Post by thread:pot on the lcd?

At 10:44 PM 1/29/97 -0500, you wrote: >hey guys & gals > How do I eliminate the pot for the contrast adjustment on my lcd >display "minimum part count" Thanks in advance. Tony M. Well, if you can afford the board space for two resistors, just figure out where you want the voltage set at (determine with a pot hooked up) and make a voltage divider using two resistors. It's been a while since I looked at the schematic, but that should work. -Todd ____________________________________ Todd Peterson (spam_OUTtpetersonTakeThisOuTnetins.net) E-LAB Digital Engineering, Inc. Embedded Control & Integrated Circuit Solutions ELECTRONICS RESOURCE DIRECTORY at: http://www.netins.net/showcase/elab Take a look at our new EDE300 PC Interface IC on our web site!

Tony M. asksed: << How do I eliminate the pot for the contrast adjustment on my lcd display "minimum part count" >> If your LCD is the for the standard temperature range, not the wider automotive range, you may find you can simply ground that pin. I've seen it recommended to bias it up toward +5v by 330 ohms out of 10k (about a sixth of a volt). In my application, grounding it worked equally well. YMMV. Shel Michaels

Todd Peterson wrote: > > At 10:44 PM 1/29/97 -0500, you wrote: > >hey guys & gals > > How do I eliminate the pot for the contrast adjustment on my lcd > >display "minimum part count" Thanks in advance. Tony M. > > Well, if you can afford the board space for two resistors, just figure out > where you want the voltage set at (determine with a pot hooked up) and make > a voltage divider using two resistors. It's been a while since I looked at > the schematic, but that should work. > > -Todd nowadays I'm a dallas fan , so I'd suggest a dallas digital pot with a user friendly interface and on-screen-display I guess an lcd should be near somewhere :) bye charley

At 10:44 PM 1/29/97 -0500, you wrote: >hey guys & gals > How do I eliminate the pot for the contrast adjustment on my lcd >display "minimum part count" Thanks in advance. Tony M. > > I have an idea that I have not tried yet. Use the PWM output of the pic to adjust the LCD contrast. Put the output to an RC circuit with a time constant that allows the PWM to keep the Cap charged but also allows the voltage to be changed. Connect the + side of the Cap to the adjust pin of the LCD. Like I said, I haven't tried this yet. Norm

At 10:44 PM 1/29/97 -0500, you wrote: >hey guys & gals > How do I eliminate the pot for the contrast adjustment on my lcd >display "minimum part count" Thanks in advance. Tony M. I have used a pot in a few applications for an LCD display. The problem is that the contrast changes with temperature. We find that when we send a unit to a customers site, the customer has to open up the unit and readjust the contrast. We have gone to an electronic contrast developed from a PWM output of our processor. Obviously not minimum parts count, but I guess you get what you pay for. Brian

At 08:12 AM 1/30/97 -0600, you wrote: >At 10:44 PM 1/29/97 -0500, you wrote: >>hey guys & gals >> How do I eliminate the pot for the contrast adjustment on my lcd >>display "minimum part count" Thanks in advance. Tony M. >> >> > >I have an idea that I have not tried yet. Use the PWM output of the pic to >adjust the LCD contrast. Put the output to an RC circuit with a time >constant that allows the PWM to keep the Cap charged but also allows the >voltage to be changed. Connect the + side of the Cap to the adjust pin of >the LCD. Like I said, I haven't tried this yet. > >Norm The problem with that technique is that it doesn't have a low enough impedance to drive the LCD properly. The normal temp range units require a voltage in the range of 0 to .5VDC. You can use your technique but follow it with an OPAMP. We have used that technique for years. Brian

>At 10:44 PM 1/29/97 -0500, you wrote: >>hey guys & gals >> How do I eliminate the pot for the contrast adjustment on my lcd >>display "minimum part count" Thanks in advance. Tony M. > >I have used a pot in a few applications for an LCD display. The problem is >that >the contrast changes with temperature. We find that when we send a unit to >a customers site, the customer has to open up the unit and readjust the >contrast. >We have gone to an electronic contrast developed from a PWM output of our >processor. Obviously not minimum parts count, but I guess you get what you >pay for. > >Brian Tony, what sort of display is it that you are using? With a small 16x2 display like the sharp LM16A21 a diode to ground (0.7V) lifts the contrast nicely, and that particular display is not especially temperature sensitive. If you just ground the pin the display darkens noticeably at higher temperatures. On larger graphic displays we use a simple thermistor based circuit to flatten the contrast drift with temperature. I think there was a series on this in EDN a few years back, might be worth checking the back issues. The display in this case is an EL backlit 320x240 sharp which varies a fair bit with temperature. Either way eliminating the need for the user to do something is more beneficial than simply reducing component count.. to do both is ideal. Ray Gardiner, Shepparton, Victoria, Australia .....rayKILLspam@spam@dsp-systems.com Technical Director DSP Systems http://www.dsp-systems.com Private e-mail to:- rayKILLspamnetspace.net.au

Ray Gardiner wrote: {Quote hidden}> > >At 10:44 PM 1/29/97 -0500, you wrote: > >>hey guys & gals > >> How do I eliminate the pot for the contrast adjustment on my lcd > >>display "minimum part count" Thanks in advance. Tony M. > > > >I have used a pot in a few applications for an LCD display. The problem is > >that > >the contrast changes with temperature. We find that when we send a unit to > >a customers site, the customer has to open up the unit and readjust the > >contrast. > >We have gone to an electronic contrast developed from a PWM output of our > >processor. Obviously not minimum parts count, but I guess you get what you > >pay for. > > > >Brian > > Tony, what sort of display is it that you are using? > > With a small 16x2 display like the sharp LM16A21 a diode to ground (0.7V) > lifts the contrast nicely, and that particular display is not especially > temperature sensitive. If you just ground the pin the display darkens > noticeably at higher temperatures. > > On larger graphic displays we use a simple thermistor based circuit > to flatten the contrast drift with temperature. I think there was a > series on this in EDN a few years back, might be worth checking the > back issues. The display in this case is an EL backlit 320x240 sharp > which varies a fair bit with temperature. > > Either way eliminating the need for the user to do something is more > beneficial than simply reducing component count.. to do both is ideal. My sentiments exactly and thats exactly the advice I was seeking PIC list strikes again. Im using a 2x8 no backlight Thanks Tony M. > > Ray Gardiner, Shepparton, Victoria, Australia .....rayKILLspam.....dsp-systems.com > Technical Director DSP Systems http://www.dsp-systems.com > Private e-mail to:- EraseMErayspam_OUTTakeThisOuTnetspace.net.au

Brian Hackett wrote: {Quote hidden}> > At 08:12 AM 1/30/97 -0600, you wrote: > >At 10:44 PM 1/29/97 -0500, you wrote: > >>hey guys & gals > >> How do I eliminate the pot for the contrast adjustment on my lcd > >>display "minimum part count" Thanks in advance. Tony M. > >> > >> > > > >I have an idea that I have not tried yet. Use the PWM output of the pic to > >adjust the LCD contrast. Put the output to an RC circuit with a time > >constant that allows the PWM to keep the Cap charged but also allows the > >voltage to be changed. Connect the + side of the Cap to the adjust pin of > >the LCD. Like I said, I haven't tried this yet. > > > >Norm > > The problem with that technique is that it doesn't have a low enough impedance > to drive the LCD properly. The normal temp range units require a voltage > in the > range of 0 to .5VDC. You can use your technique but follow it with an OPAMP. > We have used that technique for years. > > Brian I had'nt realized the contrast (Vee) pin drew any current. I'll have to Quantify that to start with I suppose and here I was trying to reduce the part count and the general concensus is "more silicon arh arh arh"I gave up replying individually as this seems to be a popular subject.I had in mind something in the nature of a thermistor voltage divider.Or even 2 fixed resistors at a recommended voltage? Thanks people Tony M.

Shel Michaels wrote: > > Tony M. asksed: > > << How do I eliminate the pot for the contrast adjustment on my lcd > display "minimum part count" > >> > > If your LCD is the for the standard temperature range, not the wider > automotive range, you may find you can simply ground that pin. I've seen it > recommended to bias it up toward +5v by 330 ohms out of 10k (about a sixth of > a volt). In my application, grounding it worked equally well. YMMV. > > Shel Michaels I will check that out thanks.

hoss karoly wrote: {Quote hidden}> > Todd Peterson wrote: > > > > At 10:44 PM 1/29/97 -0500, you wrote: > > >hey guys & gals > > > How do I eliminate the pot for the contrast adjustment on my lcd > > >display "minimum part count" Thanks in advance. Tony M. > > > > Well, if you can afford the board space for two resistors, just figure out > > where you want the voltage set at (determine with a pot hooked up) and make > > a voltage divider using two resistors. It's been a while since I looked at > > the schematic, but that should work. > > > > -Todd > > nowadays I'm a dallas fan , so I'd suggest a dallas digital pot > with a user friendly interface and on-screen-display > I guess an lcd should be near somewhere :) > > bye > charley Cute I was concerned with user access to contrast adjustment with only a two button interface 8)

Tony Matthews wrote: > > Cute I was concerned with user access to contrast adjustment with only > a two button interface 8) why two ? I did a big sys with menu system you change selection by pushing the button short select by pushing it long if you have to make an inteface one looong push could reset the counter to "factory default" short pushes inc long ones dec so you'll need only one button . bye charley

> I had'nt realized the contrast (Vee) pin drew any current. I'll have to > Quantify that to start with I suppose and here I was trying to reduce > the part count and the general concensus is "more silicon arh arh arh"I > gave up replying individually as this seems to be a popular subject.I > had in mind something in the nature of a thermistor voltage divider.Or > even 2 fixed resistors at a recommended voltage? Thanks people Tony M. The current draw from the Vee pin is, from my experience, pretty negligible on most LCD units. While your RC needs to have at least some "oomf" to it, if you're PWM'ing at 1KHz (easy in software), you would probably be fine with R==22K, C=1uF [rc=22ms] If your PWM is faster, you could use a 0.1uF monolithic cap instead if desired. BTW, on the 16C924, the LCD charge pump can keep the display up even after VDD has been removed from the device (the display fades about 1-3 seconds later)--I don't know whether this is harmful to the LCD (is the oscillator still going?) but it sure is wierd...

> > If your LCD is the for the standard temperature range, not the wider > > automotive range, you may find you can simply ground that pin. I've seen it > > recommended to bias it up toward +5v by 330 ohms out of 10k (about a sixth of > > a volt). In my application, grounding it worked equally well. YMMV. This is a small advantage of the 1/16-duty displays over the 1/8-duty displays (though on the 1/8 duty displays you could always, I suppose, program them for 1/16 duty...); on the 1/8 duty displays, grounding Vee will produce a display with an overly dark background while on some 1/16 duty displays this is not a problem.

hoss karoly wrote: {Quote hidden}> > Tony Matthews wrote: > > > > Cute I was concerned with user access to contrast adjustment with only > > a two button interface 8) > > why two ? > I did a big sys with menu system > you change selection by pushing the button short > select by pushing it long > > if you have to make an inteface one looong push could reset the counter > to "factory default" > short pushes inc > long ones dec > > so you'll need only one > button . > > bye > charley Charley, s'good idea except maybe the frustration factor. I'll think about it. Thanks. Tony M.