I have to face a new project and, before getting started, I thought to seek for the help of more advanced users here, so thank you very much in advance for your help!
I need to build a special suit for a spectacle with LEDs.
I would need to place around 200/300 (still to be decided) LEDs (3mm and 5mm) on a bodysuit for a spectacle and light them up in various ways.
The LEDs will be arranged in strips of LEDs in parallel (I know that I will light up the whole strip all together, that's not a problem).
My overall idea would be to connect the various LEDs strips to a Li-Po battery and to control each line with a MOSFET driven by Arduino (this way I should be able to use PWM I guess).
Going to the questions:
As crazy as it might seems it is easier here to find LEDs already cabled with a resistor rather than a plain LED.
Would it be a problem to put the all in parallel, all with their own independent resistor or shall I necessarily find them unmounted and place a single resistor at the top of each strip? Obviously I guess I would still need to place another resistor at the top of the line to compensate for the battery voltage which wouldn't necessarily be 12V (as for the LEDs in the previous link).
Could I run Arduino (I was thinking of the Nano to save space) from the battery with a Step-Down?
Do you think with a regular 4/6 AA battery pack I would be able to light up the LEDs for 4/6 hours in a row, so to use this instead of the more dangerous Li-Po battery?
I think that's all for now but if you have any suggestions on things I am missing please let me know.
Sorry if I might have said some silly things but I am a beginner with limited experience.
Start caculating the total current needed. 200 LEDs consuming 20 mA each sums up to 4 Amp. Multiply that with 4-6 hours and You need a 16 - 24 Ah battory. That's half of a small car battory, not exatly a few AA cells.
thank you for the hint!
I am a novice and I am a bit lost in these calculations...how do you calculate the battery life based on the Amp?
I could change battery every half an hour, or less, if needed.
Do you think that could work for a 2/3 cell Li-Po battery?
Apart from that is it okay to run Arduino from a Li-Po with a Step Down like the one I've posted?
Ale_V:
I am a novice and I am a bit lost in these calculations...how do you calculate the battery life based on the Amp?
I could change battery every half an hour, or less, if needed.
Call it battory capacity, the amount of energy it can deliver. That's measured in the unit Ah, Ampere hours. This or that many amperes during this or that many hours. 2 Amp during 3 hours == 6 Ah.
Do you think that could work for a 2/3 cell Li-Po battery?
You need to check 2 things: 1) How much current does the cell deliver. 2) How many Ah?
Apart from that is it okay to run Arduino from a Li-Po with a Step Down like the one I've posted?
That step down converter is okey but it has an absolute maximum of 3 Amps. I suggest You divide the LEDs into minimum 2 equal sections giving each section its own converter, and one converter for the controller. Ale [/quote]
Here is the scheme I thought of...
I've divided the current between Arduino and the LEDs just for safety, the second step down to 7V is pretty mush useless I guess.
I've drawn just a series of LEDs but the final project will have different, each one with his own step down converter and MOSFET connected to an Arduino PIN.
I've found a 2S 7.4V 6200mAh battery, that, considering that I'll need the LEDs sort of blinking should last long enough for my purposes (considering I'll change battery if needed).
A couple of questions:
If I set the output of the Step Down converter connected to the LEDs directly to 3.0V (which is, if I understood correctly, the correct tension for white LEDs) I could avoid all the resistors connected to the LEDs right (100 Ohm in case of 5V if I want to deliver 20mAh to the single LED)?
If I opt for a reverse connection of the LEDs (I don't know if that is the appropriate term in English, I mean when you connect the catode to the digital pin and the anode to 5V) could I avoid the MOSFET or I would fry Arduino in any case?
Bonus question...
As I work with batteries for the first time I was wondering how shall I set the step down converter: using the battery I expect the voltage to lower and lower, will the step down always output 5V while the input is over 5V or will it decrease while the battery decrease its voltage? Also, how shall I calculate the resistors if the input tension varies because of the battery? Sorry for the silly question, I'm new to all these things...
Don't worry too much about battery voltage dropping. When they really start to drop there is not much energy left in them. Also note that deep discharging LiPo cells ruins them.
Make 5 volt for the controller and LED voltage for the LEDs using a second converter.
The wiring is in principle okey. You might want to use more than one FET to awoid having too hot components.
Regarding resistors and voltage for the LED chains, check the data spec for the LEDs. However I think You will need several parallell circuits of batteris and FETs to handle the large sum of Amperes used.
I didn’t get it (sorry for my english), shall I or shall I not, set the step down converter for the LEDs to 5V?
Also, would I have any benefit using a 3s Li-Po rather than a 2s Li-Po if both have the same mAh?
Finally, if I set the voltage for the LEDs to 5V, would I have the correct voltages to make the MOSFET (the one in the scheme in particular) work fine? I guess I need a certain difference between the gate and the other pin...
Ale_V:
I didn’t get it (sorry for my english), shall I or shall I not, set the step down converter for the LEDs to 5V?
What's the spec for the LEDs? Do they have resistors built in or not? A common single LED handles 20 mA, has a forward voltage like 2.5 volts. That leaves 2.5 volts to be dropped across the resistor = 120 Ohm.
Also, would I have any benefit using a 3s Li-Po rather than a 2s Li-Po if both have the same mAh? 3 Li-Po contains more energy than 2 but 2 would work.
Finally, if I set the voltage for the LEDs to 5V, would I have the correct voltages to make the MOSFET (the one in the scheme in particular) work fine? I guess I need a certain difference between the gate and the other pin...
Use a logic level N channel MOSFET. That will work. Resistors calculated versus the LED voltage sets the maximum current.
I still have to buy the LEDs and I was considering different solution, both with integrated resistor and without.
Still, they will be normal 5mm LEDs (most common type, so I assume 20 mAh).
I am getting confused, shouldn’t I use the resistor to bring the voltage to 3V (therefore calculating the resisto based upon the incoming voltage, i.e. 5V —> 100Ohm) so to light up the LED without too much power?
If the incoming voltage, regulated with a Step Down converter, is already 3V (as I can set it directly with the converter) should I still need a resistor?
Actually, if you have not purchased the LEDs yet, you should consider using MAX7219s to drive them in matrices of 8 by 8 with individual control (but not individual dimming) of every LED.
These run from your regulated 5 V and several can be chained for multiples of 64 LEDs which are multiplexed and give excellent brightness, so your 300 would require five chips and be controlled with just 3 Arduino pins.
I advise using the MAX7219 matrix modules available in eBay or Aliexpress in partially assembled form - you just ignore the matrix which comes with them (thought you could assemble one to practice on) and wire them to your own 8 by 8 matrices of LEDs.
While they are quite bright (especially with white LEDs), they only draw 360 mA per 8 by 8 matrix.
I’ll definetively try the MAX7219 for other projects, in this specific case I must be able to dim the LEDs.
When you say in 8x8 matrix, you mean that I can control every single LED or just a 8 LED strip?
I’ve checked the pre-assembled module and they come in two fashions: with all the components to be soldered together and the ones with everything mounted (LEDs matrix as well). In this latest case, can I remove the matrix or is it usually soldered too?
Pre-assembled LEDs wise I can only find LEDs mounted for 9/12V voltages. They actually sell the same combination of LED/resistor for both 9 and 12V, is it normal? I mean, can I use both 9V and 12V with the same resistor? In this case I guess I’ll need to use a 3s Li-Po battery directly connected (without any step-down converter) to the LEDs, correct?
Have You been thinking about the weight, the stiffness of that suit? It could feel like a full metall harness modell the old horse riding knights. Maybe bullet proof due to all wires....
How about washing that suit? How long will the wires and their connections survive?
Dressing a plastic doll will work fine but being used a living, moving human being... I have doubts about the reliability.
Ale_V:
I’ve checked the pre-assembled module and they come in two fashions: with all the components to be soldered together and the ones with everything mounted (LEDs matrix as well). In this latest case, can I remove the matrix or is it usually soldered too?
You have to look carefully. You do want the ones with the separate components; the SMD chip, capacitor and resistor have already been mounted for you. Those come with socket pins so you can mount and remove the matrix. If you are going to wire your own matrix (all LEDs individually controllable), you do not want to mount the sockets as then you have the same problem as an Arduino UNO; you can only stick wires into the sockets and that is just not reliable.
The ones fully soldered may or may not even have the socket pins, the matrices are often soldered directly into the PCB and you cannot alter them in any way (including making the connecting pins usable!) and they are pretty useless.
Ale_V:
For how crazy it might sounds...it will be used once only.
Surely not!
Ale_V:
Anyhow I came up with a solution to remove the LEDs easily to wash everything...
Velcro?
The solution for the inconvenient sockets on the UNO - if you ever must use one; the Nano is certainly more useful - is to insert a pin header and solder your wires to that. The pin header will hold firmly in place.
thank you very much for the answers!
I've been busy with work and I did not have time to investigate further...!
I am now progressing in the project, I have one more question, being not an expert...
I've checked the datasheet of the MOSFET I am using, does ID (Drain Current) 32A @ 25C° means I can run up till 32 Amp through it at the same time? Not that I'll ever go up to 32A of LEDs, but just to be sure..!
In the first post you say you want to use strips of LEDs, as you want rows to light up all at the same time anyway. Now that's all individual LEDs. Big difference! Lots more wiring, as you have to wire up all LEDs one by one instead of just gluing on a strip (which often comes with sticky backing already). Very flexible for sure but also a LOT of work.
A typical LED strip comes with current limiting resistors in place. Many are designed to operate at 12V, 3s LiPo is 11,1V nominal and will light them up quite well. 4s LiFePO4 (a 10 Ah battery weighs maybe half a kg and is not particularly large) is just over 13V and will make the thing shine just a bit brighter. LED lifetime will suffer, but you're talking months, not hours. For you this may be the perfect solution.
The LED strips can be controlled and dimmed easily with a MOSFET, and are available in RGB version so you can change the colour. Add a small 5V buck converter to power the Arduino and other logic. I would want to power the LEDs from the batteries directly, if possible.
Yet another alternative is individually addressable WS2812B LEDs. Those also come on strips, but appear only available in 5V version. So you need a buck converter to match.
