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SmashTV LCD brightness


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Tim,

  Is it possible to change the value of the "current limiting" resistor to get some more brightness out of 2X20's that you sell? If so, which one is it?

Raph,

It is possible to gain a bit on the brightness, but know that too much current can do more than just toast the LEDs, it can heat up the glass enough to cause loss of contrast and permanent distortion. 

Since I have never tinkered in this direction with these I don't know how much is too much, so some careful experimentation is required on your part.  :)

That said.....

R7 on the display is used for current limiting, by jumpering over it or replacing it with a jumper you will get a little bit more brightness out of it.  To easily see how much, crank the brightness pot on the core and temporarily bridge the resistor using a small pair of needle nose pliers for a quick non-permanent jumper.  If this gets you where you want to be on brightness stop here.  :)

If you need more brightness (heeding my warnings above about current and heat), you can change the value of R4 on the CORE.  R4 limits the current to a range most useful with common backlights.

How much is too much?  I have no idea with these....But in past experimentation with LED backlights on other LCD modules I know that with most it's possible to overheat the glass to the point of distorting the image way before the LEDs die from overcurrent.  Fire it up, let it run for an hour watching for change in image quality, color distortions around the ends of the glass, etc.  The modules I have abused this way in the past were warm or hot to the touch under this kind of abuse.

These displays have 6 LEDs each, with three along each edge lighting a diffuser.  The LEDs are through-hole, so you can see the leads/connections on the backside of the module.

Let's make this reply a two-fer :

I'm having the same issue. Checked all the values, made sure my cable was wired correct, and i'm not getting a backlight.

THX,

JD

This is not quite the same issue JD, it sounds like your backlight is either not getting any voltage or there is a defect on the LCD module itself.  Some easy testing to determine what's up is to put your meter leads on pins 15 and 16 at the module, set for DC voltage.  Pin 15 is negative and pin 16 is positive, if the polarity is reversed the LEDs won't light.  If you see +4-5vdc here the LCD module has issues, check/reflow the soldering on R7 (one side first, let it cool, then the other side else you will remove it).  If still no backlight send it back to me and I'll replace it.

If you don't have 4-5vdc here, there is a wiring or connection fault, check everything associated with the brightness circuit on the CORE (solder connections ok?  Transistor in the right direction?  correct resistors in the right places?)

Let me know what you find....

Best

Smash

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thanks smash.

Mine is really low (you can barely see that it is on with normal room lighting) so dont think i'm trying to push it too bright.

Here is the thing though, i am using R3 boards. Is the trimpot functionality the same? For some reason, I thought that one pot was for normal LCD contrast and the other one was for the backwards ones.  I tried 2 types of LCDs on two of my cores and neither pots affect the brightness

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Update.

It does look like my brightness control is working but it is a TINY adjustment (barely visible, thats why i thought it didnt work).  Shorting R7 does improve it to acceptable levels though.  Will do some stress testing today to see how hot the whole deal becomes

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if you plan on shorting... i highly suggest using a resistor between 1~10 ohms for a *little* protection

thx,

jd

Keep in mind the CORE brighness circuit does some current limiting, so shorting R7 on the display gets you to the same situation as with typical LCDs without a limiting resistor on board....most displays without a limiting resistor just have the LEDs wired to pins 15 & 16.

I guess what I'm trying to say is that I -think- these displays will run fine with R7 shorted as long as the backlight power is coming from the usual CORE setup......If the current is too high a 10ohm won't protect it anyway.

Back to the kit mill....  ;)

Best

Smash

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Hey Smash, while we're on the topic of your LCD's, I was wondering if you were planning to get the Amber and Red backlight versions in. I really want a red one for my project, but i'd like to get it from you as I know it will work.

Failing that, I was thinking of changing the LED's for different colours. If it works anything like a Cell Phone backlight, that should be reasonably simple right? I see you mentioned the LED's are through hole, does that mean they're full size and not SMT LED's??

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