Okay /diy/, longtime reader, first time poster. First of all, forgive my non-standard MS Paint wiring diagram. I've been Googling my ass off to learn about how to make circuits, and I feel like I've learned just enough to get myself hurt or damage my car, so I'm coming to you to check my work and tell me how wrong I am.
I want to add ambient footwell lighting to my car. I have four LED strips that I will wire in parallel. Each has 15 LEDs, for a total of 60--I'm not sure if they're 5050 or 3528, but the difference shouldn't be important. At the most, if they're 5050, each should want about 0.24 watts of power, so at most, my circuit will be drawing 14.4 watts, and for a car where everything is 12V, that's a current of just 1.2 amps, which the circuit I'm borrowing can handle.
I'm tapping into an unused accessory socket in the car that's connected to the dome light. One end is a hot, constant 12V of juice, the other end runs to ground through a factory NC switch system. Door opens? Lights come on. All doors closed? Lights go out.
I could plug these in and be done, but I want to add a switch that will give my install an independent "On or Off or Door" choice that the dome light itself has, Middle position is off, or I can switch it down to be door-dependent, or i can switch it up to be constantly on.
But, at the same time, I want the lights to be as bright as possible when they're dependent on the door, and I want them to be dim when they're constantly on. That way, I can have them as nice, always-on ambient light while I'm actually driving.
By my very novice thinking, I can accomplish this with a DPDT switch, and a resistor of some kind bridging the positive wires between the input and the "on" output of the switch. From what I've read, that resistor has to be parallel to the LEDs, not in line with the LEDs, which is why I've got it diagrammed as such.
Can this work as pictured? What, if anything, do I have wrong?
In case the wiring diagram doesn't make it clear, I'm accomplishing the "always on" by running the negative side of the switch's "On" output straight to ground, bypassing the factory dome switch. When I do this, if I want the lights dimmer, I know I need to give them less current, so what kind of resistor do I need to put in to accomplish this? I figured a 10 Ohm resistor would add 10 Ohms of resistance, effectively doubling the resistance of the circuit by my figuring, and therefore making the lights half as bright.
I know only enough to know I'm probably wrong about this.
I'll be bumping with cute animals and additional questions. Stuff like: Is parallel the way to go for connecting the strips together? I've heard LEDs actually cause a voltage drop, so I figured wiring them in series would make them progressively dimmer and not matching.
I'll also add that I have a multimeter, and only a very rudimentary knowledge of how to use it.
i cant says YES or NO coz the switch is a mystery. you need to redraw it with a standard notation so we can tell what connects to what inside the switch. use either the left or right representation in the pic provided. personally i prefer the right coz you can follow the current flow, instead of it being a black box.
>>1004573
I think you'll want a circuit more like pic related. First off, you only need a spdt switch, not a dpdt. Just be sure to get one that is described as a on-off-on switch, not an on-off-mom or mom-off-mom. Mom stands for momentary. I included an example digikey part number for you.
Next of all, you need to limit the current going to the leds. If it was all one big series circuit, I would recommend a 680 ohm resistor for r1 because that would result in a total current of about 20 mA, but since you broke it in to 4 parallel branches, I divided that by 4 and assigned the closest standard resistor size. The best way to drive leds is with a constant current supply, but given that this system is running on a battery, you will probably be okay with just a current limiting resistor. If your leds can handle more than 20 mA, lower the value of r1. For example, if your leds can handle 100 mA, at 12V, r=e/i, so r=12V/0.1 = 120 ohm, then divide that by 4 for the 4 parallel branches and you would have r1=30 ohm.
Finally, you see that I included r1 to limit the current to the max allowable current, but after the switch I included a second resistor to limit the current even more if you have the switch in the always on position. This way, the leds will be on at full blast when the switch is in the door position, but will be dimmer because of the reduced total current when the switch is in the always on position. I selected 2.2 kohm for this value because I think it's a good starting place. But you can change the value of r2 to anything between, say, 470 ohm (bright) and 10 Kohm (very dim).
Hope this helps.
>>1004654
Oops. I mislabeled the switch in the diagram. It should be spdt, not dpst.
>>1004654
This circuit will work fine if you can understand it but the values of R1 and R2 are way off. R1 isnt needed at all, it should be a short circuit, coz the strips already have internal resistors for each LED. And R2 should be around 10-ohms at 10W.
>>1004660
Ah, ok. I've never used led strips before. So disregard r1 and use a 100 ohm potentiometer for r2 so you can adjust the brightness. Something tells me you're going to want them pretty dim if you're driving with them on.
>>1004654
>>1004660
This is absolutely awesome. One thing I am super novice about is resistors, how they work, and where to put them.
I think I was mistaken because I had seen, when using LED turn signals, people have to install a resistor so the car's computer doesn't see the small load and mistakenly think its bulb is blown. The resistor there is wired in parallel, to just "use up" whatever juice the LEDs don't need.
In this case, I don't need a resistor just to use up extra power, I need one to choke the circuit down to dim the lights, and that's why I can put it in series with the lights.
Also, placing the resistor after the load... I know how, in the real world and on paper, that works, but it's still intuitively confusing for a noob.
So how do I properly calculate the resistor I need? I figured the circuit was naturally at 10 ohms of resistance just plugging the numbers I DO have into the basic equations. But how do I figure out a good resistor to use to make the lights a certain amount dimmer?
> But how do I figure out a good resistor to use to make the lights a certain amount dimmer?
trial and error. you can predict how it will react, so you might wanna get 3 10-ohm resistors, and use diff combinations to get the right brightness.
1 resistor = 10 ohms, 2 in series = 20 ohms, 3 = 30 ohms
or two 10 ohms in parallel, then in series with one 10 ohm = 15 ohms.
etc etc, lots of combinations possible. one thing, tho, they'll get hot so you need to take care of that, like finding a metal box.
using a pot is great but pots than can handle that much power are big and expensive.
alternately, you can use various strength car bulbs instead of resistors. a 12W bulb for example is roughly equivalent to a 10-ohm resistor.
>>1004663
By that same token that I don't get resistors, I also don't get potentiometers.
I can find, for cheap on ebay, a PWM dimmer dial rated for 12V up to 8A:
http://www.ebay.com/itm/Manual-Dimmer-Switch-for-LED-Strip-Light-12V-8A-Mountable-with-Terminals-/251985183083
If I just run the +/- in and +/- out to be in line with the main wires after I've joined up all my LEDs, that should accomplish the job, right?
>>1004669
>how do I calculate the resistor I need?
Ohms law is all you need here. I think the real problem that your having, and that most beginners have, is understanding what is a series circuit and what is a parallel circuit. This circuit has a little bit of both, so you need to understand which part of the circuit you are talking about and then identify if you should be treating it as series or parallel.
For example, when we are talking about the total current going through the circuit, we are talking about a series circuit that just so happens to have one component in it that can further be broken down in to parallel branches. If you are looking at a single led then you have to analyze it as a parallel component because it just so happens to be part of the big series element that can be broken down further as parallel branches.
In a series circuit, you can put a current limiting resistor anywhere in there and it doesn't matter where; it will behave just the same. If you want to limit current in a parallel branch, you have to consider how that will effect other branches of the parallel section. It's a little weird at first, but I'm sure you will get it.
>>1004671
That isn't exactly a potentiometer. A potentiometer is just a simple resistor that you can physically change the value of. What you linked to is an active circuit that turns on and off very quickly. The knob controls how long it is on versus how long it is off. It switches so quickly that your eyes can't see it switch. It will work for your application though.
>>1004680
Absolutely fantastic. As you can imagine, my degree of soldering skill is on par with my level of understanding of electronics, so the fewer connections I have to make, the better--and I don't trust T-tap splices enough to go that route.
I imagine the PWM dimmer has the advantage over a static resistor or potentiometer of not generating any waste heat. It'll involve another step every time I want to switch modes, but I'd say that's worth it for the added level of control.
Any other tips before I set out this week to start tinkering? Stuff like "disconnect the car battery's negative terminal before doing anything" or "don't run with a hot soldering iron"?
>>1004684
My tip is don't worry too much. I've run LEDs off my fuse panel to light the trunk. Just put in a simple switch, no resistors. Lasted years
Just run a wire from your car's roof light, that way switching isn't an issue. And run the accessory power to your car light circuit to make up for it
>>1004732
>>1004573
>Can this work as pictured? What, if anything, do I have wrong?
You should use a current regulator instead of a resistor. For better load balancing, all of the LEDs should be wired in series. Use a boost converter to get the necessary voltage.
