Arduino Red on Black LCD. - Page 1
Forum section: electronics / beginners

author: neo
author_rank: Super Contributor
timestamp: September 18, 2017, 06:05:56 am
content: I have a red on black LCD(http://www.ebay.com/itm/LCD1602A-5V-Red-Character-Dot-Matrix-LCD-Display-Module-16x2-Black-Background/391821451918?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=057872.m2749.l2649)and it is working perfectly with my arduino and a DS3231 except in light, bright light especially but any light brighter than a dim glow will cause it to look washed out. Even holding it under a lamp was enough to make it look like the back light was not even there, i am assuming this to be the result of it having a red backlight which has to be very very dim to look right.I currently have a 20k resistor current limiting the back light to make it look just the right shade of red, so to the point is there anyway to use this LCD in anything except a dark room?Also another relevant question, the contrast pin is meant to, as far as i know, use a voltage divider in the form of a potentiometer to adjust the contrast however on this one it requires a direct connection to ground, anything else and there is only a blank screen.

author: Fire Doger
author_rank: Regular Contributor
timestamp: September 18, 2017, 08:53:59 am
content: Other LCDs like this can work too with a resistor to ground for contrast (around 300Ohm or 1K, I don't remember exactly), direct short doesn't sound good.If you can't see chars because contrast is very low you can put the light pipe with the red led on a regular blue/green (blue has white chars, green has black chars)

author: neo
author_rank: Super Contributor
timestamp: September 18, 2017, 09:12:55 am
content: The contrast as of yet has not been a problem, the only problem as of yet is that it cannot be viewed in light because of its color, and i like its color.

author: Fire Doger
author_rank: Regular Contributor
timestamp: September 18, 2017, 09:28:38 am
content: So if contrast isn't an issue then what's your question? We can't turn the sun off.My guess is that the chars are very dim, that's why you need a very dim back light and that's why you cant use it in room with light. A typical resistor for LED is 220-330 Ohm. If you can't set contrast to work with 100% back light then its a faulty LCD.

author: stevelup
author_rank: Regular Contributor
timestamp: September 18, 2017, 09:55:40 am
content: 20k seems way to high a value for current limiting on the backlight LED.Measure the voltage across the backlight and see what you're getting, then check it matches the data sheet.

author: tooki
author_rank: Super Contributor
timestamp: September 18, 2017, 10:04:06 am
content: Quote from: neo on September 18, 2017, 06:05:56 am1. A 20K resistor for the backlight LED sounds very high if we are talking about a 5V source. Most likely you are severely under-driving the backlight, in which case it doesn't stand a snowball's chance in hell of being daylight-visible. You mention tweaking it to "just the right shade of red" but I suspect you don't have that luxury. You could of course use an ambient light sensor to adapt the backlight brightness via the Arduino, using PWM to drive the backlight LED.2. I experimented with trying to change the backlight LED color on a regular white-on-blue 1602LCD, and was really surprised at how poorly it performed with any color other than white. The digits barely reached any kind of usable brightness, the overall contrast was extremely low. (I don't know anything about the physics of this, so I can't explain the "why".) Anyway, I wouldn't put it past some vendors to do it anyway, despite the poor performance. So it may be worth checking out a different vendor.3. On most character LCDs, the contrast pot goes between V+ and ground with the wiper output to the contrast input. However, on some (mostly large-character ones, low-voltage ones, or extended-temperature models), contrast actually needs a negative voltage, in which case the pot goes between V+ and V-, with the wiper again going to the contrast input. This is something you could try.If you do end up needing the negative voltage, chances are the LCD module has the footprint on the back for an ICL7660 chip and associated caps. It's there normally for 3.3V usage, but is also useful for the other types that need a negative contrast voltage. In that case, the contrast pot is wired as a 2-pin variable resistor between ground and contrast input.I wish I could find the schematic of the 1602 modules I once found, it explained this better than I do.

author: neo
author_rank: Super Contributor
timestamp: September 18, 2017, 11:35:49 am
content: Quote from: tooki on September 18, 2017, 10:04:06 amIf i turn the brightness of the back light up it works fine until i hit 10K at which point i being to be able to see individual pixels

author: neo
author_rank: Super Contributor
timestamp: September 18, 2017, 02:29:16 pm
content: I connected the contrast through a 1.4k resistor to ground any higher and it drops off completely and adjusting the brightness in a well lit room has lead to reasonably acceptable results.