Our high school theater is hosting a production, and a big part of the set is a giant turntable. It's going to be cool and all, but I've been challenged with controlling LED strips on the turntable over DMX, and I can't use any wires. Any excess wiring cannot be stretched across the turntable, either, and the lighting will be in three generalized areas.
I've already looked into standardized LED controllers combined with wireless DMX, and then connecting it all to deep cycle car batteries, but our theater isn't too hot about spending the money for all the equipment. We've also looked into renting, but it seems like what we need is too custom, and we'd be buying the LED strips so it would be great to be able to reuse them at any time.
So, after much thought (and still contemplating) I've decided that we could build three boards that have some form of PWM driver on them to switch MOSFETs. The three boards would all need to be controlled digitally, through SPI or I2C. I'm open to other methods of control. This digital line would be mastered by a raspberry pi, and it would connect to another raspberry pi over a standardized wireless protocol (maybe wifi?). I can script. The second raspberry pi (in the booth) will connect over DMX to our lighting board. The lighting board is an ETC Ion, and I'm almost certain the DMX is completely standard.
The lighting strips that we'll be dimming are from amazon, http://www.amazon.com/gp/product/B00W0VEOP4?ref_=cm_sw_r_awd_XlWawbR5CFVDD. Each strip is RGB. In each of the three general zones, we'll have three subdivided zones. So on each board that I make for each general zone, I'll need 9 channels of PWM/9 MOSFETs.
As far as PWM drivers go, I've been searching through different DIY websites like adafruit and through different manufacturers, like NXP and Ti. I'm trying to find a driver that can get a decently high frequency. All I've been finding are SMD chips. I found a SMD chip already attached to a breakout board sold by adafruit, meant to control servos and it would probably work for this application, but is was $15 and the chip on it was only a couple bucks. I need a DIP chip because there is no way I'd be able to solder the SMD with any method. I'm not good enough to do that. Besides a specified PWM driver to do the job, I've also considered using a couple of microcontrollers, like the ATmega328P-PU which has become standard through arduino, and ordering them with the bootloader already programmed in and using a couple arduinos I have lying around to program them. However, it would require extra components, more work, the board would be more complex only to waste the full potential, and it nears the cost of the adafruit breakout board. Not to mention, I'd need two of them because each controller can only power 6 (hardware based) PWM outputs.
My first question: what should I use as a PWM driver? Where can I get one?
For the MOSFETs, I'll need some pretty high powered devices. Each generalized area will have 15 amps worth of LED strip, at 12v each. I'm not knowledgeable on RGB leds, so I don't know if it'll pull full current with only 1 color on. But just to be safe, It'd be nice to have MOSFETs that can handle 6 amps each. I'm hoping it'll also help to overshoot with the MOSFETs with the whole heat situation thing, where switching accuracy decreases. I'll probably be able to add a standard 12v computer fan to the enclosure, maybe two, and bolt some washers onto the MOSFETs for extra heat dissipation.
My second question: what MOSFET should I use? Switching side doesn't matter, but I think I'd prefer high side. I'd prefer a common MOSFET.
My third question: what kind/what size capacitor should I use/should I use one to balance/average the PWM output from the MOSFET? The show will be recorded using high quality cameras, and I don't want the camera capturing a frame where the LEDs are off and a computer to pick that frame while downsizing the framerate (whatever that is called). It'd be bound to happen more than once.
My fourth question: since the MOSFET needs to have such a high amperage, would it be correct to assume I'll need a transistor to switch it on and off? Also, should I use some form of isolation? I read a thread in the this forum, http://www.avrfreaks.net/forum/how-use-opto-couple-pwm, and it said that octocouplers generally are not fast enough for high frequency PWM.
For power to the entire setup, each area is going to have to have deep cycle car batteries. We'll be using standard car battery chargers, and they'll have to last for 4 to 5 hours. We'll probably be getting huge 80-100 AH batteries, and I'm hoping that'll cut it. On top of the batteries, I've been scrounging ebay for a decent buck/boost DC DC converter to get as much time out of them as possible and make sure we don't burn out the LEDs.
My fifth question: does anybody know of any good buck/boost converters to make sure the batteries will last as long as we need them, and will we destroy the batteries if they drop to too low a voltage?
That's pretty much it. If you read this all, THANKS A TON, because it was pretty long-winded for a forum. If you can suggest anything or answer any of my questions, that would be GREAT. Please remember, I'm just a high school sophomore, so I won't understand high level electrical engineering terms.
Best Answer
The problem with everything that you have listed here is that your borderline to a full custom solution and you have left very little to compromise with. But i'll try to fill in all the blanks.
The PWM driver i would recommend is this one https://www.sparkfun.com/products/10615 It can support the voltage directly up to 17v. This would eliminate the need for opto couplers.
You want a N-Channel MOSFET and to switch it on the low side. N-Channels are cheaper and have lower ON resistance. Find one that can support the current and voltage that you need.
Don't use a capacitor. The capacitor looks like a short to the MOSFET when it's not charged. This can over current the MOSFET.
You are partially correct. The gate of the MOSFET looks like a capacitor. You will need a resistor in series with the gate to make sure you don't over current the driver. Beyond that as long as the gate can be pulled sufficiently high enough the current of the driver only effect the switching times.
Your math doesn't add up here. You state that each channel will draw 3A and at 27 channels you get roughly and hour of run time from the 100Ah battery. Using a boost converter may actually lessen that amount run time because there will be efficiency losses related to the converter. Also a converter that's capable of the kind of current that you want would be way over your budget. Run straight from the battery.