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Looking for some help on picking a microcontroller.

Former Member

So, I am working on a project that runs 100-120 LEDs in parallel. I would like to be able to use a little programming to add effects. For example, i would like to use pwm to make them fade on, when i power on the device. I tried doing this with an attiny85, but that maxes out at 40ma of current. so even a test string of 10 LEDs was way too dim. I need a microcontroller that is programmable and can handle 2 amps of current. I have an arduino uno and am familiar with a few different attiny chips. But, i'm not afraid to pickup a different development board. I'd also like the microcontroller to be able to comfortably fit inside of a round tube with an internal diameter of .75" to 1". all suggestions, or microcontrollers to read up on are welcomed.

Thanks, Derek

  • Keep using the current micro, but change your interface. Drive the LED current through a BJT transistor circuit to achieve the load current. Look up interfacing to microcontrollers with transistors for high current loads.

  • in reply to michaelwylie

    Either a BJT or a MOSFET.  If you have an old PC PSU you will find FETs with a 2A rating. A FET will run very much cooler and so need a much smaller heatsink.

    I've not come across microcontrollers with more than 40mA per part, not pin (25mA/pin), but I mainly use Microchip.

  • it is not the micro you need help with but the need for a driver capable of handling 2A with the additional thermal requirements of PWM. I suggest an easy to mount MOSFET such as this:

    http://uk.farnell.com/international-rectifier/irliz34npbf/mosfet-n-logic-fullpak/dp/9102566

     

    It has an insulated package which can easily be clamped to a metal heatsink.

     

    Peter

     

    PS. Microcontrollers cannot drive 2A!

  • Ok, first of all why should you stick with 2A of current? any specific reason?

    Any typical 5mm through hole LED can lit up bright with less than 50mA and not more than 3V maximum, and you said you are going to connect LEDs in parallel, hmmmm again you are passing 2A current through LED and going to resist the current with high resistor value not less than 10k for each LEDs...which is unnecessary here...Series of LEDs needs high voltage and high current.

     

    You can achieve this project with simple mobile charger which is 5V and 250mA, typically. I am sure you are already own one... use BJT transistors as a switch, better use this schematics TI and Würth Elektronik LED RoadTest+ - The Schema - 5/10 instead of 4 put 2 LEDs in series with 1K resistor and connect the pairs in parallel as much as you want... each series will get 5V and each LED will get 2.5 volt and 250mA  which is already maximum limit of a typical LED to lit up bright...connect the microcontroller as I described in my blog, the microcontroller voltage may differ from MSP430FR4133... but thats not the issue... Leave out the ambient light sensor part in my schematics and as well just use one BJT NPN transistor...instead of the 12V in my schematic it will be 5V mobile charger for your project, BOOM it's done... no high voltage and not heavy current. ATTiny is more enough for this project image or I would say any microcontroller image

  • in reply to tonyboubady

    I'm working with LED's in parallel. Not in series. LED's in Parallel require the same voltage (2.2-3.4 depending on color), but much higher current. passing 2A through 100 LEDs will give no more than 20ma to each individual LED which will give me the brightness i desire

  • in reply to artful_bodger

    Thank you. i actually had a 10 year old emachine running linux die the other day. it was a weak computer i used only for coding arduino. I actually planned on taking it apart for the psu today. I'll look into this.

  • in reply to michaelwylie

    Thank you. I'd actually started looking into transistors for this the other day. But i was having difficulty finding helpful resources. I figured there wasnt going to be a microcontroller that could handle this by itself. But i thought i'd post this as a backup plan anyway.

  • in reply to Former Member

    It is not as simple as you think....

     

    LEDs have a negative temperature co-efficient so as one warms up it will drop its resistance and therefor increase its current, having 100 in parallel (Especially if not from the same batch, and matched for the job) can lead to failure as one led will allways consume a little more current due to a slightly lower forward voltage drop, if the current is not limited to each LED with a resistor then you will get a run away situation and the LEDs will start to fail, some will be bright, some will be dim

     

    You should have a small resistor in series with each one if you want to do the job properly and for it to be reliable

     

    If the LEDs are placed in series then the current can remain the same but of course the overall forward voltage goes up for each LED int he chain, by having say 10 LEDs in series and a single resistor of say 10-20R in the chain, then paralleling 10 of these chains will give the same result but with far more efficiency, of course you still have the risk that one LED going open will take out that chain, but one going short will have very little effect.

     

    To Clarify LEDs are current devices, not voltage devices, the forward voltage is provided as a guide but the forward current is what should be controlled to get a more even light from all the LEDs

  • in reply to Robert Peter Oakes

    Im making an LED string blade for a lightsaber. As much as i'd like to

    break it down into 2 or 3 series of LEDs they wont fit properly without all

    being built into one big string. Also i have seen youtube videos where this

    works. I dont know about life expectancy. But i know it will work alright.

    I just needed a different way to power it because i dont believe in paying

    65$-100$ for premade led drivers when i can work out my own circuit for

    5$-10$