the relay on. If the lights are on the fog gets power, it they're off it doesn't. Simples.

Now, ignore the relay and fog light and just look at the switch, diode and buzzer.
When the switch is on and IGN is on, both sides of the buzzer are at 12V, so no current flows through the buzzer and it stays off.
When the switch is on and IGN is off, the + side of the buzzer will be at 12V and the - side will be at 0V. So current will flow through the buzzer and it will sound.

The diode is needed to stop current flowing from IGN through the buzzer and relay coil when the fog switch is off and IGN is on.

Ahh I see! You are quite brilliant. One question though! Why does the piezo buzz if a circuit isn't made to the negative side of the battery?:confused:

Only if the ignition switch changes from live to ground , most will change from live to unconnected not ground.

TT

Ah ha, you've both discovered the slightly dubious subtlety in my scheme :o

It's correct that IGN is typically switched from battery to unconnected.
However, there are going to be several things on the car that are powered off IGN.
These will be connected between IGN and GND, and effectively drag IGN down to GND when it's switched off.
The buzzer will sort of leech current through these loads, which is the dubious part. However it requires so little current that it gets away with it...

Those electrons are trickey little buggers!:D