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Sensors, Actuators, and Robotics » LCD display with LED backlight

April 17, 2009
by Kevin
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After damaging my original LCD display :-( I replaced it with an LCD display with LED back light (not the one included with the newer kits. I bought one from another supplier.)

I cut the ribbon cable off of the old LCD and soldered it to the new LCD and connected power for the back light. I noticed that my temperature reading was dancing all around similar to when I used a wall wart power supply. I was able to filter out the wall wart noise with the addition of another bypass cap but I cant get rid of the noise when back lighting the LCD at full power. I have to add about 25 ohms in series with the LED back light to get it to settle down. With the 25 ohms in series the back light is a little dimmer than I would like. Not a show stopper though - but I would like to figure out a solution.

Any one have any ideas?

April 18, 2009
by mrobbins
(NerdKits Staff)

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Hi Kevin,

The LED backlight probably isn't intended to be run at 5.0V -- this is at least true for our newer 20x4 LCD modules, which have a forward voltage of 4.2V at 180mA (typical rating). The "dancing temperature reading" is probably due to drawing too much current either from your power supply or voltage regulator. If it's the voltage regulator, be aware that you can use a larger resistor in series with the LED backlight, but then connect it to the battery/wall wart directly, instead of going through the 5V regulator. This will significantly reduce the amount of heat being dissipated in the voltage regulator itself, which may prevent overheating and the oscillating effect you are seeing. Let us know what happens!

Mike

April 22, 2009
by Kevin
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Yeah, you were correct Mike. The LED backlight VF should be 3.8 - 4.4 VDC with a typical rating of 4.1VDC 160mA.

Thanks

May 19, 2009
by smwulph
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How do you wire up the new 4x20 LCD's LED backlights?

May 19, 2009
by mrobbins
(NerdKits Staff)

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Hi smwulph,

Take a look at the last page (Appendix C) of the NerdKits Guide. Basically, you put it in series with an included 33 ohm resistor, but take a look for a photo and more explanation.

Mike

May 19, 2009
by smwulph
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Yet another RTFM moment, Mike. Thanks for the pointer still :)

June 30, 2009
by BobaMosfet
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smwulph

You can also use a potentiometer to control your LCD contrast.

December 12, 2010
by mbobak
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Not sure what the etiquette is here on starting a new topic or reopening one that's been dead over a year.

I was reading about the LCD backlight, and that it couldn't be done using the 9V battery, as it drew too much current. It recommends converting to AC power, if you want the backlight to work.

What about using USB power? Will the backlight put too much demand on the USB power supply? I understand it will drain my laptop battery that much faster, but I'm not worried about that, as I'm usually plugged in to AC when I'm at home experimenting w/ my NerdKit. So, could running the backlight on USB power cause any kind of damage?

Thanks!

-Mark

December 12, 2010
by met_fredrik
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Depends on how much current the display youre talking about draws. As far as I know the max current draw from a laptop USB port is typically 150-200 mA, but I've read that it's possible to get as much as 500 mA with drivers. And also, some people on the net says it will damage your USB port if you draw to much.

December 13, 2010
by hevans
(NerdKits Staff)

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Hi mbobak,

With the LED Backlight running with a 33ohm resistor the way we described in the guide it is drawing about 30mA of current. With that draw your battery will not last too long, but I would give it at least an hour or so.

Running the backlight off your USB Power will not cause any damage assuming everything is properly connected. Make sure the current limiting 33ohm resistor is there to prevent a current overdraw.

Humberto

December 14, 2010
by mbobak
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The display I'm talking about is the standard 4x20 LCD that comes w/ a NerdKit.

I'm looking the data sheet, shouldn't it list somewhere the max current draw? Can someone explain to me, where to find that information? I'm trying to get better at reading data sheets, but at this point I'm pretty clueless.

-Mark

December 14, 2010
by hevans
(NerdKits Staff)

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Hi mbobak,

Datasheets are kind of like code documentation, sometimes you get an awesome one that tells you everything you need to know in a well laid out document, sometimes they don't give you much to work with and you have to figure most of it out. There really isn't a well made datasheet for the LCD in the kit, what you are seeing in the downloads section is the datasheet for the controller on board the LCD. This gives you all the information you would need to operate the LCD, but not much about the electrical characteristics or the backlight. The manufacturer of the LCD was able to tell us the backlight has a 4.2V forward voltage and from there we are able to do the math you find on page 82 of the guide to find the proper resistor to use and how much current that will draw. Hope that helps.

Humberto

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