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Author Topic: Easiest configuration to power 200 LEDs  (Read 1120 times)
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Estonia
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My project is to attach LEDs to the rims of my bike wheels and I would like _minimum_ 50 per side of a wheel which equals 200 for the entire bike. I would also like to be able to get a decent battery life out of it.

I know there are way to re-generate power and everything but time is limited (its a birthday present) and modifications to the bike should be very minimum. This means that I would like it to be battery powered (AA or 9V) and each wheel will have to have its own power supply.

What would be the best configuration of batteries / LEDs to get the maximum battery life without having too many batteries to attach to each wheel?
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Valencia, Spain
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Battery life is going to be important so I'd multiplex them using a MAX7219 chip for each side of the wheel. That way you can have 64 LEDs per side but only 16 LEDs will ever be on simultaneously so battery power draw will be reasonable.

You'll need an Arduino to drive the chips. A Pro Mini or Nano is a good size. It also means you can sequence/flash the lights

(and also keeps this project on-topic for an Arduino forum).

Throw in a Hall sensor and you can sync them to the turning of the wheel - make it look like the wheel isn't turning (or going backwards).
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Manchester (England England)
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Battery life is going to be important so I'd multiplex them using a MAX7219 chip for each side of the wheel. That way you can have 64 LEDs per side but only 16 LEDs will ever be on simultaneously so battery power draw will be reasonable.
You don't get something for nothing, so if you multiplex the LEDs they will be dimmer. If you want to save battery current then make the resistors larger. An LED that is flashed on / off on a 50% duty cycle is just as bright to the eye as one with half the current going through it full time. There is no saving of current if you want the same brightness.

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Throw in a Hall sensor and you can sync them to the turning of the wheel
Can you synchronise the scanning of a MAX7219 chip?

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Valencia, Spain
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Throw in a Hall sensor and you can sync them to the turning of the wheel
Can you synchronise the scanning of a MAX7219 chip?

No, but you don't really need to.

The MAX7219 scans at 800Hz so you'd have to pedal *really* fast to get anywhere near the Nyquist frequency.
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Manchester (England England)
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so you'd have to pedal *really* fast to get anywhere near the Nyquist frequency
Well my calculations work out at about 10MPH to get to something you can see.

Anyway it doesn't actually save anything multiplexing so there is little point.
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Estonia
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Ok I have 2 micros at the moment but it really seams excessive to use a controller for this project. Could I really benefit from using these? I would have to use 2 arduinos, 1 for each wheel.

I know it is a bit off the Arduino topic but it does seam like just having them constantly on would give the best cost-to-output situation with relatively the same battery life. I figure I would power the Arduinos with 1 9V each.

So, in your guys honest opinions, what would be the best idea? I am looking for the cheapest and easiest long-term but with good quality idea. If the outright cost is high, that is alright as long as it saves on battery life and everything in the long run.

I can't go too crazy though, this is supposed to be a birthday gift which is due in 1 week from this moment and getting supplies is time consuming (multiple days).
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If I were doing the project I would use a lytheim battery at 9V and have groups of three LEDs and one resistor. I would run the LEDs at 10mA.
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Lacey, Washington, USA
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Yikes. 200 LEDs at 2V (for Red or Yellow, higher for Green, Blue, or White) at 10mA each: 200 x 0.02 x 2 = 8W And that is with 100% efficiency from battery to LEDs, which you'll never get. That'd be about 1A from a 9V battery, or about 1.5A from four AA cells.

How about a clear disk with fewer LEDs shining into its edge? And maybe C or D cell alkalines, or a Lithium pack?
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Estonia
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If I were doing the project I would use a lytheim battery at 9V and have groups of three LEDs and one resistor. I would run the LEDs at 10mA.

So you would recommend the circuit that I have attached in my image here?

@polymorph - I don't quite follow you. I am starting to realize that I really need to trim down on the number of LEDs that I will be powering since battery life is really going to be an issue but I don't quite understand your calculations or what they mean.

Edit:
With the circuit I have attached, how could I calculate the battery life of a single 9V with such a setup? Meaning, If I have 1 9V how can I see its estimated battery life with 1 group, 2 groups, 3 groups, etc. I am looking to find the sweet spot of number of LEDs to battery life.

Edit 2 @Grumpy_Mike: If I understand your idea, it would be to have the 3 LEDs in _series_ instead of parallel. Wouldn't this make the 1st LED in the series very bright (and possibly burn it out) with the remaining 2 dimmer? I will do tests at home as soon as I can but thats how I understand it.


* 3-circuit.jpg (11.04 KB, 537x403 - viewed 27 times.)
« Last Edit: July 05, 2013, 04:59:28 am by shiznatix » Logged

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You could have one power supply - and have the circuit intermittent by having pins on the wheel strike a PCB on the frame.  Wouldn't be perfect and needs some more thought on how it would wear down...
It would be intermittent at very low spinning speeds
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Is the idea really to have the LEDs on the RIM of the wheel ( NOT down the spokes)? To gain the illusion of a ring of light around the rim as the wheel is going round? In which case a) you don't need any micro control and b) you probably don't need anything like 50 per rim to get a nice ring illusion at any half decent speed - probably 6 would work great and save you a ton of power....
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Estonia
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@Marvin Martian - I had this same idea but due to time constraints, it just isn't possible.

@tocpcs - You are correct in the idea and I am starting to lean your way in regards to the number of LEDs. I will do some experiments this weekend and see what I come up with
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Is the idea really to have the LEDs on the RIM of the wheel ( NOT down the spokes)?
How does that work - what about the brakes?
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Is the idea really to have the LEDs on the RIM of the wheel ( NOT down the spokes)?
How does that work - what about the brakes?

I have disc brakes on my bike. Even if using "old style" caliper-on-rim brakes....could just attach LEDs where spokes enter rim.

My question is....how to get power from battery to a rotating wheel. Where to mount the battery....without throwing wheel balance all off?
 — some kind of "brushes" or spring contacts?
— or use two batteries opposite each other on each wheel?

If time & resources where no constraint, some sort of custom induction system would be awesome!  smiley-lol
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Valencia, Spain
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My question is....how to get power from battery to a rotating wheel. Where to mount the battery....without throwing wheel balance all off?
 — some kind of "brushes" or spring contacts?
— or use two batteries opposite each other on each wheel?

Brushes are doomed to failure. You have to balance it.
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