LED Backup Light Upgrade

[kuksul08]

Hey guys!

In my car (BMW 3 series), the backup 'light' if that's what they want to call it, is a crappy 21W bulb that barely makes any light. There are two of them. The supply voltage is 12V as would be expected - the multimeter measured about 11.97V.

I want to put an LED in there so that I can actually see things when I back up, and to have something unique. I have already removed the glass bulb from the base, so the interface with the car is good. Since it's pretty much unlimited power, I don't need to bother with a regulated circuit, right? I plan on using a resistor in series.

My question is - what is my best option for LED choice, and optimization of the optics here? The current reflector probably will not work that well for an LED. I am considering an XR-E, or MC-E. I don't have experience with anything else. I am thinking it will be best to do a 2s2p MC-E setup to limit the resistor size needed, but I am open to any ideas.

Here is what I have to work with.

:wave:

 

[msmemt]

Hmm, funny you should post this because I was just searching for advice of what kind of resistors I would need to install an MC-E to a 12v source for the same reason... to upgrade my reverse lights. The emitters themselves seem to be hard to find, do you know where to buy them?

 

[kuksul08]

Yeah, I have ordered from Deal Extreme before. The prices and products are good but it can take forever. Also Cutter.au is another source, but more expensive. They did however just lower their prices, so about 15USD for an MC-E on a star, plus $10 shipping I believe.

 

[msmemt]

well I bought a couple MC-E K bins emittors and OP reflectors... we'll see how this works out!

 

[lonesouth]

I would use a couple of bukpucks (1 per side), mount whatever LED you have laying around or XR-E R2s if you are buying on a piece of aluminum flat stock and put em in. I've got some bucktoots I'm planning on doing the same for park/stop lights in an early bronco.

 

[J_C]

Unless you bought flood reflectors you may find the beam too narrow, half the time I find I am not backing up straight but rather to the left or right so I need to see a pretty wide angle. You might start out without the reflectors to see what it's like, but then heatsinking could require making something... at least if you aren't backing up for a long distance you won't have to worry about managing a sustained ~ 12W from the LED, something minimal could handle the few seconds most people are backing up but it would be nice for more margin if you were ever in a situation where you had backed up and held the brake while waiting or talking, whatever the situation might be.

 

[kuksul08]

Thanks for the replies. I am sure some buckpucks would work, but they are pretty expensive, and arent they typically for a varying voltage source, where you want the current to be constant? Since I have constant voltage, I was thinking a large resistor would work.

(12V Source - 6.8V 2s2p MC-E) = 5.2V

5.2V = (1400mA)R -> R=3.7 Ohms

P=(1400mA)(5.2V) = 7 Watts ... so I would need a fatty resistor

Something like this: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=3.9W-10-ND

 

[Diesel_Bomber]

The electrical environment in cars is not constant-voltage. You can have 12.7 without the engine on, up to 14.7 with the engine on, and anywhere in between. Make sure you choose your resistors with the engine-on voltage in mind, and give yourself a safety margin.

If you're only getting 11.9v at the bulb, you have wiring issues, especially if that's the case when the engine is on.

 

[John_Galt]

Also, don't forget a thermal mass for the LED. You don't want to fry the LED the first 5 seconds you have it lit up.

 

[kuksul08]

The electrical environment in cars is not constant-voltage. You can have 12.7 without the engine on, up to 14.7 with the engine on, and anywhere in between. Make sure you choose your resistors with the engine-on voltage in mind, and give yourself a safety margin.

If you're only getting 11.9v at the bulb, you have wiring issues, especially if that's the case when the engine is on.

Shouldn't there be a voltage regulator after the battery to ensure the safety of the electronics? Please correct me if I'm wrong. Either way, my measurement at the bulb was with the car off.

I definitely will need to take voltage fluctuations into account. So... I've decided to use an LM338 voltage regulator (Current regulation in this case). It can account for the minor voltage fluctuations, provide up to 5A (although I will only need 1.4A), and will allow me to get a smaller power resistor. Hopefully this will limit size, and only cost a few $.

http://www.reuk.co.uk/buy-LM338T.htm

Also, don't forget a thermal mass for the LED. You don't want to fry the LED the first 5 seconds you have it lit up.

Definitely. I am going to basically build a plug for the LED to sit on... of some sort, although I am not too worried about the heat. The car will only be in reverse for a few moments. But yeah... safety factor

 

[J_C]

Generally speaking an approximation of working voltage to use on automobile equipment that is used when the car is running (like reverse lights) would be 13.8V. If it changes +-1V or so that isn't a big difference when you are going to wire 2S2P and only run it for a few seconds at a time. It is not likely to be regulated, that is the one beauty of the simplicity in incan bulbs that they just have to pick one comfortable at peaks of 14.V. Same with most automobile electrical parts, motors and such I mean, are designed to run ok off the slightly varying voltage while ICs will have regulation stages before them.

Current regulation with a linear regulator will give you constant light output (or at least close, temperature is a factor too) which is desirable for a consistent result but the running voltage will be a bit higher, the alternator steps up duty and voltage average as a result when battery voltage would've otherwise been lower, but don't expect any decrease in total size using a linear regulator for this application. On the contrary it will need to be quite a bit larger than one or two resistors as you have to put a heatsink on the regulator and find a place to mount that while resistors can be air-wired.

The heatsink on the page linked, they are quite wrong in suggesting it will support an LM338 up to full load rating. That heatsink is good for maybe 3W at most in a climate controlled environment not in a hot outdoor environment, and with a regulator dropout of 2.5V or so (dropout could vary a little from this, consult the datasheet), that's no more than 3/2.5 = 1.2A. You'll probably want an enclosure for it too. I'd expect all that to take up at least 2 cubic inches and usually more than that since minimally sized enclosures are difficult to mate with projects unless you build your own or get lucky. Either way it's going to be far larger than an couple power resistors or possibly even an LED specific integrated current regulator.

You might see if DX or KD has a switching regulator you can use, but pick one with only ceramic or tantalum capacitors not electrolytic so the outdoor environment isn't an issue, then stuff it into a plastic bottle lid and squirt some epoxy in when finished to have a DIY potted driver.

Since the LED is producing heat too and the space inside the bulb is not much, personally I would put either the resistors or other regulation on the wiring harness just before the socket, not in the bulb. However then you have a permanent mod where you or someone else can't just put a standard bulb in anymore if the day came when someone wanted or needed to replace it. Ideally the LED bulb would run for many years, but in practice it may depend on how hot it gets.

 

[kuksul08]

Generally speaking an approximation of working voltage to use on automobile equipment that is used when the car is running (like reverse lights) would be 13.8V. If it changes +-1V or so that isn't a big difference when you are going to wire 2S2P and only run it for a few seconds at a time. It is not likely to be regulated, that is the one beauty of the simplicity in incan bulbs that they just have to pick one comfortable at peaks of 14.V. Same with most automobile electrical parts, motors and such I mean, are designed to run ok off the slightly varying voltage while ICs will have regulation stages before them.

Current regulation with a linear regulator will give you constant light output (or at least close, temperature is a factor too) which is desirable for a consistent result but the running voltage will be a bit higher, the alternator steps up duty and voltage average as a result when battery voltage would've otherwise been lower, but don't expect any decrease in total size using a linear regulator for this application. On the contrary it will need to be quite a bit larger than one or two resistors as you have to put a heatsink on the regulator and find a place to mount that while resistors can be air-wired.

The heatsink on the page linked, they are quite wrong in suggesting it will support an LM338 up to full load rating. That heatsink is good for maybe 3W at most in a climate controlled environment not in a hot outdoor environment, and with a regulator dropout of 2.5V or so (dropout could vary a little from this, consult the datasheet), that's no more than 3/2.5 = 1.2A. You'll probably want an enclosure for it too. I'd expect all that to take up at least 2 cubic inches and usually more than that since minimally sized enclosures are difficult to mate with projects unless you build your own or get lucky. Either way it's going to be far larger than an couple power resistors or possibly even an LED specific integrated current regulator.

You might see if DX or KD has a switching regulator you can use, but pick one with only ceramic or tantalum capacitors not electrolytic so the outdoor environment isn't an issue, then stuff it into a plastic bottle lid and squirt some epoxy in when finished to have a DIY potted driver.

Since the LED is producing heat too and the space inside the bulb is not much, personally I would put either the resistors or other regulation on the wiring harness just before the socket, not in the bulb. However then you have a permanent mod where you or someone else can't just put a standard bulb in anymore if the day came when someone wanted or needed to replace it. Ideally the LED bulb would run for many years, but in practice it may depend on how hot it gets.

lovecpf

I didn't take into account a heatsink for the transistor. I have played with one similar before, and it got warm, but never to the point of needing a heatsink. I wonder how much of that voltage drop through it is actually released as heat.


I am going to think a bit more about this... maybe sketch some things up while keeping all your suggestions in mind. Thanks!!!

 

[J_C]

100% of the voltage drop is released as heat, that is what makes it a linear regulator. There are still advantages to using one with a variable voltage power source, but with the automobile running your power source voltage won't vary much, IMO not enough to be concerned about unless you were using a configuration with very slim margins, for example if you were trying to run 3 x series of 3W LED @ 3.6V each, then the sum total of 10.8, being only 1.2V away from a 12V nominal voltage, would make a deviation of 1V almost 50% change for the purposes of picking a resistor.

Assuming your alternator, battery, and wiring are in good shape, I suspect you will find a reasonably steady voltage closer to 13.V while the car is running and recommend you have the car running to take additional voltage measurements. When the car isn't running, would it really matter if reverse light output were a little dimmer? I don't usually see people turn off the ignition while the car is in reverse gear anyway...

 

[kuksul08]

Opinions? Suggestions?

 

[J_C]

I could be wrong, but I suspect that reflector is too narrow a beam for backup lights.

Given the volume the entire assembly takes up, and the low mass of the heatsink, it looks like it could support a 3W LED, not a 4 x array of them. If you can effectively thermally couple the (brass?) bulb base to the heatsink you should have a significant improvement but still not enough. Filling the bulb base with thermally conductive epoxy would increase durability a bit but also increase mass, useful for high power short duration light operation, but possibly still not enough.

If I were going to use a reflector I'd use one that is metal (assume that one isn't?) with a good thermal bond to the heatsink, or pill if it starts out as some sort of drop-in you modify. THEN I think it would be enough for brief operation at 5W total from the MC-E.

I should mention I am thinking conservatively, "what if" you leave your reverse lights on for more than a few dozen seconds, and to have it last the life of the car. In that case I suspect even a single 3W LED is pushing the limits of the pictured design so if I had that bulb in front of me, I'd probably see how it did with a single XR-E/etc at 500mA, but I know that is not bright enough to meet your goal.

Sorry if I seem critical of the design, it just isn't enough for the job IMO.

 

[-Virgil-]

Talk about reinventing the wheel!

#796 bulb, direct swap for P21W, 780 lumens from a filament in the right place for the optic it's going in.

 

[kuksul08]

I could be wrong, but I suspect that reflector is too narrow a beam for backup lights.

Given the volume the entire assembly takes up, and the low mass of the heatsink, it looks like it could support a 3W LED, not a 4 x array of them. If you can effectively thermally couple the (brass?) bulb base to the heatsink you should have a significant improvement but still not enough. Filling the bulb base with thermally conductive epoxy would increase durability a bit but also increase mass, useful for high power short duration light operation, but possibly still not enough.

If I were going to use a reflector I'd use one that is metal (assume that one isn't?) with a good thermal bond to the heatsink, or pill if it starts out as some sort of drop-in you modify. THEN I think it would be enough for brief operation at 5W total from the MC-E.

I should mention I am thinking conservatively, "what if" you leave your reverse lights on for more than a few dozen seconds, and to have it last the life of the car. In that case I suspect even a single 3W LED is pushing the limits of the pictured design so if I had that bulb in front of me, I'd probably see how it did with a single XR-E/etc at 500mA, but I know that is not bright enough to meet your goal.

Sorry if I seem critical of the design, it just isn't enough for the job IMO.

THANK YOU for the critique, this is exactly what I'm looking for .

Regarding the reflector, it has a 'wide' beam, which is a 48 degree angle. Due to the nature of the emitter, it is generally hard to focus it well, and there is a lot of spill. I know this from experience, using a 'smooth spot' optic with an MC-E. Even with the 'spot', the light tends to flood quite a bit. Of course, I will only find out once I can test it, but I have a feeling the wide beam angle will diffuse enough.

Regarding the heat, I agree it is not sufficient for sustained operation. I am still trying to look at other heatsinks, as this one is rated at a mere .5W or 1W with no forced convection, I think... From my experience with a bike light (with an MC-E in 4P with 500mA per die), the heat is really not a big issue when dealing with short run times.

The reflector is metalized polycarb, I could look into a metal one to provide more surface area for heat transfer though :thumbsup:

Once again, I appreciate it. This is my first 'sketchup' of the idea.

Talk about reinventing the wheel!

#796 bulb, direct swap for P21W, 780 lumens from a filament in the right place for the optic it's going in.

Haha, yes, I have seen that before. They claim 7W more than before, so that would mean 28W, and 100% more light output. The 1156 bulbs suck so badly, I'm not sure how that could convert to 780 lumens. Also this one will use about 9 watts. It doesn't really matter since it's a car with an alternator, but hey less power draw is always good.

Either way, this project is less of reaching the goal as it is the process of reaching that goal - that is, the research, design, and fabrication processes


:twothumbs keep 'em coming!

 

[J_C]

Since you are no longer relying on the internal backup housing reflector you might as well shoot for a heatsink that extends out away from the socket as much as possible if the rear lens comes off the housing for access, but I suspect it does not? If you cannot get the lens off the housing and must install the bulb through the rear/trunk with the socket detached from the housing, I would take a different design.

I would turn the old bulb base into nothing more than a plug for the existing socket, but I would leave the socket out of the housing hole. So then you would have the existing socket, the bulb base as a plug mating with that socket, and the resistor still inside that bulb base, all of this remaining in the trunk cavity not inside the light housing.

It would then take two wire leads to the LED, that LED mounted on the end of a solid aluminum or copper rod of the largest diameter you can fit through the housing hole, minus the small amount of space needed to string the wires beside it, or drill a hole through the length of the rod and slip the wires through that instead - length of rod measured based on how far into the housing you want the LED to be. I don't know how much machining you want to do. You could use a die and tap to cut a couple sets of threads into the rear end of the rod, drill a couple holes in a mounting plate, put heatsink grease on the rear of the rod and add bolts to secure the rear end of the the rod onto the mounting plate.

Drill a couple holes at each side of the mounting plate and slip that rod into the light housing, and secure it with screws on each side of the hole. Might want to first drill pilot hole for the mounting plate to rear trunk wall screws, I'm not sure if you can follow what I'm describing and if you can, how much of the basic bolt-it-down-with-new-holes idea needs elaboration.

That would give you a lot more mass to 'sink away heat for the short duration you'd have the backup lights on. How much heat for how long I don't know, but if you had to you could extend the mounting plate as far as you wanted on the inside rear of the trunk? Little less tidy having to tuck the original bulb sockets out of the way, but I feel it will be a much improved heatsinking to not have to rely on only what you can affix to the end of the original bulb base as a mount.

 

[kuksul08]

Since you are no longer relying on the internal backup housing reflector you might as well shoot for a heatsink that extends out away from the socket as much as possible if the rear lens comes off the housing for access, but I suspect it does not? If you cannot get the lens off the housing and must install the bulb through the rear/trunk with the socket detached from the housing, I would take a different design.

I would turn the old bulb base into nothing more than a plug for the existing socket, but I would leave the socket out of the housing hole. So then you would have the existing socket, the bulb base as a plug mating with that socket, and the resistor still inside that bulb base, all of this remaining in the trunk cavity not inside the light housing.

It would then take two wire leads to the LED, that LED mounted on the end of a solid aluminum or copper rod of the largest diameter you can fit through the housing hole, minus the small amount of space needed to string the wires beside it, or drill a hole through the length of the rod and slip the wires through that instead - length of rod measured based on how far into the housing you want the LED to be. I don't know how much machining you want to do. You could use a die and tap to cut a couple sets of threads into the rear end of the rod, drill a couple holes in a mounting plate, put heatsink grease on the rear of the rod and add bolts to secure the rear end of the the rod onto the mounting plate.

Drill a couple holes at each side of the mounting plate and slip that rod into the light housing, and secure it with screws on each side of the hole. Might want to first drill pilot hole for the mounting plate to rear trunk wall screws, I'm not sure if you can follow what I'm describing and if you can, how much of the basic bolt-it-down-with-new-holes idea needs elaboration.

That would give you a lot more mass to 'sink away heat for the short duration you'd have the backup lights on. How much heat for how long I don't know, but if you had to you could extend the mounting plate as far as you wanted on the inside rear of the trunk? Little less tidy having to tuck the original bulb sockets out of the way, but I feel it will be a much improved heatsinking to not have to rely on only what you can affix to the end of the original bulb base as a mount.

I understand. I could go to great lengths making a massive heatsink, drilling and machining parts, but my goal is to make a very simple plug and play module with the few parts I have. The goal is to have something that anyone could install without any additional modifications.

 

[kuksul08]

http://autolumination.com/1156_1157.htm

uh oh! hahahaha...
Anyone know what kind of LEDs they're using on these? Up to 48 of them on some versions... I wonder how bright they are.