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?

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

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

How do you wire up the new 4x20 LCD's LED backlights?

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

Yet another RTFM moment, Mike. Thanks for the pointer still :)

smwulph

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