It's been 20 years since I studied electronics, and in that time I've managed to forget most of what I was taught in uni. I am starting to dabble again, and have picked up an Arduino Uno and a length of WS2812 LEDs. I also have two 4AA battery holders and one 9v battery holder. I am working on a wearable project for the kids, and would like to incorporate just 16 of the LEDs from the stip (which I will cut off of and the main strip). I plan to use the FastLED library with the brightness set to a value of 128, so at max brightness, I am assuming a draw of 30mA per LED for a grand total of 480mA.
Now here is where I am a little unsure as how to proceed; as it is a wearable project, I need to supply power to both the Arduino and the LED strip. What I would very much appreciate is your guidance on is how best to achieve this, using the items I have to hand, and how to wire them up. I was thinking of using the 9v battery to power the Uno only, and for the LED strip, I was thinking of using 4AA NiMH to power all 16, or perhaps splitting that and using 2 x 4AA NiMH battery packs to drive 8 LEDs each. As I said, I am slowly finding my way back into this whole realm of electronics, so please bear with me.
Certainly use Ni-MH batteries in the holder to provide 4.8 V, not 6 V or more.
Powering the UNO (wrong device for a wearable - use a Nano. In fact, use a Nano in preference to UNO for everything for which you do not have a specialised UNO shield) from a 9 V battery is total nonsense. It runs on 5 V - 4.8 will do just fine - and the on-board regulator is pretty useless.
Connect your battery pack via the switch to the terminals of the WS2812 strip using twin cable ("figure 8"). Now connect all three connections from there back to the Arduino 5 V terminal with a 3 wire ribbon. This fulfils the requirement that power wires run together as a pair and signal/ control wires run together.
For 16 LEDs there should be no problem but for long strips (what is long? Well, 50 or more I suppose) you must connect power to both ends (equally/ in parallel). It is recommended to have a 470 Ohm resistor in series with the data line at the input of the LED strip, more important if there is a long separation. (What is long? Maybe a foot. This probably is less important given that you do keep all the wires properly paired.)
Paul__B:
Powering the UNO (wrong device for a wearable - use a Nano. In fact, use a Nano in preference to UNO for everything for which you do not have a specialised UNO shield) from a 9 V battery is total nonsense. It runs on 5 V - 4.8 will do just fine - and the on-board regulator is pretty useless.
Connect your battery pack via the switch to the terminals of the WS2812 strip using twin cable ("figure 8"). Now connect all three connections from there back to the Arduino 5 V terminal with a 3 wire ribbon. This fulfils the requirement that power wires run together as a pair and signal/ control wires run together.
Thanks for the feedback to you both. For the above, I hadn't considered the Nano, but that is mostly because I only have the Uno to hand currently. The project itself is a robot costume, and considering it is mostly carboard box based, I figured the Uno would be sufficient as I can hide it somewhere on the costume. I should have provided info on the LED strip as well, so here it is: WS2812B 5050 RGB LED Strip 5M 150 300 Leds 144 60LED/M Individual Addressable 5V | eBay
If I understand what you have described correctly, you are saying to take the + & - from the battery pack, via a switch, to the + & - terminals at the start of the LED strip. The 3 wires going from the LED strip going back to the Arduino are the +, - and data I am assuming. Can I take from this that you are suggesting just one 4xAA battery pack (using NiMH batteries) to power the Uno and 16 LEDs?
Thank you both. And if memory serves me correctly, connecting one power supply at the start of the led string and one at the end would satisfy the parallel requirement?
Now while I was showering, I was cogitating on some further observations.
It is generally considered undesirable to put two battery packs in parallel as they may not be at the same state of charge and you might get some form of "cross-charging". Particularly for a long strip, connecting one at each end would tend to minimise this as the resistance of the strip itself balances them out.
It is probably not a good idea to break the power rail along the strip (obviously you cannot break the ground rail) to segregate it for power supplies as you get a "phantom powering" concern where the data line bridges. Mind you I have done this often enough with no drastic result.
If you are not discharging the battery pack ridiculously quickly - clearly you will just have to try it out - the best approach may be simply to provide switching on both battery packs and swap from one to the other when necessary.
Thanks for the additional cogitation If I understsnd correctly, what you are suggesting is what I have thrown together in Fritzing; power the system using one pack and leave the other powered off. If/when the need arises, switch to the other power pack.
If my calculations are correct, and I use half-power for the LEDs (30mA each * 16 = 480mA) and allow 25mA for the Uno (based on using one PWM out), I am looking at 500mAh. I don't think that would run down the pack too fast, and even then, this calculation assumes those 16 LEDs are on constantly at half-brightness, which they will not be, as it will just be a repeating sequence of colours.
It's also best practice to put a resistor between the Arduino pin (220R~470R) and the strip, and a large cap (100uF~470uF) across the power lines, close to the strip.
If I understsnd correctly, what you are suggesting is what I have thrown together in Fritzing; power the system using one pack and leave the other powered off. If/when the need arises, switch to the other power pack.