At 03:11 PM 3/8/2005 +0000, you wrote: >John J. McDonough wrote: > >And yes, 10 ohm seems way too low to me. I generally use whatever I happen > >to have, but never less than 1K. Sure they didn't intend 10K? > >A little background information on this... > >The Vo input to the LCD module (wiper of the pot) is actually a power >supply rail for the liquid crystal drivers within the controller IC on >the module. Fortunately, the current requirement is usually low enough >that a 10K potentiometer is a good enough voltage source. > >For larger LCD panels which require more Vo current, a voltage follower >circuit is preferred, to avoid the need for a lower-value potentiometer >which would waste more current. Some LCD modules include this: others >expect the designer to provide it externally. Check the data sheet for >the module (if you can find it!). > >In the absence of a full data sheet, a possible clue is that an extended >temperature module which requires a separate Vee input (fixed negative >supply voltage, as distinct from adjustable Vo) usually has the voltage >follower on board (hence the need for this Vee supply) so that only a >high-value external potentiometer is required. > >In any case, 10 ohms is much too low a potentiometer value. 10K should >work fine for a small-to-medium sized LCD module. >-- >Ian Chapman >Chapmip Technology, UK To go even further, the contrast voltage, in general, should vary with temperature in order to maintain constant contrast on the display. The PIC can measure the temperature and vary the voltage as required, or a simple analog circuit can be used (with reduced flexibility and accuracy, of course). The specific LCD datasheet should have the required temperature characteristic detailed. Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist