Character LCD at 3.3V instead of 5V - Page 1
Forum section: electronics / projects

author: gnuarm
author_rank: Super Contributor
timestamp: February 16, 2021, 09:38:11 am
content: A project I'm working on started with an Arduino so everything was developed around 5V. Now a redesign of the electronics is using 3.3V circuits for 90% of the device, but the 2x16 character LCD modules are still 5V. I contacted the manufacturer and they don't have a mechanically compatible unit running on 3.3V.However, the data sheet for the controller chip indicates it runs off any voltage down to 2.7V. So it will work at 3.3V for sure. I guess the issue is the LCD itself may not be suited for 5V. When I've looked at LCDs in the past they seem to be made for one or the other voltage. Not sure what they optimize differently. But without simply trying a module on 3.3V (I don't have one myself), anyone know if these things are actually voltage agnostic?Of course the backlight will be dimmer on 3.3V and we can run that on 5V. I'd just like to not use an interface chip to convert between 3.3V and 5V logic. I would ask someone on the project to try it, but mostly people are rather busy with other stuff.

author: Psi
author_rank: Super Contributor
timestamp: February 16, 2021, 10:14:15 am
content: If you need 5V at low current you can make a charge pump out of a spare mcu output.That will get you a 5V rail from your 3.3V supply.

author: Nusa
author_rank: Super Contributor
timestamp: February 16, 2021, 10:23:56 am
content: Here's the conversion instructions for those LCD modules from 5V to 3.3V. The boards are already set up for it.https://www.codrey.com/electronic-circuits/hack-your-16x2-lcd/Or you can buy them that way in the first place, usually for about the same price. Just search for "3.3V 1602 LCD modules" (or 2002 or 2004 if you prefer those sizes). Whichever vendor you're talking to now may not want to bother, but they are easily found.

author: gnuarm
author_rank: Super Contributor
timestamp: February 16, 2021, 10:51:58 am
content: Quote from: Nusa on February 16, 2021, 10:23:56 amThat's the problem. You can buy lots of LCD modules, but they are not all interchangeable mechanically. I found two or three and every one had differences in either the board/mounting dimensions or the display itself having different sizes of characters or positioned slightly differently.The guy heading up the project is a mechanical engineer, so he sees the mechanical parts of the project as being complete and not subject to change. I'm not willing to dig through a hundred data sheets to find a compatible display without even knowing one exists. Given the info on that web page, it seems the LDC display itself is designed for 5V and so is not compatible with 3.3V drive.The crazy thing is that while the manufacturer of this display says they don't sell a 3.3V version, I can see on the image of the back PCB they have the footprint for the 7660 chip to make it work on 3.3V! Isn't that odd, they built in the feature, but never used it in a product.Go figure!

author: Ian.M
author_rank: Super Contributor
timestamp: February 16, 2021, 11:21:17 am
content: So all you need is a negative bias supply for Vo (i.e the bottom end of the contrast pot) to about -2V. *ANY* negative rail you've already got will do the job with an appropriate dropper resistor. No negative rail? If you've got a spare PWM capable pin on your MCU, a charge pump to generate the negative bias can be as simple as two caps and a dual (series) diode. Otherwise you are going to need that 7660 circuit or similar on your main PCB.@Nusa: Link bookmarked. *MANY* thanks!

author: Renate
author_rank: Super Contributor
timestamp: February 16, 2021, 01:39:54 pm
content: Slight tangent here...In regular 5V operation I see everybody using a contrast pot with +5 on the high end.Why do they do that?It makes the pot have no useful range.The LCD spec sheets that I have seen all indicate a variable resistance from the low end of the (internal) resistor divider chain to the low supply (for 5V gnd).I use 5K as a two terminal and it works fine.

author: madires
author_rank: Super Contributor
timestamp: February 16, 2021, 02:48:53 pm
content: Yep, you can adjust the voltage across the resistor ladder by a potentiometer between V0 (Vee) and Gnd. I have no idea why or when we started using the "standard" way with a 10k pot between Vcc and Gnd.

author: coromonadalix
author_rank: Super Contributor
timestamp: February 16, 2021, 03:22:08 pm
content: Good trick for the charge pump added .....https://www.codrey.com/electronic-circuits/hack-your-16x2-lcd/All the lcd are not always identicals, even based on the 44780 chipset, i use 2x40 models and was struck by clock timing isuessome of them refused to start ? now we have a good suplier and never had any problems since

author: hexreader
author_rank: Frequent Contributor
timestamp: February 16, 2021, 03:29:31 pm
content: The 2x16 character LCDs that I use (MikroE modules) can run with 5V supply, but work happily with 3.3V signals.Depends on LCD module and phase of the moon, but if you have 5V and 3.3V available, then supply power to LCD module with 5V, and drive with 3.3V signals.YMMV

author: gnuarm
author_rank: Super Contributor
timestamp: February 16, 2021, 08:42:30 pm
content: Quote from: hexreader on February 16, 2021, 03:29:31 pmThat does not meet the spec on the signal levels. 0.7 * Vcc is as high as 3.675V worst case. Can't reach that level with a 3.3V driver.All the power levels are available. I'm trying to avoid using a level shifter chip.

author: gnuarm
author_rank: Super Contributor
timestamp: February 16, 2021, 08:59:15 pm
content: Quote from: coromonadalix on February 16, 2021, 03:22:08 pmYeah, thanks for the link. I've seen that, but I'm not trying to hack a display. I'm designing a product in which a particular display was chosen to work with the Arduino, then the mechanical engineer in charge refused to change the LCD because he would have to change something in the case. So we seem to be stuck using a 5V part with 3V logic.

author: Miti
author_rank: Super Contributor
timestamp: February 16, 2021, 09:05:47 pm
content: Quote from: gnuarm on February 16, 2021, 08:42:30 pmWhat about using pull ups and MOS-FETs, something like I2C level shifters, but in one direction only? See attached

author: gnuarm
author_rank: Super Contributor
timestamp: February 16, 2021, 09:49:19 pm
content: Quote from: Renate on February 16, 2021, 01:39:54 pmYeah, I see that. The VO signal for the adjustment is the resistor divider chain from 5V providing the voltages for the multiple driver signals. As you say, giving this the range of the full 5V span makes no sense. A variable resistance to ground or V- is shown in the data sheet for the display driver chip, ST7066U in my case.In the most recent iteration of the schematic they've removed the pot and use a pair of resistors setting Vo to... well, there's no way to tell what voltage that will be since we don't know the value of the resistors inside the module. With that value being unknown knowing what size pot to use as a variable resistor is also dependent on an unknown.I suppose if the module maker says use a voltage divider, we need to use a voltage divider. I saw one project where they used a PWM output to set the voltage.One thing I realize is that maybe it makes sense to use a 7660 chip on our boards to generate a negative Vo. That will let it run off 3.3V safely and if someone adopts 3.3V displays they only need to add a jumper and leave off the inverter circuit to work with it. Since this is in place of using a level shifter chip or two, it is easily justified and probably cheaper.

author: amyk
author_rank: Super Contributor
timestamp: February 17, 2021, 04:17:37 am
content: You can drive the LC directly at a lower voltage, all you'll get is lower maximum contrast (which is usually not a problem since the contrast adjustment is normally not set to the far end of the range) and slightly slower refresh rates, but since you are talking about passive matrix, you are unlikely to care about the latter.

author: gnuarm
author_rank: Super Contributor
timestamp: February 17, 2021, 05:04:24 am
content: Quote from: amyk on February 17, 2021, 04:17:37 amI'm not sure 3.3V is enough for proper contrast. It appears the LCDs used on the 5V displays need about 3.7V to 4V on the VO line which I think is the terminus of a resistor chain setting the several voltages which are used for the display multiplexing. I've found several web pages about this and the module we are using has a place for the 7660 chip, so that would seem to be what they do for a 3.3V display... even though they don't sell one.Just tried simulating a voltage inverter using diodes, but it generates significant current spikes when the diodes turn off. I'm using Schottky diodes to minimize the voltage drop. Are they worse in this regard? The current can be limited to a few mA, but with the spike being so narrow the harmonic content will be huge and there are sensitive analog circuits on this board. Maybe I should stick with the 7660? I guess it doesn't make spikes because the FETs switch on and off without shoot through.

author: blowtorch
author_rank: Newbie
timestamp: September 28, 2021, 01:19:47 pm
content: I know this is old but I had a similar problem and found a solution so thought to post in case it helps somebody else.Problem and fix #1Previously, we had specified a system around a particular LCD module then had a problem with supply. Not wishing to spend plenty $$$ on changes to the mold for the product we managed to find a different module that only needed a change to the PCB layout, to slightly move the relative position and get it to fit.Problem and fix #2I have a bunch of unused 5V modules, and also wanted to use the 5V LCD model on 3V3 and had the problem with setting contrast. The logic is perfect at 3V3, so it was only the contrast that needed to be set. Anyway, my proposed solution was to use a negative voltage on VO, but when setting up a test circuit to determine the optimum voltage, I realised that the 5V version of the module was perfectly clear, withperfectcontrast with VO simply pulled to ground. So, I have several of these boards running 24x7 for testing at the moment, and to test any unwanted contrast changes with changes to temperature etc.

author: gnuarm
author_rank: Super Contributor
timestamp: October 02, 2021, 04:07:47 am
content: I remember that the contrast voltage at 3.3V was nearly within spec. The problem is the variation in voltage, temperature and the differences from LCD unit to unit. These factors may not be a problem for your application. For others it may not be optimal. That's why the negative voltage generator is designed into the circuit boards of many LCD modules.

author: tooki
author_rank: Super Contributor
timestamp: October 02, 2021, 08:27:41 am
content: FYI, it also depends on the LCD size. I have some large-character 1602 LCDs, and they require a negative contrast voltage even at room temp.I’ve done the ICL7660 “mod” on regular-size 1602 displays before, quick and easy fix for proper 3.3V operation.