Wiring lights

[Tem]

Been scratching my head all day long and I can't figure this out.

I have a pair of daytime running lights and here's what I want:
1) They are off when lights are off
2) They are on when tail lights are on and headlights off
3) They are off when headlights are on


I found from BGB that the Body ecy gets info about both lights, so I figured I'll use that to trigger the relay. Pin10 gets info about headlights and it has 12V when the lights are off and ground when they are on. Pin8 has accordingly 12V when tails are off and ground when they are on.

So:
1) Both pins have 12V
2) Pin10 has 12V, pin8 has ground
3) Both pins have ground

But if I put a relay between pin10 and pin8, the relay doesn't trigger, but instead I get 12V on both pins  s:? :? s:?

After Googling I suppose I should use a diode...but where and how?  s:? :? s:?

[aaronjb]

You want a rudimentary OR gate, Tem, like so (I think):



Hopefully that all makes sense enough  s:) s:)

The diodes will need to have a reverse-voltage capacity of at least 12v, obviously, but that's really the only property you need to worry about, I believe..

I'm sure, if I'm madly wrong, someone will yell and correct me in a second  s:) s:)

Apologies for the *terrible* coil icon in the relay, btw, freehand with Paint was never easy..  s:) s:)

[Tem]

Wouldn't that also trigger the relay at #1?

"1) Both pins have 12V"

[aaronjb]

Wouldn't that also trigger the relay at #1?

Whoops! Sorry Tem, I read that, but it didn't sink in.. let me have another think  s:) s:)

[aaronjb]

Right.. so what you actually need is..

Code Select Expand

Pin 8 | Pin 10 | Result
------+--------+--------
   0  |    0   |    0
   1  |    0   |    1
   1  |    1   |    0
   0  |    1   |    0


You could do that with some logic gates, but probably the easiest would be:

Connect pin 8 to a normally-open relay (switches on when a current is applied), and connect pin 10 to a normally-closed relay (switches off when a current is applied).

Connect 12v to the input of the first relay, and connect the output of that to the input of the second relay, and then the output of the second relay to the lights..

I think that would do it without any IC's involved.. Unless anyone has any better ideas I'm missing?

[Tem]

I was afraid I'd have to use two relays...

(XOR would do that, but it would be too much trouble  s:? :? s:? )

[aaronjb]

Yeah - it's not ideal, but it's probably the best way.. Afterall, if you start using some funky electronics for the logic switching you'll not only have to drop the input levels down to TTL level with resistors, but you'll have to use it to drive a relay anyway, so you'd only be losing one relay and gaining electronics..

Much easier to sling some relays together with auto wire than to make up the stripboard and IC's etc.. Not as nice, of course, but..  s:) s:)

[SteveJ]

Yeah - it's not ideal, but it's probably the best way.. Afterall, if you start using some funky electronics for the logic switching you'll not only have to drop the input levels down to TTL level with resistors, but you'll have to use it to drive a relay anyway, so you'd only be losing one relay and gaining electronics..

Much easier to sling some relays together with auto wire than to make up the stripboard and IC's etc.. Not as nice, of course, but..  s:) s:)

If you use 4000 series CMOS logic there is no need to drop to TTL levels you can quite happily operate up to 18V as long as you clean up the supply rails. All you will need is a small NPN transistor on the output to drive the final relay.

[SteveJ]

Code Select Expand

Pin 8 | Pin 10 | Result
------+--------+--------
   0  |    0   |    0
   1  |    0   |    1
   1  |    1   |    0
   0  |    1   |    0


I was afraid I'd have to use two relays...

(XOR would do that, but it would be too much trouble  s:? :? s:? )

That doesn't look like an XOR to me  s:? :? s:?  that would be -

Code Select Expand

Pin 8 | Pin 10 | Result
------+--------+--------
   0  |    0   |    0
   1  |    0   |    1
   1  |    1   |    0
   0  |    1   |    1


[Tem]

That doesn't look like an XOR to me  s:? :? s:?

You're right, but since the only requirements are these three, XOR is enough  s;) s;)

Code Select Expand

Pin 8 | Pin 10 | Result
------+--------+--------
   0  |    0   |    0
   0  |    1   |    1
   1  |    1   |    0


There's no occasion, where Pin8 would be high and Pin10 low, so the outcome of that wouldn't matter.

I'll go for the two relays anyway, just have to wait till monday to get them  s:? :? s:?

[Tem]

Connect pin 8 to a normally-open relay (switches on when a current is applied), and connect pin 10 to a normally-closed relay (switches off when a current is applied).

Connect 12v to the input of the first relay, and connect the output of that to the input of the second relay, and then the output of the second relay to the lights..

Oh well...doesn't seem to be so easy after all  s:? :? s:?

When measured against ground, Pin8 has 12V when lights are off and 0V when the tails are on. Also according to BGB Pin8 is grounded when tails are on.

But when I put a relay between 12V and Pin8, assuming it would trigger when I turn the tails on...it doesn't. And measuring against ground, both pins of relay have 12V, yes even the one connected to Pin8.

Uh...huh...any ideas?