Pages: 1 [2]   Go Down
Author Topic: Nice way to power an LCD backlight  (Read 1499 times)
0 Members and 1 Guest are viewing this topic.
Offline Offline
Edison Member
*
Karma: 116
Posts: 2205
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
Why try to discredit a circuit design by asking "what happens if you don't provide sufficient input voltage"?

Trust me when I say that no one here is important enough for me to discredit, you included. I was simply pointing our your design's limitations so that more people can use it successfully.

Quote
The mistake I made

is to think that you can design a circuit without limitations.

The most successful designs are those that made the best compromises for their respective applications.
Logged

Worst state in America
Offline Offline
God Member
*****
Karma: 23
Posts: 665
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

This post is for smusante in reply to his PM to me. I posted here rather than answering the PM so that others may make use of the info.

Quote from: smusante
I saw your post about the way you hooked up the Optrex DMC20261 display. I have one of these and cannot make it work like all of my other displays. Could you post on how you hooked it up, what library you use, and what voltage level it needs? I can only get a backlit screen. No characters show up no matter where I turn the 1K pot. I am using the standard LiquidCrystal arduino library. I don't have this problem with my 16x2 or my 20x4 displays. I'm stumped.

Thanks,

Steve

Steve, the newer Optrex displays require a NEGATIVE VEE supply. In the past, they worked properly with the VEE pin tied to ground. Now they require about -3.5 volts. Here is a circuit that I used to generate the negative VEE supply using an Arduino PWM output (it MUST be a PWM pin).

Use this circuit, then set the PWM pin to analogWrite(pin, 127) to generate a 50% duty cycle square wave to drive the inverter. If the LCD display ends up too dark (i.e. too much contrast), you can decrease the negative voltage by using a smaller value on the PWM pin (i.e. analogWrite(pin, 64)). Use any value between 0 and 127 to get the contrast you want. Going above 127 is pointless since 128 to 255 is equivalent to 127...0. smiley

(edit to add): Capacitor values are not critical. 1uF to 100uf will work equally well. The diodes are not critical either. They can be 1N4148, 1N914, even 1N4001. The resistor isn't critical either (maybe not even necessary). I used it to limit the current surge on the AVR pin when the circuit is first powered up and the caps are discharged. You can probably get away without it.


* v-minus_generator.jpg (57.51 KB, 1040x496 - viewed 19 times.)
« Last Edit: December 12, 2012, 12:08:47 pm by Krupski » Logged

Worst state in America
Offline Offline
God Member
*****
Karma: 23
Posts: 665
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
Why try to discredit a circuit design by asking "what happens if you don't provide sufficient input voltage"?

Trust me when I say that no one here is important enough for me to discredit, you included. I was simply pointing our your design's limitations so that more people can use it successfully.

Quote
The mistake I made

is to think that you can design a circuit without limitations.

The most successful designs are those that made the best compromises for their respective applications.


(1) I said you were trying to discredit the CIRCUIT DESIGN, not the DESIGNER. I do not claim to be anything special.

(2) I never said the circuit was without limitations. Of course it won't work on 7 volts. It won't work on 120 volts either. So yes I guess it has limitations.

(by the way, in case you don't know, posting text in purple is the way [on another board I frequent] of marking the text as sarcasm).  smiley
Logged

Worst state in America
Offline Offline
God Member
*****
Karma: 23
Posts: 665
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset


Your drawing show you grounding the center pin of the 7805. I don't think that is the proper way to wire up a 7805 as a constant current source.

http://talkingelectronics.com/projects/30%20LED%20Projects/images/ConstantCurrent-2.gif

Lefty

Current in a series circuit is equal at any point. Take the drawing you posted the link for and move the LED from the ground side to the plus side of the series circuit and tell me what the difference is.
Logged

Offline Offline
Newbie
*
Karma: 0
Posts: 1
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Krupski,

Thanks for that info. I was able to quickly verify that the display is not dead by using a 9V battery in reverse through a 10K pot. Now I cannot get any activity so I am still not out of the woods. I am using the LuquidCrystal library. Is there a special Optrex library with different parameters?

Thanks for your help.
Logged

Worst state in America
Offline Offline
God Member
*****
Karma: 23
Posts: 665
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Krupski,

Thanks for that info. I was able to quickly verify that the display is not dead by using a 9V battery in reverse through a 10K pot. Now I cannot get any activity so I am still not out of the woods. I am using the LuquidCrystal library. Is there a special Optrex library with different parameters?

Thanks for your help.

The DMC-20261 works fine with the stock "LiquidCrystal" library.

BTW, good idea testing the negative VEE with a pot and 9V battery... clever.

Hopefully you only have something mis-wired and not a blown display module.

Good luck!

-- Roger
Logged

Pages: 1 [2]   Go Up
Jump to: