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Newbie needs help with LCD backlight dimming

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Mongoose

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Hello all,
First, let me say this is a great forum. I've been reading and searching for a couple of weeks trying to find out how to do what I want to do, and the wealth of information on here is staggering.

What I need is a little direction. I don't expect you to give me a fish, just show me where to start fishing...

What I have is a 2x24 Character LCD that I am installing in my car. What I would like to do is use my existing car dimmer lines to dim the backlight to match the other displays in my car (AC/Clock/Radio/etc...).

The backlight is being fed 3V (~50 mA) which is full bright.

I have two wires that dim the other displays in my car. When I turn the headlights on, one wire goes from 0 to 12V. This cuts the other display illuminations in half. (i.e. dims them to approximately 1/2 of "day" illumination) I would like to use this input to bring the input voltage to the LCD backlight down to ~ 1.5V.

The other line stays at 0V, until I start further dimming down the other displays (with the car dimmer control), and then it runs up to 9V at full dim. I would like to use this input to bring the input voltage to the LCD backlight down to 0V (or close to it).

I looked at using a voltage divider on each input line, and then a couple of ICL7660s to invert the voltage. But I can't seem to get them all to work correctly together to bring the 3V LCD input down to close to 0V. I'm (attempting to) simulate it all in CircuitMaker 6.2c.

Can anyone get me pointed in the right direction? I'd rather not get into timers and PICs if that can be helped. But if that's what it takes, then I guess I'd better start reading.

TIA,
-Jason

Just to clarify, the data input/control to the LCD is already taken care of (not by me obviously), I just need help with the backlight control.
 
The best ways of doing it are PWM and constant current.

LEDs shouldn't be driven from a constant voltage source so a variable voltage supply isn't really the right way to do it.
 
Hero999 hit the nail on the head, as he almost always does :D

Although, I have seen 'consumer' products dimming LED's (for maybe 2 dimming levels) using basic analogue, varying the current....its not really a good way of doing it. I will say this though, providing you have a current limiting resistor in series with the LED backlight, using two different voltage levels WILL change the brightness, but you will have to experiment with what levels work, its not as simple as 'halfing the voltage to half the brightness'!

For just having 3 levels...fully ON, fully OFF, and 'somewhere in between' it would probably be over kill to use a PIC, or fully controllable PWM. I think a good half way point would be a simple 555 timer, or possibly a two level current source.

A 555 timer can be used for PWM, with a standard astable circuit, connect your '0-12v dimming' control line to pin 5 of the 555 via a voltage divider (if the 555 is powered by 5v, don't give it 12v on any pin!).....or even simpler, connect the 'dimming' control line to the 'reset' pin. When the 12v is high, the timer will run at its configured frequency, and duty cycle (ergo: with the correct part values, it'll be 'dim') and when the 12v source is 0V, the 555 will stop running, and leave its out 'high', (ergo: full brightness). If the control line works the other way, then a simple transistor inverter would do.

I'm just spitting out idea's here man, its what I do :D If you need anything more, I have some free time at the weekend for a rough schem.

Yet another idea: Although even new cars still insist on using crappy 'bulbs' in the dash for lighting, if there are any LEDs in the dash, you could check those to see if they are dimmed by current, or PWM. If its PWM, then you could just 'tap' that line, use that to turn on/off a transistor with the power to your backlight. (don't do it direct, as you need 3v, and the LED's circuit in the dash may be muich higher).

Well thats food for thought at least....at most I'd say a NE555 timer chip, with a few resistors/caps and possibly a transistor or two........at the very least maybe a transistor and a few resistors....to be honest I'm half asleep so working out specifics ain't on the cards right now :eek:

Cheers,

Blueteeth
 
Thanks guys.

Much thanks guys, I think I understand where to start/go from here. I'll give it a shot and report back if I've got more questions.

-Jason
 
Blueteeth said:
varying the current....its not really a good way of doing it.
I disagree, there's nothing wrong with varying the current, it's varying the voltage that's more likely to cause you problems.

The only thing is that varying the current is normally harder than simply using PWM which is also more predictable and is why it isn't normally done.
 
Oops, 'varying the voltage' thats what I meant :eek: Thats what I get for posting at 2am :D Sorry to cause any confusion. I agree with the PWM thing though, probably the best way to dim LED's

-Blueteeth
 
Connect the (+) line of the back light to the (0, 12)V line through a diode like 1N4004 and the (-) line through a resistor to the 0...9V line. Be sure not to mix the back light (-) with the signal ground.

If your back light needs say 3V at 50 mA for full brightness, then the resistor should be (11V/0.050A) ohms (your 12V is actually closer to 14V). This example would be 220 ohms, 1 watt. If you only have 1/4 watt resistors it would be (4) 1K ohm resistors in parallel.

If you want the back light also to be ON when the headlights are OFF, use the ACCessory line instead of the headlight power.
 
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