Post by thread:[PIC] 10 ohm pot to control contrast on a 16 char

In the book "Programming and Customizing PICmicro® Microcontrollers (2nd edition)", it shows a circuit for connecting a PIC to a 44780 LCD (Figure 2-25 on page 278 if you are interested). What I don't understand is it shows a 10 Ohm pot connected in a simple voltage divider fasion (One pin to VCC (5v), one pin ground, and the wiper pin to the contrast control pin on the LCD). My understanding is that .5A will be wasted though the pot (I = E / R = 5 / 10 = .5). It will also be dissipating 2.5 Watts. My spec sheet shows that the max current drain of the LCD is 4 mA. So my question is doesn't 10 Ohm seem way low? Maybe 10k or 100k? I checked the errata for the book at http://www.myke.com/pic-book.htm and it doesn't list this issue. Thanks. - Mark Bellamy

Mark Bellamy wrote : > In the book "Programming and Customizing PICmicro® Microcontrollers > (2nd edition)", it shows a circuit for connecting a PIC to a 44780 LCD > (Figure 2-25 on page 278 if you are interested). > > What I don't understand is it shows a 10 Ohm pot connected in a simple > voltage divider fasion one pin to 5v, one pin ground,... Bad proof reading... Try with 10k. Funny a "2nd edition" still has that error... :-) :-) Jan-Erik.

----- Original Message ----- From: "Mark Bellamy" <spam_OUTmbellamyTakeThisOuTgmail.com> To: "PICLIST" <.....piclistKILLspam@spam@mit.edu> Sent: Saturday, March 05, 2005 4:34 PM Subject: [PIC] 10 ohm pot to control contrast on a 16 char x 2 line LCD? {Quote hidden}> In the book "Programming and Customizing PICmicro® Microcontrollers > (2nd edition)", it shows a circuit for connecting a PIC to a 44780 LCD > (Figure 2-25 on page 278 if you are interested). > > What I don't understand is it shows a 10 Ohm pot connected in a simple > voltage divider fasion (One pin to VCC (5v), one pin ground, and the > wiper pin to the contrast control pin on the LCD). My understanding > is that .5A will be wasted though the pot (I = E / R = 5 / 10 = .5). > It will also be dissipating 2.5 Watts. My spec sheet shows that the > max current drain of the LCD is 4 mA. > > So my question is doesn't 10 Ohm seem way low? Maybe 10k or 100k? > > I checked the errata for the book at http://www.myke.com/pic-book.htm > and it doesn't list this issue. Must be an error. 10K should be fine, but 100K may work ok too. Just grounding the pin should work as well, but it might be a tiny bit off on the viewing angle, it depends upon the specific LCD.

Mark, Jan-Erik, > Mark Bellamy wrote : > >>In the book "Programming and Customizing PICmicro® Microcontrollers >>(2nd edition)", it shows a circuit for connecting a PIC to a 44780 LCD >>(Figure 2-25 on page 278 if you are interested). >> >>What I don't understand is it shows a 10 Ohm pot connected in a simple >>voltage divider fasion one pin to 5v, one pin ground,... > > Jan-Erik Soderholm wrote: > Bad proof reading... > Try with 10k. > > Funny a "2nd edition" still has that error... > :-) :-) Thanks, I've added this error to my 'private' list of errata for the abominable edition of this book I ever received (Second edition). (http://www.robh.nl/picsoft.html#predko) Regards, Rob. -- Rob Hamerling, Vianen, NL phone +31-347-322822 homepage: http://www.robh.nl/

On Sat, 5 Mar 2005 17:34:14 -0500, you wrote: {Quote hidden}>In the book "Programming and Customizing PICmicro® Microcontrollers >(2nd edition)", it shows a circuit for connecting a PIC to a 44780 LCD >(Figure 2-25 on page 278 if you are interested). > >What I don't understand is it shows a 10 Ohm pot connected in a simple >voltage divider fasion (One pin to VCC (5v), one pin ground, and the >wiper pin to the contrast control pin on the LCD). My understanding >is that .5A will be wasted though the pot (I = E / R = 5 / 10 = .5). >It will also be dissipating 2.5 Watts. My spec sheet shows that the >max current drain of the LCD is 4 mA. > >So my question is doesn't 10 Ohm seem way low? Maybe 10k or 100k? > >I checked the errata for the book at http://www.myke.com/pic-book.htm >and it doesn't list this issue. > >Thanks. > >- >Mark Bellamy Is it _REALLY_ not obvious that it should be 10K...???

Note that some LCD displays (I have a couple of Seiko units that are this way) actually require a small negative voltage for the contrast - my Seiko's require about 1 volt negative. I didn't have a -ve supply available, so I used a PIC pin to generate a square wave at about 50 Hz (that was the interrupt rate I was using), then a couple of diodes and 10uf electrolytic capacitors to make a charge pump -ve voltage source. At 05:34 PM 3/5/2005 -0500, you wrote: {Quote hidden}>In the book "Programming and Customizing PICmicro® Microcontrollers >(2nd edition)", it shows a circuit for connecting a PIC to a 44780 LCD >(Figure 2-25 on page 278 if you are interested). > >What I don't understand is it shows a 10 Ohm pot connected in a simple >voltage divider fasion (One pin to VCC (5v), one pin ground, and the >wiper pin to the contrast control pin on the LCD). My understanding >is that .5A will be wasted though the pot (I = E / R = 5 / 10 = .5). >It will also be dissipating 2.5 Watts. My spec sheet shows that the >max current drain of the LCD is 4 mA. > >So my question is doesn't 10 Ohm seem way low? Maybe 10k or 100k? > >I checked the errata for the book at http://www.myke.com/pic-book.htm >and it doesn't list this issue. > >Thanks. > >- >Mark Bellamy >

From: "Larry Bradley" Note that some LCD displays (I have a couple of Seiko units that are this way) actually require a small negative voltage for the contrast - my Are these Hitachi 44780 compatible, or are they graphical displays? Seiko's require about 1 volt negative. I didn't have a -ve supply available, so I used a PIC pin to generate a square wave at about 50 Hz (that was the interrupt rate I was using), then a couple of diodes and 10uf electrolytic capacitors to make a charge pump -ve voltage source. If one is already present in a circuit, a max232 is a possible source of a negative voltage. At 05:34 PM 3/5/2005 -0500, you wrote: {Quote hidden}>In the book "Programming and Customizing PICmicro® Microcontrollers >(2nd edition)", it shows a circuit for connecting a PIC to a 44780 LCD >(Figure 2-25 on page 278 if you are interested). > >What I don't understand is it shows a 10 Ohm pot connected in a simple >voltage divider fasion (One pin to VCC (5v), one pin ground, and the >wiper pin to the contrast control pin on the LCD). My understanding >is that .5A will be wasted though the pot (I = E / R = 5 / 10 = .5). >It will also be dissipating 2.5 Watts. My spec sheet shows that the >max current drain of the LCD is 4 mA. > >So my question is doesn't 10 Ohm seem way low? Maybe 10k or 100k? > >I checked the errata for the book at http://www.myke.com/pic-book.htm >and it doesn't list this issue. > >Thanks. > >- >Mark Bellamy >

At 09:39 AM 3/6/2005 -0500, you wrote: >Note that some LCD displays (I have a couple of Seiko units that are this >way) actually require a small negative voltage for the contrast - my >Seiko's require about 1 volt negative. I didn't have a -ve supply >available, so I used a PIC pin to generate a square wave at about 50 Hz >(that was the interrupt rate I was using), then a couple of diodes and >10uf electrolytic capacitors to make a charge pump -ve voltage source. The requirement for a negative contrast voltage is typical of wide temperature range LCD character displays. Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" speffKILLspaminterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com

----- Original Message ----- From: "Spehro Pefhany" <.....speffKILLspam.....interlog.com> Subject: Re: [PIC] 10 ohm pot to control contrast on a 16 char x 2 line LCD? > The requirement for a negative contrast voltage is typical of wide > temperature range LCD character displays. In fact, "extended temperature" is pretty much synonymous with "needs a negative supply". Fortunately, the negative supply requires very little current, so a charge pump as you used is fairly simple to cobble together. Not all Seiko displays are extended temperature. Seiko, like most suppliers, provides both extended temperature and "normal" displays. 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? --McD

These are small Seiko 16x2 character displays with the KS0070 controller (44780 compatible). I got them from EIO (surplus company) for $6.00 each, with LED backlight. I use IC charge pumps for -ve voltages for op-amps and the like. I've never build a circuit with a 232 on it, but that is worthwhile keeping in mind. At 09:00 AM 3/6/2005 -0600, you wrote: >Are these Hitachi 44780 compatible, or are they graphical displays? > > >If one is already present in a circuit, a max232 is a possible source of >a negative voltage. > Larry Bradley Orleans (Ottawa), Ontario, CANADA

John J. McDonough <EraseMEmcdspam_OUTTakeThisOuTis-sixsigma.com> 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

At 03:11 PM 3/8/2005 +0000, you wrote: {Quote hidden}>John J. McDonough <mcdspam_OUTis-sixsigma.com> 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" @spam@speffKILLspaminterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com