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Thread: Auto brightness with PIC (PWM) and LDR for Lilliput 701 LED Backlit

  1. #41
    Variable Bitrate
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    Sounds about right.

  2. #42
    Raw Wave
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    Reegler - V & A shouldn't matter.
    Ideally PWM the ground connection and use a MOSFET where 1A to 60A aren't much different...

    Voltage is only a problem if it exceeds 50V, or if they are common anode displays, but then a "high" MOSFET will do the job... a P-Ch can be at any voltage (up to 50V etc) and turned on by the original N-ch MOSFET or open-collector PWM output.

    Or what have I missed?

  3. #43
    Variable Bitrate rEegLer's Avatar
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    No you're right about that but my backlights are powered from a 12V regulator and the darlington transistors have a saturation voltage of about 2V so if your backlight requires 11v to run then the regulator will need to be changed. Overall not a big deal.
    Love

  4. #44
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    On the current limiting resistor thing...if your using PWM and driving the LED at 100% duty cycle, isn't the LED always on? Wouldn't you then need to limit the current?
    What I've been finding says to ignore the PWM and calculate the resistance like normal. R=(Vsupply - Vdrop)/mAh

    I was doing a little testing with a single LED and a 5volt supply, and while it worked for a little while, if you left it at 100% duty cycle it burnt out after about 2mins.
    I found I needed the resistor unless you keep the duty cycle down.

    So we'd need to know how many LED's, parallel or serial, their maximum current. When I take my display apart I'm going to have a look at the LED's. I doubt I'll find part #'s on them, but I bet we could make some assumptions about their max current.
    Set the PWM to be 100% duty cycle and adjust the resistance to get an acceptable current draw.
    Keep it on the low side to start to be safe.

    davidk
    Last edited by davekra; 03-28-2011 at 11:55 PM.

  5. #45
    Maximum Bitrate Mickz's Avatar
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    Quote Originally Posted by stretch4x4 View Post
    How hard would it be to take two inputs for the brightness? Haven't finished thinking this through at all but you could have a light sensor to do most of the brightness control then if it is too different you could tweak it with a pot.. Might lead to conflicts though for example the screen is too bright still so you turn down the dash and then have no dash lights.. Hmm yeah probably not useful..
    .
    I have auto adjust combined with inbuilt Dash light Control of the screen brightness with a LED backlit Tranreflective LCD. Its a set and forget system and adapts to every driving condition I have encountered Never had to adjust the dam screen again

    At the moment brightness is changed in software. (Input hardware interface is the same reading LDR and PWM dash bright voltage. Only have to add a PWM REG if needed. When I get a chance Ill see if the LEDS are actually changing in my unit and change or combine it. I doubt my LED backlight is varied. Mainly interested in power saving as Software control is perfect at the moment.

    To use both control inputs you need:
    1. A means to calibrate the control point to match the normal night time dash light control position. Most dash lights controls change the Dash brightness gradually around the normal night driving position so it easy to set up. This allows really good control without affecting Dash backlighting in other words they both track the dash control correctly.

    2. Simply detect when the vehicle lights are on/off and have the screen controlled only by the LDR/photo diode during the day.
    Surface Pro 2 128GB portrait mode, Win8.1, Reverse camera, Dual 10HZ GPS RX's for Speed Display & Sat Nav, FM-DAB & Phone Modules, iDrive interface. T-Screen HVAC control, custom microcontrollers, microcode and FE. Previous Car-PC Project

  6. #46
    Variable Bitrate
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    I think I get you, although the system would need adjusting for me as I drive during the day with my lights on..

  7. #47
    Raw Wave
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    Quote Originally Posted by davekra View Post
    ...if your using PWM and driving the LED at 100% duty cycle, isn't the LED always on? Wouldn't you then need to limit the current?
    Yes - you said it - it is always on hence the PWM does not limit the resistor. You limit the max PWM duty cycle if the PWM needs to limit the current.


    But you can ignore PWM for resistance calculations (PWM voltage drops ignored) - just calculate the resistance as normal, the PWM = 100% is full bightness, 50% is half etc.
    Or HALVE that resistance for (say) double the current, then have max brightness at the MAX of 50% PWM, half bright at 25% PWM etc. Go above 50% PWM and blow the LEDs....


    Get rid of Darlingtons and use MOSFETs - why have a 2V drop, and worry about gain?


    Did I mention the MIC502 fan controller chip? It handles 2 inputs.... (Just cut out its first 64 cycles of full brightness.)


    Most LEDs are 20mA.
    Peaks vary though, but are are typically 10-fold (200mA) though 5-fold might be a safe assumption if lacking data (ie 100mA for 20mA nominal), and I'd assume 10mA or 10% duty-cycle max.

  8. #48
    Variable Bitrate rEegLer's Avatar
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    I've been really busy at work the past week so sorry for the late updates!

    I redesigned the circuit board to work with the PICAXE 18M2 controller because it has 2kb storage and more PWM outputs which allow for future development. It's also only a $1 more than the 14M controller but is much more sophisticated. Again, if anyone is interested in a production board, I would need about 10 people to be in. The cost would be $22 + shipping. The reason for the $2 increase is for the larger controller and slightly larger size of the PCB. You can PM me if your interested.
    Love

  9. #49
    ibf
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    Why not use PIC 12F or even 10F series on SMD package? would save you space as well as minimise the cost. You will have to program in ASM but it should be fairly straight forward since most PIC has built in timers and ADC.

    The spare digital I/O can be used to adjust the brightness, UP/DOWN buttons for brightness perhaps.

    Like sparks said use MOSFETs

  10. #50
    Variable Bitrate rEegLer's Avatar
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    I use PICAXE controllers because they are extremely easy to program. Two resistors, serial interface, and their Programming Editor, that's it. I also want the design to be customer friendly so that they could potentially edit the code and suite it to their needs if they are not able to do so through other means (like a POT or buttons). I do like SMD style packaging and I agree that it would save a significant amount of space while adding functionality but I like being able to swap chips if needed. One statically charged finger and the any controller is usually rendered useless.

    With the new 18M2 design, I added another ADC input for a second LDR which would allow for a smarter dimming process (high LED if LDR1>XXX AND LDR2>XXX for example).

    I also agree with using mosfets. I had a couple darlington arrays lying around so that is what I used for my prototype. And my LED backlight caps out at 9.6V so a 2V saturation was fine with me.
    Love

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