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[on:]

I saw these character displays with RGB back lights very recently but they were all custom-made for these two stores and I couldn't find one on ebay. Here are my two questions:

1) What do you need an RGB back light for?
2) Where do you get one for a better price?

Adafruit:
http://www.adafruit.com/products/399


sparkfun:
http://www.sparkfun.com/products/10862

BTW, it seems sparkfun screwed up by ordering some common-cathode displays, not pin-compatible with regular LCDs on the part that pin 15 is A pin 16 is K (sparkfun's has pin 15 K and pin 16 A)

[Reply #1 on:]

1) What do you need an RGB back light for?

1) Aesthetics
2) Say you had the need to indicate a warning; under normal conditions, use a green backlight - but in the event of the warning, flash it red (or leave it solid red)...

2) Where do you get one for a better price?

Ebay?

[Reply #2 on:]

Ebay is disappointing. No hit for "character lcd rgb":

http://www.ebay.com/sch/i.html?_from=R40&_trksid=p5197.m570.l1313&_nkw=character+lcd+rgb&_sacat=See-All-Categories

If you try "character lcd", there's too many hits to sift through.

[Reply #3 on:]

Maybe not a better price, but possibly a better selection.  http://www.newhavendisplay.com/index.php?main_page=index&cPath=329&zenid=rlht917r1bk5sgpo2hvq70c6t4

Don

[Reply #4 on:]

BTW, it seems sparkfun screwed up by ordering some common-cathode displays, not pin-compatible with regular LCDs on the part that pin 15 is A pin 16 is K (sparkfun's has pin 15 K and pin 16 A)

There are others out there like this.  There are also some with the power on pins 1 and 2 reversed.

Don

[Reply #5 on:]

Ebay is disappointing. No hit for "character lcd rgb":

http://www.ebay.com/sch/i.html?_from=R40&_trksid=p5197.m570.l1313&_nkw=character+lcd+rgb&_sacat=See-All-Categories

If you try "character lcd", there's too many hits to sift through.

Interesting - I tried several different searches and couldn't find anything; I was almost certain such a thing would be on Ebay if anywhere...

This is what I would do (it would take a while):

1) Go thru all the listings, and find the "top N" rated sellers (where N is 1-10)
2) Contact these three people
3) Ask them if they could supply you with such an LCD (give a link to the one you know, plus any other information)

Hopefully, you'll get some responses, and the quality (or lack?) of their English will tell you whether they have understood what you want, and whether they can supply it; you may have to "sweeten" the deal by ordering more than one (maybe 10 or more), but perhaps they could do it, and then put the rest up on their Ebay store or something (?). If you let them know that others may be interested (and if you get good service, you'll mention them here?) - they might just swing it.

[Reply #6 on:]

This any good?

http://www.ebay.co.uk/itm/ART-New-LCD-2x16-F-LED-b-l-RGB-HD44780-H-30mm-ABC016002F00-MIRGB-R-/320795737775?pt=LH_DefaultDomain_212&hash=item4ab0ea7eaf#ht_754wt_952

Price is very cheap!

[Reply #7 on:]

Good tips thanks cr0sh! I got one good lead but the minimal order is 50 pcs. That's too many. I was just interested in getting a couple units.

@matinzk, thanks. The display has the double-row connection on the side instead of on the top, which I prefer.

I'm thinking of using this RGB display for my serial panel but that means I won't be able to reserve the I2C pins for future I2C interface since two more LEDs need two more pins.

@floresta, you're right. I was mainly thinking about the pin-15 being A and pin-16 being K. SPE sells this regular LCD:

http://www.sparkfun.com/products/255

It's got the pin assignment I'm used to.

[Reply #8 on:]

What about the chinese wholesale sites like alibaba ? I looked but did not spend much time at it. They may have some.

[Reply #9 on:]

I'm thinking of using this RGB display for my serial panel but that means I won't be able to reserve the I2C pins for future I2C interface since two more LEDs need two more pins.

What are you using all the other pins for? Have you considered sharing some of the LCD pins with other I/O functions?

[Reply #10 on:]

What about the chinese wholesale sites like alibaba ? I looked but did not spend much time at it. They may have some.

@copiertalk, Is that a wholesale only site? I just need a couple to play with.

@dc42, I have not thought about that. So do you mean maybe using the 4 LCD data pins to drive the columns of the matrix keypad? Should be doable without much modification of my original code, right? I don't sense the keypad and update LCD at the same time.

[Reply #11 on:]

@dc42, I have not thought about that. So do you mean maybe using the 4 LCD data pins to drive the columns of the matrix keypad? Should be doable without much modification of my original code, right? I don't sense the keypad and update LCD at the same time.

That's the sort of thing I had in mind. For a keypad I would connect the columns to 4 of the LCD pins via diodes (cathode to pin, anode to keypad column), and the rows to 4 Arduino inputs with internal pullup enabled. If there are any pushbuttons or rotary encoders as well, you could use the 5th LCD output pin and some more diodes to multiplex them onto the same 4 input pins, if you need to save some more pins.

Provided you are not using interrupts to drive the LCD or to poll the keypad, and neither the keypad polling code nor the LCD driver code assumes that output pin states are preserved between calls, they should not interfere. You may have to modify the keypad polling code to set all 4 outputs high each time you start polling the keypad, because the LCD driver may have left them in a different state.

The one pin you can't share is the one driving the LCD EN input - but you probably knew that already.

[Reply #12 on:]

I haven't thought about using diodes. So are they there to isolate what happens on the LCD pins from the keypad or are they isolating what the keypad does from the LCD? I know that the LCD could write on the data pins but I don't think I need the LCD to do that. I have a display buffer on the arduino.

[Reply #13 on:]

I haven't thought about using diodes. So are they there to isolate what happens on the LCD pins from the keypad or are they isolating what the keypad does from the LCD? I know that the LCD could write on the data pins but I don't think I need the LCD to do that. I have a display buffer on the arduino.

If you think of the keypad as a matrix of switches, then if you press 2 keys in the same row at the same time, you will be shorting two columns together. Diodes in the column lines prevent this. The keypads I have dismantled use a graphite film on the bit you push to connect the contacts together, so it may be that the resistance when pressed is high enough that the diodes are unnecessary - you could check with a multimeter.

The keypad library page in the playground has this (very recent) comment:

1.8 2011-11-29 - Tom Putzeys : Non active columns are inputs with internal pull-up enabled

So the problem of columns getting shorted together is avoided in the latest version of the library by only setting one column pin to be an output at a time. OTOH the LCD library needs them all to be outputs at the same time. So, if you want to share the keypad column pins with the LCD, then I think you will need to use diodes on the column lines. You'll also need to either use an earlier version of the keypad library that keeps the column pins as outputs all the time, or else set all the column pins to outputs between polling the keypad and writing to the LCD, and set them all back to inputs with pullups enabled between writing to the LCD and polling the keypad.

So it's possible to share the pins between LCD and keypad, but not quite as easy as it appeared at first sight.

[Reply #14 on:]

This any good?

http://www.ebay.co.uk/itm/ART-New-LCD-2x16-F-LED-b-l-RGB-HD44780-H-30mm-ABC016002F00-MIRGB-R-/320795737775?pt=LH_DefaultDomain_212&hash=item4ab0ea7eaf#ht_754wt_952

Price is very cheap!

I bought one of those and it works OK. It came with two 150 ohm and one 100 ohm resistor for the backlight, but didn't say which colour the 100 ohm resistor was for. The data sheet doesn't specify the maximum backlight currents. After some experimenting I found that the best colour balance was obtained when I used it on the red led, however I can't be sure that the resulting current (28mA) is not over-driving it.

The brightness of the backlight is not brilliant (especially when red) but OK. Since the leds all take less than 40mA, the series resistors can be driven direct from 3 Arduino PWM pins, making it easy to select any backlight colour.