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LED Strobe light — Parallax Forums

LED Strobe light

GeeksGoneBadGeeksGoneBad Posts: 100
edited 2012-12-29 15:02 in Propeller 1
I am thinking I want to make an LED strobe light - I am already using the Propeller in this project so it makes sense to use the prop to fire the strobe?

I *think* this is the circuit I want (I designed it myself so if it's wrong, it's my fault) I have a slew of different mosfets on hand and that's the key word here - on hand - I want to make this by Saturday :) so I don't have time to wait for a part to get shipped... I picked the IRF511 just because it was a part that popped into my head - but I would like recommendations on the best thing to use in that spot :)

yes? No? whatcha think?

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Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-10-25 12:34
    A couple things:

    1. Your LEDs better not have a forward voltage greater than about 1.6V, or they will not light. IOW, white or blue LEDs will not work, but you may be okay with red.

    2. The IRF511 has too high a gate voltage requirement to be driven from the Prop. (I wish RadioShack would quit carrying them.) A better choice would be an IRF3708. But check the datasheets for the parts that you have. Ignore the gate threshold voltage spec, and look for a chart or table that gives the source/drain resistance for a gate drive of 3.3V.

    -Phil
  • CircuitsoftCircuitsoft Posts: 1,166
    edited 2012-10-25 12:47
    What color LEDs? White? You won't run 7 white LEDs off 12V. You can just make 3 work. I would recommend something like the follwoing:

    strobe.png


    The two MOSFETs in parallel are just to make the circuit simulator I'm using behave better. Anyway, that circuit will dump a large amount of current through the LEDs, but will be limited to the energy stored in the cap. That way, you can safely, severely over-drive the LEDs, and as long as the energy in the cap is low enough, you won't damage them.
    strobe.png
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  • GeeksGoneBadGeeksGoneBad Posts: 100
    edited 2012-10-25 12:51
    I am using Red LEDS :)
  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2012-10-25 12:54
    Halloween strobe I presume.

    Yes basically it should work just fine.

    However, that MOSFET the IRF511 can't be driven directly by the prop. It has a 10V gate.
    You can make a simple gate driver for the IRF511 with an NPN transistor.
    250 ohm on the base with 1.5K on the collector to 12V. Should work but not tested.

    You can generate the pulses by:
    1. Setup the counters as a numerically controlled oscillator, NCO. (For a Halloween strobe the jitter is inconsequential.)
    2. Using software to generate the square wave.

    I would use FemtoBasic to output the square wave.

    Duane J
  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2012-10-25 12:55
    GeeksGoneBad wrote: »
    I am using Red LEDS :)
    Red is preferred as it doesn't interfere with night vision.

    Duane J
  • GeeksGoneBadGeeksGoneBad Posts: 100
    edited 2012-10-25 13:00
    Yes it's a Halloween project :) and they are actually going to be inside a box, not directly visible, and I was thinking that i would just turn them on and off quickly, I don't know that I need anything fancy like some squarewave hehe - that's over my head ;)
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-10-25 13:15
    Circuitsoft,

    In your circuit, you need a resistor in each string of LEDs. Otherwise some strings will be brighter than others, due to variations in the the LEDs' forward voltages.

    -Phil
  • Martin_HMartin_H Posts: 4,051
    edited 2012-10-25 16:27
    Phil Pilgrim (PhiPi) wrote: »
    In your circuit, you need a resistor in each string of LEDs. Otherwise some strings will be brighter than others, due to variations in the the LEDs' forward voltages.

    So that's why it is done, that's good to know. A fellow forumista pointed me to the LED array design wizard and it always puts a separate resistor on each string. I followed the advice, but always wondered why.
  • CircuitsoftCircuitsoft Posts: 1,166
    edited 2012-10-25 19:16
    PhiPi, individual resistors are necessary when driving relatively small currents, but the circuit I laid out is designed to drive pulses of several hundred milliamps through the LEDs. At that current level, they self-equalize pretty well.

    I bought a "12V 3W" LED from dx.com, and when driving it at 15-20mA, I can see significant differences in light level between various diodes within the module. However, when I bring it up to 270mA (design current), the brightness evens out due to the inherent resistance of the LEDs and bonding wires becoming more significant than the logarithmic I/V curve of the diode itself.

    GeeksGoneBad: Using red LEDs, you can put 5 in series for my circuit.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-10-25 19:19
    Circuitsoft wrote:
    However, when I bring it up to 270mA (design current), the brightness evens out due to the inherent resistance of the LEDs and bonding wires becoming more significant than the logarithmic I/V curve of the diode itself.

    Are you sure it's that and not just your rods and cones becoming saturated? :)

    -Phil
  • CircuitsoftCircuitsoft Posts: 1,166
    edited 2012-10-25 19:24
    The LEDs were brighter/dimmer in sections, and those sections evened out as far as I could see through the paper. Either way, I can't imagine a problem even if there were some, even up to 40%, imbalance.
  • JonnyMacJonnyMac Posts: 9,007
    edited 2012-10-26 09:48
    I designed a high-current LED driver for an EFX-TEK customer. In the end I used a TC4427 between the Propeller and the MOSFET to ensure solid gating (we used 9v to the TC4427). The circuit works very well and includes 12-bit (counter assisted) PWM control of LED brightness. BTW... the LED module we're controlling runs at 33V and 3A. It consumes nearly 100W and puts out the equivalent of 500W of light.

    I know it's an extra part, but the TC4427 costs about $1.50 and allows you to control the gate voltage well above what the Propeller output provides. We (EFX-TEK) have produced about 2000 of these boards for our customer who is very happy with them.
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  • GeeksGoneBadGeeksGoneBad Posts: 100
    edited 2012-10-26 11:19
    thanks everybody for the replies and advise :) I ordered a few of those gate drivers Jon - thanks for the PN!
  • Mark_TMark_T Posts: 1,981
    edited 2012-10-26 14:23
    Firstly you need to find out the maximum forward voltage of your LEDs at full forward current and across a reasonable temperature range. If driving from 12V ensure the LEDs in a string take no more than about 10 to 10.5V. The rest goes into the current-setting resistor. What current do your LEDs take?

    Secondly junk that ancient part, the IRF511 is not really up to the task (its a truly ancient device too). A logic-level MOSFET with a 0.05 ohm or less Rds(on) spec will be much more suited - you don't want a heatsink do you? Just make sure the LEDs and resistors will take their power dissipations OK.

    Another way is to use an LED driver chip (I just happen to have been playing with a CAT4104 recently - resistor-programmable in 100mA to 1A range, 0.5V drop out, 3V3 logic drive compatible, fast PWM compatible, something like 25V max LED chain IIRC - constant current means no resistor needed for one chain. Probably only SMT though.)
  • GeeksGoneBadGeeksGoneBad Posts: 100
    edited 2012-10-26 15:54
    Mark_T wrote: »
    Secondly junk that ancient part, the IRF511 is not really up to the task (its a truly ancient device too). A logic-level MOSFET with a 0.05 ohm or less Rds(on) spec will be much more suited - you don't want a heatsink do you? Just make sure the LEDs and resistors will take their power dissipations OK.

    unfortunately I have 130 of the IRF511 so I WILL be using them any chance I get LOL :) and if they're not as fancy as the latest / greatest, that's OK with me ;)
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-10-26 16:02
    GeeksGoneBad wrote:
    unfortunately I have 130 of the IRF511 ...
    Wow! *_* Did your local RadioShack have a fire sale? :)

    You should be okay with them, as long as you use a gate driver chip.

    -Phil
  • GeeksGoneBadGeeksGoneBad Posts: 100
    edited 2012-10-26 16:16
    LOL even better - I used to work for an electronics distributor and I get *nearly* anything I want that's in stock for a few hours of programming :)

    I know I posted it here before but I have my parts all cataloged at http://www.makertrader.com (which is a site I made to keep track of what I have and share with friends) :) (oh and anything marked "Warehouse" is at my old job that I can get...

    Yups Mouser shipped my gate drivers today so I should have them - I figure they are just easier to use, right?
  • Mark DietrichMark Dietrich Posts: 7
    edited 2012-12-29 15:02
    If you use a CREE star, 1-watt, which is really bright, it draws 350 ma. I made one , using a 555 timer and a LUXDRIVE driver. Burned out the driver, as it should not be turned on and off in rapid succession for very long. Found out the right thing to use to drive it is a LUXDRIVE Buck Puck. It is made for flashing applications. Less than 1" square, can use external pot for 0-100% intensity control if needed. Look at the spec sheet on that to see if this will help you make a better flashing strobelight. I believe that the CTRL pin can be driven from the MCU (*) you want to use. 0 volts = bright, 5V = OFF.
    I am still a newbee, trying to master the BS2. Something to look at anyway.
    Happy New Year
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