Single LED Standlight Tailight Help

[rjtedge]

Hi all,

I've been searching the wealth of information on these forums (first time poster) and I am sure I have read the answer to my questions many times, but it has clearly gone over my head.

I currently have an Edelux front light connected to my Shimano 6V3W Dynamo Hub. I havve the housing of a dynamo taillight and want to build a simple (single 5mm red LED) standlight circuit for it that connects to the Edelux's taillight socket. I also already have a 5.5V 1F supercap that came from the original taillight that I would be happy to use.

Can some one tell me what I need to make this circuit work?

Cheers!

Rob

 

[Steve K]

hi Rob,

I've been perusing the thread on standlights....
http://www.candlepowerforums.com/vb...Will-this-circuit-work-)&highlight=standlight

... and it's mostly focused on taillights that are more powerful than the average 5mm LED.

There's a circuit on the top of page 3 of the thread that might be a good basis for a circuit. You would need to rectify the power coming to the taillight first. You would also need something to limit the current through the LED.

If space & budget permit it, I'd suggest upgrading to a somewhat larger LED. Most taillights are designed to draw 60mA, so I'd recommend an LED of that size.

What are your space constraints? That might affect the final answer...

In the meantime, I'll see if I can sketch out anything better. :)

edit: I've been looking at some of the major distributors for a high output LED in a 5mm or 10mm package, but don't seem to come up with anything decent. I'm leaning towards recommending a Cree XP-C and add a suitable optic.

 

[Steve K]

I'm back... I thought I'd do a quick simulation to see if the circuit works the way I think it does. This was complicated by needing to come up with a model for the Cree XP-C red LED. I played around with the model parameters, and came up with something that seems to hit the voltages and currents fairly well.

Anyway... here's the schematic:
http://www.flickr.com/photos/kurtsj00/9259463940/in/photostream

The schematic is very similar to what is shown on page 3 of the standlight thread. Since the LEDs are red instead of white, there wasn't a need for a bunch of diodes to drop the capacitor charging voltage.
I should probably mention that the light is only drawing current from the dynamo for one half of the AC waveform. It makes for a simpler circuit that way, and when you are slowing down for an intersection, the taillight changes from a continuous "on" to a "flash" operation.

Putting this into the LT Spice software, I drew this schematic:
http://www.flickr.com/photos/kurtsj00/9259472208/in/photostream/

The primary difference is that I added a 6v DC voltage source on the left. It was easier than switching an AC voltage source on and off. :)
The 6v source is on for 60 seconds, and then off for 60 seconds.

The output from the simulation? I graphed the capacitor voltage and the current through the lower LED. It took around 30 seconds for the cap to fully charge and the LED to reach full current. This will be longer when connected to the AC voltage.. perhaps a minute?
When the voltage is disconnected, the lower LED starts off at 40mA, and decays down to 13mA after a minute.
http://www.flickr.com/photos/kurtsj00/9256722695/in/photostream/

all in all, it seems like decent performance for a simple circuit.

 

[find_bruce]

Thanks Steve, I am in the process of building some lights to run off my shutter precision dynamo & had all but given up on a standlight, but your post now has me going back to rethink it. Note sure if I can fit 2 led's in the small housing I have, but worth exploring. Even better I already have everything except the resistors

 

[Steve K]

What does your taillight look like, and how big is the supercap?

If you need a bigger taillight, I do have an old Schwinn generator set that I'd be happy to get out of my parts box...

 

[N8N]

Subscribed... I actually am running an off the shelf B und M taillight with stand light but it uses a battery not a capacitor... I put Eneloops in there but it would be nice if the headlight would recharge the battery to make it no-maintenance. Previously I had cheap CTA non-LSD NiMH in there and I don't remember ever charging them, and light still worked, so apparently draw is super low... I just leave it on auto and let it do its thing.

 

[find_bruce]

What does your taillight look like, and how big is the supercap?

If you need a bigger taillight, I do have an old Schwinn generator set that I'd be happy to get out of my parts box...

Very generous of you Steve, but postage to Australia will probably cost more than it is worth.

I am using an Easy2Led housing which has 25mm diameter space. I am putting the led in the shallow end which gives me a 21.5mm deep space to put the supercap - 1 farad 5.5 v roughly 18mm diameter x 5 mm thick, and other bits & pieces.

The red leds I have are on 20 mm stars & while I have read about people trimming stars, I am a bit nervous about it. After a bit of a search I found I can just order some 10mm stars from cutter if I stuff it up.

My final touch is adding a spherical diffuser (aka the ball from a deodorant) :)

I

 

[Steve K]

that Easy2LED housing looks pretty nice! For a taillight that may only draw 60mA at 6v, the heatsink may be overkill when it comes to dissipating the 0.36 watts. No matter..

If you can get 10mm stars to fit, that should be fine. I was going to suggest fab'ing a copper mount, such as I did for some XP-G's for a desk lamp:
http://www.flickr.com/photos/kurtsj00/sets/72157628336517483/
but you'll need to be comfortable using a dremel tool and soldering small wires. (both are very useful skills!)

My personal inclination is to pick optics to put the light where it does the most good. For the rural roads that I ride on, I use some 5mm LEDs with fairly narrow beams. i.e. all of my threats are directly behind me. For more widely dispersed threats, I'd probably opt for oval optics, where there is more spread horizontally, but still relatively narrow spread vertically. This will really increase the brightness compared to radiating the photons equally in all directions. Granted, you wouldn't be able to tell friends "would you believe that this is the ball from deodorant!?", but it might be more efficient. :)

 

[find_bruce]

The housing is probably overkill for the front xml driven at 0.5A. I have a few of the housings from battery powered lights, worked well for 1 xml & a triple xpg, so the cheapest thing is to re-use them.

I took some brave pills picked up my aviation snips and trimmed the stars to suit. Despite my hesitation, it seems to have worked well. I struck while the soldering iron was hot & now have a nice set-up with 2 easy2led housings at the front each with an xml, and 1 easy2led housing at the rear with 2 red xpes, using Martin's circuit 7 less the tuning capacitors and adding in the circuit you suggested.

Not too worried about optics at the rear for 3 reasons.

(1) this is for my commuter & my commute is on city streets where the threat is from the sides as much as the rear.

(2) I previously ran 1 xpc at the rear on battery power & I found that at 700mA it was simply to bright to the rear & I fitted the diffuser &

(3) Not aware of anyone selling optics for a butchered side by side twin :)

I am actually pretty happy with the way it has turned out good light out the front & rear. I will do some testing over the week & post up some pics, along with what I think is the wiring diagram.

My initial observation is that the smoothing capacitor, 4,700uF 16v, is working much better with 4 leds than it did with just the 1 xml. I presume this is because it is now operating at a more appropriate voltage 11.3v (2 x 3.35 + 2 x 2.3).

At this stage there is no front standlight - I still use a battery powered helmet light so that does the trick nicely.

 

[Steve K]

Hey, you changed headlights! You started the thread with an Edelux, and now have a home-brewed light. Did you tweak the tailliight circuit to compensate for the change in the voltage across the headlight?

Even worse, you haven't posted pictures! ;)

 

[find_bruce]

I have to confess I am not the OP, but an interloper who has hijacked his or her thread. I am a bad bad man :o I promise to start my own thread setting out what I have done, with pics & no doubt my errors will be obvious to all, starting with my failure to tweak the taillight circuit to compensate for the change in the voltage across the headlight :)

 

[Steve K]

putting 11v across the light that I sketched out is going to be a problem. Can you just tap across the lower two LEDs in the headlight to generate a proper 6v or so for the taillight?