Wiring Footwell LEDs: Using Diode, Switch, and Ground on Negative-Switched Circuit

the first question that sprigs to mind is WHY ???? but joking aside
why use a diode ?

the earth from your cigarette lighter is permanently earthed, how are you controlling the current for the LEDs ?

I need the diode so that when I power on the LEDs using the switch, which is grounded from the cigarette lighter, the ground isn't sent back into the gray wire as well, which would turn on my dome light.

Does that make sense?

My major concern is, is it bad to have the switch connected to the cigarette lighter as well as the dome light wires? Could one ruin the other?

Finally, would I need a diode anywhere before/after the switch to prevent any current from going back into the cigarette lighters ground? Or is that ok? Or will that not happen?

EDIT: Actually I may see what you are confused about. Both the pink and gray wire are hot, until my door is opened, at which point in time the gray wire is grounded. The switch would accomplish the same thing, by grounding the gray wire.

Hi Billy, it helps if you attach a circuit diagram when you ask a question like this, saves a lot of words and a lot of misunderstandings. JPG is OK, or GIF or PNG is neater and a smaller file.

I am attaching a couple of basic circuit ideas that will help you. A regular white LED takes about 3.3 volts, and 20 to 30 milliamps. We set the required current level with a resistor.

The best simple circuit for 12 volts is a string of 3 LEDs in series (get the polarity right), and a resistor to limit the current by dropping the spare volts. Add more strings in parallel for more light.

You are right about needing a diode in some car circuits to prevent current going back where its not wanted.

If you post up an image of your intended circuit, we will tell you where they go.

Sorry I am not an artist, but how does this look?

Mostly what I need to know:

Can I use just the diode? Could either the dome light or cigarette lighter get messed up?

Would I need a second diode on the cigarette lighter side?

Or is all that wrong and I need a relay?

And overall, does that look right?

Thanks for the help!

I'm trying to install some LEDs in my footwells.

I have a pink wire, which is my positive, and a gray wire, which is my negavtive. This is a negative switched circuit.

So, is it ok to hook it up like this?

1 Positive of LEDs to pink, negative to gray.

2 Then, use a diode on the gray wire, with the stripped end pointing away from the leds back to the gray. Now, on the non stripped side of the diode, add a wire which connects to a switch. Then, connect the other side of the switch to the ground on my cigarette lighter?

The first part works I know. But can I just use a diode? Or do I need a relay? I know the second step functions I just want to know is it safe?

Also, if the second isn't safe, would it be safe to ground it to the car instead of into the cigarette lighter?

Thanks!

Oh- finally, if that does work, does the ground of the cigarrette lighter function as a ground 24-7? Or only when the car is on?

EDIT: See below for diagram.

The best I can tell, you don't need the diode, but you do need to follow Frank's advice -- unless these are "12Volt" LEDs (i.e. LEDs that include an internal current limiter that is set for 12Volt operation), then you don't need any current limiting.

The reason why I don't think the diode is necessary: You are getting a direct connection to the positive side of the battery via the pink wire and you are getting a direct connection to the negative side of the battery via the cigarette lighter (if I understand your circuit diagram). Because of this, there can be no current path through the dome light (or any other "appliance") from the LED circuit, no matter what state it's in.

UNLESS: the idea is for the footwell lights to come on when the dome light comes on [you assumed we all knew how "footwell lights" are supposed to work ;], Then you _do_ need the diode.

Now, not sure that you mean by "safe"? Do you mean safe as in "fire hazard" or "shock hazard" or ??? If so, then it's fire hazard safe as long as you take steps to prevent a short. And the voltages are not nearly high enough to pose a shock hazard in any scenario that I can think of (I mean, if you were doing open heart surgery in the back seat and one of the wires dropped into the open cavity and touched the heart while some part of the patients bare skin was in contact with the bare metal of the car's chassis--then, it could be a hazard. but what are the odds of _that_ happening?).

If "safe" means will it fail in the presence of your hot date, then not due to the integrity of the circuit, but I can't vouch for Murphy, Fate, or Jealous Gods (is she hot enough to tempt divine favor?)

First, I apologize for not mentioning this, but they are commercial strips, wired as 12v strips meant for a car.

Second, sorry for not making it clear, I do want them to come on when the dome light comes on.

The way the dome light works is: pink (+) is 12v always hot. gray (-) is always hot as well, until the door opens. When this happens, the gray wire is grounded, completing the circuit.

So, knowing that, I do need the diode? And then would I need on on the wire which is going back to the cigarette lighter?

And finally, is the ground of the cigarette lighter always a ground? Or only when the car is powered?

And by safe, all I need to know is, if I hook it up as suggested, would it pose a risk of fire, shorts, overheating, anything? Frying the cig lighter or dome light?

Well, I, too, apologize. I didn't take a close enough look at your schematic. It turns out that you DON'T need the diode after all. Check out my version of the circuit, below.

And, the 12 Volt LED strips DON'T need any current limiting (that's already taken care of)--so, just hook them strait up!

And I don't know if the cig lighter is always connected to ground -- probably is, but I don't have a lot of variety in my experience of cars, and my experience of cars spans the 60's, 70's and early 80s and that is only from working on my own cars. Once they got loaded up with all that computerized whatsamahookie, I started patronizing auto mechanics.

BUT, you can test the cig lighter ground. Just get a continuity tester and see if there's continuity between the cig lighter ground and the chassis when the key is off. Or/and simply connect one of the LED arrays to the cig lighter ground and to the pink wire and see if the thing lights up with the car key off.

Is S2 my switch and LS1 my lamp? And are you not using the cigarette lighter at all?

Perhaps after I completely understand your diagram that will make more sense.

Yes, LS1 is the Dome Light and S2 is your switch. I updated the diagram to show those labels.

If I interpret your diagram correctly, the only thing that "grounds the grey wire" is the door switch, right? If so, then no need for a diode. The Footwell Lights are simply across (in parallel with) the Dome light, so they are turned on or off just like the dome light is, unless S2 is open, in which case the Footwell lights are out of circuit, and the system behaves as if they were never there (i.e only the dome light comes on when the car door opens).

Thanks for the explanation. That makes sense for sure.

However, I don't need to be able to disable the footwell lights, what I want to be able to do is have it so I can turn on the footwell lights independently of the dome light. So the point of the switch is to turn on the footwell lights when the dome is off, not disable the lights when the dome is on.

Does that make sense?

_Holy carp eating catfish_! I looked at your diagram again, and realized that I'm still full of _shingle joints_!

OK, please tell me what you intend for this set up. The following is what we call, in the industry, a "Truth Table" -- let me know if I got this right (and if it isn't, please correct it):



[code lang="c" line="none"]
//| Door | Switch | Dome Light | Freewell lights |
//====================================================
//| closed | closed | off | on |
//| open | closed | on | on |
//| closed | open | off | off |
//| open | open | on | off |
[/code]

or if you can't correct it without it getting all messed up (I had to try several things to get it to format properly) just tell me what is wrong with it.

Or let me know if you don't understand what it means and I'll try to explain it.

Don't worry about it! I appreciate the help! (Also a carp eating catfish sounds scary!!)

Sorry I can't make my table look as nice like yours. Very similar, just I believe one change.

//| Door | Switch | Dome Light | Freewell lights |
//====================================================
//| closed | closed | off | on |
//| open | closed | on | on |
//| closed | open | off | off |
//| open | open | on | on |

Or I can just tell you, the only difference is that I want the lights to come on when the door is opened regardless of the switch.

Although I suppose if your table would be easier I could do it your way. I would prefer mine I think though.

Perhaps I need to see what the difference would be in terms of difficulty wiring.

OK, you were replying at the same time I was, so I missed your correction. So, it sounds like my truth table is correct, and if it is, then, you still don't need the diode. Updated diagram, attached.

The reason no diode is needed is because the footwell lights are completely independent of the dome light. All you are doing, at the dome light, is accessing the positive side of the battery. Because you are grounding the footwell lights via the cigarette lighter, the dome light isn't even in the footwell lights circuit. So, no worries!

Let me know if I still have this wrong

I just noticed that I forgot to move the "footwell switch" label, so attached is yet another diagram update...

Also, this diagram assumes that the ground at the cigarette lighter is active even when the car is switched off and the keys are out of the ignition. You'll need to do the test I suggested to determine that, and if not, then you will need to ground he footwell lights somewhere else.

With that last setup though, I don't believe opening the door will actually power the LEDs will it? The pink wire is always hot, so I think this would be the equivalent of just tapping into the cigarette lighter.

Also, I think I would rather have that one change in the truth table, I can't imagine a time when I wouldn't want the lights to come on when opening the door.

Finally, I would rather be able to leave the switch in the on position and the lights not be on all the time (this way when I turn on the car, the lights come on automatically without my flipping a switch), so I think I am planning on making that ground only active when the car is in the acc position (probably with a relay.)

Does all of that make sense?

Because the "ground" is being interrupted by the door, the Pink wire isn't really the "hot"
wire--I know that is probably confusing, but ask the question, "what makes 'hot' hot?"

Unfortunately, there are two answers to that question:
1. The pink wire is "hot" because it is at 12 Volts relative to the chassis--which acts as a "ground" return.
2. The pink wire is not "hot", because the hot side is the side connected to the switch. In fact, one could argue, at least for this one circuit, it's a positive ground system--in other words, the Pink wire is ground, and the chassis is at -12 volts (i.e. it's a "hot chassis").

So, think of it in terms of current loops. Look for the completion paths. The pink wire is connected to the positive side of the battery. The switch is connected to the negative side. When the switch closes, it completes the path from the positive side of the battery, through the dome light and back to the negative side of the battery via the door switch.

If the LED strings are connected across the dome light, then they will be energized in the same way the dome light is: when one of the car doors is opened, the dome light and anything connected across it will come on. When all of the doors are closed, the dome light, and anything connected across it, will turn off.

The only role the cigarette lighter plays is to supply a ground connection. Nothing else. Assuming that is a consistent connection to the negative side of the battery, that is all it will supply -- a connection to the negative side of the battery, no matter what state the cigarette lighter is in. If it doesn't, then it isn't the kind of connection you want.

So, now, that leaves the switch. I'm struggling to understand what the switch is for. You say you want "that one change in the truth table" but I'm not sure what change you are talking about. You, also, say, "I can’t imagine a time when I wouldn’t want the lights to come on when opening the door." That will happen if you hook the LEDs across the dome light,

But, you also say, "I would rather be able to leave the switch in the on position and the lights not be on all the time" -- so, when _do_ you want the lights to come on -- I thought, when the door is opened? And then you say, "this way when I turn on the car, the lights come on automatically without my flipping a switch" So, I still need to know when you want these lights to come on, because that makes it sound like you want them on whenever the car is "on" (or did you wanted the circuit to be "active" (i.e. with the potential to be turned on)).

Can you give me some kind of "truth table" with the three conditions: car on or off, door open or closed, switch turned on or off. There will be 8 lines (or conditions) in such a table, and it would really help to know what each of those conditions are. You can do it like this:

Switch off, Car off, Door closed: LEDs ???
Switch off, Car off, Door opened: LEDs ???
Switch off, Car on, Door closed: LEDs ???
Switch off, Car on, Door opened: LEDs ???
Switch on, Car off, Door closed: LEDs ???
Switch on, Car off, Door opened: LEDs ???
Switch on, Car on, Door closed: LEDs ???
Switch on, Car on, Door opened: LEDs ???

My understanding of these wires is they are both 12v, until the door is open when the gray is grounded. You're saying that's not right? My understanding was that in order to complete the circuit, I could take a wire coming from the negative side of the battery and tap it into the gray wire, therefore grounding it, and completing the circuit. That's wrong?

And its like this:
Switch off, Car off, Door closed: LEDs off
Switch off, Car off, Door opened: LEDs on
Switch off, Car on, Door closed: LEDs off
Switch off, Car on, Door opened: LEDs on
Switch on, Car off, Door closed: LEDs off
Switch on, Car off, Door opened: LEDs on
Switch on, Car on, Door closed: LEDs on
Switch on, Car on, Door opened: LEDs on

I just reread your reply to Sarah where you said: "The switch would accomplish the same thing, by grounding the gray wire." So, what I take from that is, you want the switch to do that same thing to the LEDs, that the door switch does to the Dome light but you also want the door switch to control the LEDs (while continuing to controlling the dome light).

So, now I see the need for the diode, but I'll wait until you supply a truth table before commenting further.

Yes, glad we are on the same page! I've been to so many places around here, and the only place who really has the knowledge to help basically said I had to give them my car and pay them to do it.

I realize my desired setup is very confusing, and I truly appreciate the help.

(Also, we posted 2 minutes apart so the truth table must not have been there when you loaded the page, its now in my previous response.)

bq). My understanding of these wires is they are both 12v, until the door is open when the gray is grounded. You’re saying that’s not right? My understanding was that in order to complete the circuit, I could take a wire coming from the negative side of the battery and tap it into the gray wire, therefore grounding it, and completing the circuit. That’s wrong?

The pink wire is always at 12 volts, but the gray wire is not. So, it isn't correct to say it is "at 12v". It is only transiently at 12v. Sounds like you are thinking of it as a "supply line"-- it's not. It's the switched side of the dome light, and as you said, when a door is open, it goes to ground. Yes, if you run a wire from the negative side of the battery and connect it to the Grey wire, the dome light will, indeed, come on. Doing that is the same thing as opening the door.

1.Switch off, Car off, Door closed: LEDs off
2.Switch off, Car off, Door opened: LEDs on
3.Switch off, Car on, Door closed: LEDs off
4.Switch off, Car on, Door opened: LEDs on
5.Switch on, Car off, Door closed: LEDs off
6.Switch on, Car off, Door opened: LEDs on
7.Switch on, Car on, Door closed: LEDs on
8.Switch on, Car on, Door opened: LEDs on

You're right, because of condition 7, you will need something like a relay to shift the pattern when the car is on. Basically, the switch controls this pattern shift and this can be implemented by simply using a relay (or transistor) in series with the switch. When the car is on, the relay closes and turns the LED on (always) unless the switch is open, in which case the LEDs follow the dome light. Also, the diode _is_ needed, see attached diagram.

Why would my diagram I put up not work for 7? Wouldn't it basically be the same as grounding the gray wire and just blocking the signal before it gets to the dome light?

I know a relay would work (I have one in now, but the dimming effect of the light makes the relay sound TERRIBLE to where I don't even want to use it.

if the light is on a timed dimmer, that changes everything a little.
so you want the leds to come on when the door opens, and an override switch to keep them on permanently.
something like this.

bq). Why would my diagram I put up not work for 7? Wouldn’t it basically be the same as grounding the gray wire and just blocking the signal before it gets to the dome light?

1.Switch off, Car off, Door closed: LEDs off
2.Switch off, Car off, Door opened: LEDs on
3.Switch off, Car on, Door closed: LEDs off
4.Switch off, Car on, Door opened: LEDs on
5.Switch on, Car off, Door closed: LEDs off
6.Switch on, Car off, Door opened: LEDs on
7.Switch on, Car on, Door closed: LEDs on
8.Switch on, Car on, Door opened: LEDs on

The diagram you uploaded would produce the result for condition7, but not for condition 5. For condition 5 it would be "LEDs on".

And what do you have the relay coil connected to? It sounds like you have it connected to the pink wire. That's the only place that I can think of that would have the kind of pulsed voltage that would cause a relay to chatter. if that is coming off the Accessory line, then I'm mystified. Why would the accessory line cause a relay to chatter. Does it chatter even if the car's engine is not running?

But, anyway, you can do this with a transistor, which won't make any noise, Two diagrams show how with bi-polar and with MOSFET (the preferred method). I don't know what kind of current your LED lights draw, so it's hard to make recommendations, but probably can't go wrong with an IRF540 for the MOSFET.

true, if the cigarette lighter is switched via the ignition, then taking a the supply for sw2 from there would solve that.

My relay is connected to the cigarette lighter and gray wire.

It doesn't run when the car isn't on as it is connected to the cigarette lighter.

Oops replied to the wrong post.

I was thinking of making a relay using the cigarette lighter ground and positive, and then when it activates, it providing the ground to the switch.

That way, it only provides a ground when the car is running.

And what exactly is the MOSFET? I have never heard of that?

And Sarah, by that do you mean if the ground is only grounded when the ignition is on my table would work?

Thanks for the assistance guys!

Connecting the relay coil across the cigarette lighter sounds like a good idea (if, indeed, that is what you said--you have a tenancy to "abbreviate" in ways that are not clear to me--such as saying "relay" instead of "relay coil" or "relay contacts", or saying "making a relay" instead of, what, "connecting a relay coil to..."? or saying "connected to the cigarette lighter", which side of the cigarette lighter? By not spelling it out, you're relying on my powers of deduction ,

If the cigarette lighter doesn't work when the car is switched off, then that is ideal (i.e. that suggests that the cigarette lighter is being fed by the Acc line).

Connecting the relay coil across the cigarette lighter ground and the grey wire makes no sense. You want the relay to engage when the car key is turned to Acc and beyond, not when the door isn't open (regardless of whether the car is on or off). If you're connecting the relay coil to the grey wire, that validates my suspicion that you're thinking of the grey wire as a source of 12V. That is incorrect. It's only a source of 12V when all of the car doors are closed. That's not a reliable source of 12V -- it's a source of the logic state: "one or more doors open or all closed". And, we've established that the pink wire isn't even a source of 12V. It's a source of "continuous, brightness-modulated, voltage". So, actually, the grey wire is really a source of: "one or more doors open or all closed AND brightness modulation when one or more doors are open".

So, I would test the cigarette lighter to see if it doesn't work when the key is in the off position, and if so, then try connecting the relay coil there, and wire the rest of up the way I show in my latest diagram (reproduced, below). In the diagram, "Acc" becomes a connection to the positive side of the cigarette lighter.

I apologize for not using the best wording, I am learning all of this as I go. I do believe that your latest diagram is quite similar to what I am trying to describe.

However, to clarify a few things:

-my cigarette lighter does not work with the car in acc.
-I'm wondering if my ground would still function though in the cigarette lighter, which I would think it would. So, I want to set up a relay in order to make it that ground is only provided to the switch (or would it be better to have the relay after the switch, or does it matter?) when the relay (coil?) is activated. To accomplish this, I would use the positive and negative of the cigarette lighter, so the coil is only active when the car is in acc, not off.

Is that what your diagram is saying?

Finally, when the door is closed, would that unground the cigarette lighter? Or no? I don't completely understand that part. I would guess yes, except for how you just tried to explain it.

Oh, and just to clarify, although not super relavent, I do have my relay currently hooked up to the gray wire, but the other side is hooked up to the positive side of the cigarette lighter. I do apologize if I stated that wrong or confusingly earlier.

Also, I will be at radioshack later today or tomorrow, so what diodes should I pick up? 3 amps? Anything else in terms of picking up a diode I need to look out for?

bq). -my cigarette lighter does not work with the car in acc.

Then that isn't a path to Acc.

bq). -I’m wondering if my ground would still function though in the cigarette lighter, which I would think it would. So, I want to set up a relay in order to make it that ground is only provided to the switch (or would it be better to have the relay after the switch, or does it matter?) when the relay (coil?) is activated. To accomplish this, I would use the positive and negative of the cigarette lighter, so the coil is only active when the car is in acc, not off.

No. You've established that the cigarette lighter doesn't provide a connection to Acc ["_my cigarette lighter does not work with the car in acc_"], So, connecting to the cigarette lighter won't give you what you want (based on the truth table you provided)--unless we define "car on" as "car's engine running" and not "car with key switch set to Acc and beyond". If that is the case (if "car on" means "car's engine running"), then, yes, by all means, connect the relay coil across the cigarette lighter.

bq). Finally, when the door is closed, would that unground the cigarette lighter? Or no? I don’t completely understand that part. I would guess yes, except for how you just tried to explain it.

Think about what you are asking: what would be the point of designing a car with a cigarette lighter that doesn't work with the doors closed? After all, how is a cigarette lighter going to work if it's ungrounded? If what you meant was, will it be ungrounded with one or more of the doors open, probably not, but it's a simple test: open one of the car doors, and then see if the cigarette lighter works--it can't work if it is ungrounded.

BTW: if you meant the latter, then, please proofread your replies!

bq). Oh, and just to clarify, although not super relavent, I do have my relay currently hooked up to the gray wire, but the other side is hooked up to the positive side of the cigarette lighter. I do apologize if I stated that wrong or confusingly earlier.

Well, no wonder it's chattering when the lights are dimmed--and I bet it only does that when all of the doors are closed. Again, were you thinking that the grey wire is a source of 12V? It's only a source of 12V when all of the doors are closed and the lights are turned up to full brightness (and maybe not even then)--IN OTHER WORDS, IT'S _NOT_ A SOURCE OF 12V. Were you thinking that the grey wire is a source of ground? It's only a source of ground when the one of the doors are open--IN OTHER WORDS, IT'S _NOT_ A SOURCE OF GROUND. The relay coil needs to be connected to either a path to Acc and ground, or across the cigarette lighter (i.e. one side of the relay to cigarette lighter ground, and the other side to cigarette lighter positive UNLESS IT'S ESTABLISHED THAT THE CIGARETTE LIGHTER DOESN'T FUNCTION DURING ONE OF THE REQUIRED STATES (SUCH AS WHEN ONE OF THE DOORS ARE OPEN) [check your _Truth Table_]--IN WHICH CASE YOU WILL HAVE TO FIND ANOTHER CONNECTION FOR "Car On", WHATEVER THAT ULTIMATELY TURNS OUT TO MEAN (It still hasn't been established if "car on" means key switch turned to Acc and beyond, or if it means "Car engine running").

bq). Also, I will be at radioshack later today or tomorrow, so what diodes should I pick up? 3 amps? Anything else in terms of picking up a diode I need to look out for?

I don't know how much current your LED arrays draw, so I have no way of determining the current rating on the diode. What you need is a rectifier diode with a PRV (Peak Reverse Voltage) of 100V or more (I would go with 400V or above--car supply voltages are riddled with voltage spikes that can damage semiconductors that aren't able to withstand those spikes, Thus, the need to select a part with high voltage capability.)

Find out the current spec for one of your arrays, multiply that by 4 and then again by around 1.5 and get a rectifier with a continuous forward current rating of that much or more. You don't need anything fancy like a "fast recovery" diode, just a plain ol' rectifier.

AHhhhhhhhhhhhhhhhhhhhhh.

My mistake. Brain is roasted right now.

The cigarette lighter DOES work in acc.

Does not work when car is off.

So when the car is off, would the ground function still? Or no?

And I do realize that the cigarette lighter is currently not ungrounded when a door is opened.

What I mean is, when the door is closed, and the gray wire functions how it does, if that indirectly is attached to the cigarette ground, would it mess up the cigarette lighter where I would need a diode? Similar to the reasoning behind needing one for the domelight.

As for the relay, the intention was for it to be grounded when the door opened. So it actually functions fine. Its just because of the dimming it clatters. The way the relay is currently set up actually is a lot more intensive then I have described, but it is irrelavent.

Knowing my absolutely idiotic mistake, how does it sound?

Goodness I feel like a doofus. Sorry again for the confusuion.

EDIT: In terms of the diode, it seems a strip of LEDs runs around .25 to .36 amps. So I'll go high, with .5 per strip. (I don't actually have a way to determine actual amount.)

.5*4=2
2*1.5=3

So, is the PRV you mentioned also referred to as Peak Invert Voltage? I see a 3 amp, 200 PIV diode at radioshack.

Actually I see one with 400 PIV for $.30 more. So that ought to work. As long as PIV=PRV