Post by thread:LCD contrast

Peter van Hoof wrote: > > Yes, I do not have the datasheet of this display but a lot of similar > displays need a negative voltage for good contrast , most will take a pot > between -5 and +5 volt > > I've got two completely unrelated questions for you all: > > 2. I have a Seiko L2432 2 x 24 char LCD module, which I am having some > > difficulty getting a decent contrast out of. I am using what appears to be > > a fairly standard usage of a 10k trimpot to control the LCD drive voltage > > (pin 3) however the best contrast occurs when this is fully turned towards > > the ground side. When I connected ground straight to the pin the display > > was slightly darker but still not upto what I would consider to be a > > reasonable standard. Anyone have any ideas? > > > > Wesley Moore Get a 1.5 volt battery and connect it positive to pin 3 and negative to ground. If it darkens the display a lot more, then that is your answer. It requires a negative bias. In fact it sounds very much like it, as I have been down this road before. If you have a max-232 or similar on board, you can steal the negative supply from that. Still needs a pot for final adjustment, or a voltage divider pair of resistors. Don McKenzie spam_OUTdonTakeThisOuTdontronics.com http://www.dontronics.com Don's Download Dungeon: http://www.dontronics.com/download.html Australian Electronics Ring http://www.dontronics.com/aering.html Win $500USD Cash. Micro design contest: http://www.simmstick.com

> > > 2. I have a Seiko L2432 2 x 24 char LCD module, which I am having some > > > difficulty getting a decent contrast out of. I am using what appears to be > > > a fairly standard usage of a 10k trimpot to control the LCD drive voltage > > > (pin 3) however the best contrast occurs when this is fully turned towards > > > the ground side. When I connected ground straight to the pin the display > > > was slightly darker but still not upto what I would consider to be a > > > reasonable standard. Anyone have any ideas? If the necessary negative voltage is small, lets say, less or around -0.7V, you can apply a regular diode from the LCD ground pin (1) to the circuit ground. Connect pin 3 (lcd contrast) to circuit ground. It means that the LCD ground will be 0.7V above ground, and pin 3 will be virtually 0.7V below the LCD ground. Of course all control and data lines from the processor will arrive with 0.7V below LCD ground (when bit = 0), but it should work. Check it out. Another solution is just steal some processor pin that has a constant high frequency, and use the following circuit to extract an aprox VCC * -0.8 Volts. If the high frequency is high impedance, just connect it via a transistor or any available gate. The capacitors value is not critical, and they define the Voltage level at the output, based on the LCD load for that negative voltage. 100pF k diode aprox 500kHz----||-----o----|<-----o-------o -Vcc*0.8 | | V --- diode --- --- 100pF k | | | | Gnd Gnd

Thanks to all who helped with my quetions. Both have been answered. It has come through load and clear that I need to generate a negative voltage for the contrast. I think I will give the diode idea a bash first as it's the easiest and I'm pretty sure this will be enough as the display isn't too far off as it is, just needs a little more oomph. Wesley

Wesley Moore (Yallara) wrote: > I think I will give the diode idea a bash first as it's the easiest > and I'm pretty sure this will be enough as the display isn't too far > off as it is, just needs a little more oomph. Use silicon signal diodes, *not* power diodes which don't work at frequencies much above 1 KHz. -- Cheers, Paul B.

Check out Micrel's MIC2660BM5. It's a charge pump in a SOT23-5 (5pin). Connect the normally positive output to ground and the ground output will be negative. Load the output to get the voltage you need or use a divider. Good luck, Todd. Paul B. Webster VK2BZC wrote: {Quote hidden}> Wesley Moore (Yallara) wrote: > > > I think I will give the diode idea a bash first as it's the easiest > > and I'm pretty sure this will be enough as the display isn't too far > > off as it is, just needs a little more oomph. > > Use silicon signal diodes, *not* power diodes which don't work at > frequencies much above 1 KHz. > -- > Cheers, > Paul B.

Paul B. Webster VK2BZC wrote: > > Don McKenzie wrote: > > > Get a 1.5 volt battery and connect it positive to pin 3 and negative > > to ground. > > Oh dear oh dear! *Negative* to pin 3 and positive to ground. > -- > Cheers, > Paul B. c'mon Paul, don't I get off easy because of my age? I'll span 3 centuries soon. :-) Don McKenzie .....donKILLspam@spam@dontronics.com http://www.dontronics.com Don's Download Dungeon: http://www.dontronics.com/download.html Australian Electronics Ring http://www.dontronics.com/aering.html Win $500USD Cash. Micro design contest: http://www.simmstick.com