new LED rear/brake light help

[Hairyeyeball]

Hi,

First off I am VERY NEW to LED lighting so you may have to bare with me!

I am trying to make a rear/brake light for my bike and need a bit of help figuring out the wiring.

I have been on another electronics forum and they managed to help me a little but came accross this forum and thought that you guys might be able to help me out a lot better........

What I want to do is run a rear light at say half the LEDs output and when I pull the brake they will light up using the full output....

from what I gathered from the other forum I came up with this....

The LED's I was going to use are these http://www.maplin.co.uk/Module.aspx?ModuleNo=35812 (the middle ones-item code N46AT)
But I also have some of these http://www.maplin.co.uk/Module.aspx?ModuleNo=2063 at home already so would these be any better or make a difference to the wiring etc...

I would be very grateful if anyone could spare the time to help me out with this.....


Thanks in advance for any help and/or advice

Trevor

p.s. I would post a link to the thread on the other forum but don't know if thats permitted or not??? so thought it would be best not to just in case.

 

[Genes]

Well, with those red leds, you don't need the resistors in the loop or external. Those 280 ohm resistors simply aren't needed. You can drive those 6 red leds in series with 12 volts and they will be reasonably bright. You may find that those 6 leds aren't bright enough even with no resistors. The specs say 2.5 volts at 30 mils, so you really need 15 volts to get them to full brightness. You may have to use only 5 leds to get the brightness you want.

If you add an external resistor, like the 47 ohm, yes, all six (or 5) will be dimmer. You may have to experiment with the value of that resistor to get the proper dimming level to suit you. Your contact switch would be "normally open" and when you hit the brakes, it shunts the resistor, making all six leds brighter.

What you may want to try is to use 6 leds and no resistors at all when riding. When you hit the brakes, have the switch contacts shunt one of the 6 leds. This will leave 5 much brighter leds. When you release the brakes, all 6 leds come back on.

 

[Oznog]

Each series string must have a resistor, and only one resistor is necessary. You've got one string with a bunch.

Gotta strongly disagree with Hairyeyeball here. The LEDs do not self-regulate, even if the official Vf is 2v and you put 6 in series it's a disaster on 12v. Either only partial brightness or they'll burn up. 4x in series=8v is about the max for a 12v battery.

You can put any number of these 4x LEDs + 1 resistor strings in parallel. Once they're tied in parallel, you can use one big series resistor to produce a lower current for "running" and have a switch short past that resistor to produce a "high" for braking. Be sure the total current is within the current rating of the switch.

 

[ellerbro]

Here's a drawing of what you want to do, though you may use a different number of LEDs. If the resistors are the same value then the current will be approximately double when you hit the brakes. Assume you want 30 mA when you hit the brakes and 15 mA normally. The LED datasheet (N46AT) shows that typical forward voltage = 1.8 V when current = 15 mA (see the I-V graph on the last page). If your LEDs are typical, then 6 in series would make 10.8 V at 15 mA. Not sure what type of battery you're using but assume it's a lead acid, so about 12.6 V when full. That gives 1.8 V left over to drop across the resistor. You must choose the resistor that will give 15 mA with 1.8 V across it. 1.8 / .015 = 120 ohms. If the other resistor is also 120 ohms, you'll have 60 ohms equivalent (two 120's in parallel) when you activate the contact switch. 1.8 / 60 = 30 mA.

Now if you hook up your battery right after charging, the voltage will be higher and you'll get higher currents. So do the above calculation using the highest voltage you'll connect to the circuit. You want to be sure the LEDs don't see more than 30 mA with a full battery. As the battery drains of course the current will decrease and your lights will dim.

If your LEDs don't follow the typical characteristics you'll have to play around with the resistor value. Start high so you don't end up burning up the LEDs. Good luck.

 

[Hairyeyeball]

Ah I think I see now..... thank you for the diagram.... will hook it all up but prob wont be for a day or 2 yet. Hopefully it'll be ok lol
All I need then is just the 2 120ohm resistors?

Thanks guys, Your stars!!!!!!!!!!!!!!!!!!

Trevor

 

[Hairyeyeball]

Just had a thought............

Would I be able to bypass the on/off switch for the brake light so that it will work without the lights being switched on? It doesn't need to be as bright as it would do if the lights were switched on. Just so it lights up when I pull the brake.

Also would I be able to run 2 lights?
one as above (tail/brake light)
and one as just a tail light from the same battery?

Like this

No problems if can't.... Might look bit nicer if can lol.........

 

[Genes]

You don't need the resistors! Period. 6 of the referenced leds at 12 volts will work just fine and like I said, perhaps not bright enough. His spec says 2.5 volts at 30 mils. I've been doing this in commercial applications for decades. If you do the very simple math they will work just fine. The resistors are ONLY required to limit the current when the voltage source is greater than the sum of the Vf's. You guys are over complicating the design.

 

[Hairyeyeball]

I know it must be frustrating for you guys and I am sorry for that.... I'm a cycle mechanic/builder not an electritian lol if I understood all this stuff I wouldn't be asking for your help lol

Genes
You don't need the resistors! Period. 6 of the referenced leds at 12 volts will work just fine and like I said, perhaps not bright enough. His spec says 2.5 volts at 30 mils. I've been doing this in commercial applications for decades. If you do the very simple math they will work just fine. The resistors are ONLY required to limit the current when the voltage source is greater than the sum of the Vf's. You guys are over complicating the design.

If I dont have the resistors (on at least the brake/tail light) to limit the initial power of the light how would they go brighter when the brake is applied?

Like i've said, i'm no good with all this stuff hence the need for the help of you guys lol

 

[Genes]

Not all Red Leds have the same specs. The ones you referenced, had a Vf of 2.5 volts at 30 mils. Some red leds have a much lower Vf at 30 mils. If you use leds that have a Vf of 1.2 volts, then you will most certainly ruin them by putting 6 in series across the 12 volt supply with no dropping resistor. With the ones you mentioned, you can put 6 of these directly across the 12 volt battery and they will work fine with no resistors. The simplest way to make them brighter when you hit the brakes is to short out one of the leds. This leaves 5 leds with a 12 volt supply; again this is using the leds you mentioned. OR, you can insert one resistor in series with the 6 leds and then short it out when you hit the brakes. You may find, that during normal riding, with the 6 leds and the resistor, that they simply aren't bright enough to suit you.

One theory is that you only spend a very short amount of your riding time braking and the remainder is spent riding with the leds dimmer. So, during normal riding, you have minimum lighting protection. You might want to think about running the leds bright while riding and then flash them very quickly when braking. This gives maximum illumination when riding and then the flashing leds attract the others attention when braking. This second scheme is more complicated, but offers more protection.

Many of my biking friends have lights that flash continuously. I'm sure this can be a bit annoying when in traffic, but that is exactly what bikers want, for drivers to be aware of their presence.

You might want to consider using just one of the red Luxeon Leds or red Cree Leds. These things are very visible at 1/2 to 3/4 miles. Also, what size and weight is the 12 Volt battery. If it is a sealed, lead acid, then it may be very heavy. If you have a minimal weight bike, the battery might weigh half of the bike's weight. If you ride long distances, then you will notice the additional 6-8 lbs pretty quickly. Using a single Red Cree Led, means you can use a much smaller battery or batteries.

 

[Hairyeyeball]

Sorry I should have said more about the bike they are going on....
It wouldnt be used all the time and not often at night, the lights would mostly be used for show but will also be used normally for quit short commutes so
i'm not to worried about the long range visabillity side of things. They are going on a custom bike that I have been doing of which the battery fits inside a compartment built into the frame.

 

[Cemoi]

Hairyeyeball, what kind of device will you use to make your (rear?) brake act as a switch?

 

[Hairyeyeball]

Hairyeyeball, what kind of device will you use to make your (rear?) brake act as a switch?

The switch I am using is this http://www.maplin.co.uk/Module.aspx?ModuleNo=20406
I have drilled a hole in the switch lever and passed the cable through it, as the cable gets pulled through a crimped on alu ring hits the lever causing it to make the contact and light up the brake light.
It will only be on the rear brake as I won't be fitting a front brake.

 

[ellerbro]

The updated circuit you posted will work fine. If the main switch is off and you hit the brakes it will be the same brightness as if the main switch were on and you weren't braking. You can connect as many of those circuits as you want to the battery.

The OP said he was going to use the N46AT LED. The datasheet shows Vf = 1.9 V at 30 mA. The drawback to direct drive (no resistor) is that you are limited to particular currents. For example, your lead acid battery will be about 12.6 V when full. If you use 6 LEDs each one gets 2.1 V. The datasheet shows that the current will be off the chart. The LEDs will fry. If you use 7 LEDs each one gets 1.8 V. The datasheet shows I = 13 mA at this voltage. So you are stuck with 13 mA or less with direct drive. Using a resistor you can achieve whatever current you want, but at the expense of efficiency (some power is being consumed by the resistor whereas with direct drive all power is being consumed by the LEDs).

I assume you have a meter so you can determine the voltages and currents? It's important to make sure the current is not exceeding spec. Calculations will get you started but you need to verify the end results.

Since resistors are so cheap I recommend you buy lots of different value ones. If you use different LEDs, want a different current, use a different type battery, or your LEDs are off spec you will need to use different resistors as required.

Here's a rear bike light I recently made.

 

[Hairyeyeball]

Jolly jolly! Thats what I like to here. I think I will stick to the updated circuit then lol
I know where I am with that one lol
I started to get a bit confused with all the different variations I could do.
I will stick with the LEDs(N46AT), battery etc... I quoted in previous post as the lights will be permanently fixed to the bike.
My brother has a meter so I will get him to check everything is ok.
I'm liking the lights you made... Once mine are done i'll post them on here for you to see, hopefully they'll be done by the end of next week! along with everything else lol as I am hoping to take the bike to a cycle meet on the 21st.... fingers crossed lol
Thanks again for all the help......

 

[Cemoi]

The switch I am using is this http://www.maplin.co.uk/Module.aspx?ModuleNo=20406

I assume you'll be waterproofing it using e.g. heat shrink tubing?

I have drilled a hole in the switch lever and passed the cable through it(...)

Could you please post a couple of pictures once your device is ready?

 

[Hairyeyeball]

I assume you'll be waterproofing it using e.g. heat shrink tubing?

Could you please post a couple of pictures once your device is ready?

Yeah, It will all be weather proofed etc... The actual switch will be contained inside a little plastic box with just the switch lever exposed. But all the connections will be heat shrunk to protect them from the elements etc.

I will take photos of the completed lights and parts etc for you to see. I'll post them on this thread when they are finished. won't be for a for days(maybe about a week) because I got a lot to finish on the bike which needs to ready by the 21st.

 

[Cemoi]

I will take photos of the completed lights and parts etc for you to see. I'll post them on this thread when they are finished. won't be for a for days(maybe about a week)

I'd be happy to see them