LCD Peculiarities

geezer88

geezer88

Senior Member
I have been playing with a four line LCD that I picked up in a surplus store. It came with no documentation, but an online search of the sticker information turned up this:

http://www.varitronix.com/Product/character/VL-FS-MDLS20464BD-09.pdf

Yep, that's the one. I wired it all up and nothing happened. That's when I wrote up this touch sensor article for debugging:
http://www.picaxeforum.co.uk/showthread.php?t=18295
Using that technique I verified that the code and the Picaxe were working as expected.

Some more online searching found a reference to needing a minus power supply connection to the contrast pin (V0) on some LCDs. Well, applying -3 volts with an auxiliary battery did the trick. All works fine.

Now that is quite inconvenient to need a bipolar power supply, especially for a battery operated application. So the point of this thread is to watch out for surplus devices, or you may find life more complicated than you were expecting. I would also like to know if this need for minus voltage on the contrast pin has been found by other experimenters, or if I was just "lucky"?

tom
 
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J

john2051

New Member
Hi Tom,

Have you tried 0v on the contrast pin? A lot of them will work without the negative voltage.
Regards john
 
C

cactusface

Senior Member
LCD problem...

Hi Geezer,
Get yourself a Maxim Max660 this 8 pin DIL IC allows you to input 1.5-5.5V and take out the inverse or negative equv voltage. Or it can be used to double the input voltage... A very useful little device for battery powered stuff.. And only needs two caps!!

Data sheet here: http://www.maxim-ic.com/datasheet/index.mvp/id/1140

I usually find a 5-10K trimmer between power and Gnd to pin 3 fixes the contrast for me?

Hope it helps (Someone).. :rolleyes:
Regards
Mel.
 
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geezer88

geezer88

Senior Member
John,
Yes, that was how I wired it originally. When that didn't work, I then wired in the recommended pot and swung the pin between power supply rails, and still didn't work. By the way, the picaxe and the lcd are run off a regulated 5v supply.

I also temporarily ran the LCD off a higher voltage of about 8 volts. That worked, but I was leery of damaging the outputs of the picaxe.

tom
 
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geezer88

geezer88

Senior Member
Mel,
Thanks for Maxim suggestion. That looks like a really useful device. I could glue it to the back of the LCD and wire it "dead bug".
tom
 
hippy

hippy

Technical Support
Staff member
Typical negative voltage generators use a 555 and a couple of diodes and capacitors as a charge pump. Drive the same from a PICAXE PWM and that should work ...

[ Example Link Deleted ]
 
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G

geoff07

Senior Member
any 7660 chip (not just Maxim) would do it. The chip plus two electrolytics is all you need. +5 in gives you -5 out, and will drive a few tens of mA, though the voltage will drop if you suck out too much. I'm using one at the moment to give a -ve rail for an opamp circuit.
 
I

inglewoodpete

Senior Member
hippy said:
This is a typical negative voltage generator using a 555. Drive the 22uF from a PICAXE PWM and that should work ...
Click to expand...
I'm with hippy on this one. Try to free up a PWM pin on the PICAXE and use the square wave to generate the negative voltage you require.
 
hippy

hippy

Technical Support
Staff member
russbow said:
Quite an impressive link, Hippy :D
Click to expand...
I had read your comment, wasn't sure how you meant, and simply let that pass. Unfortunately I hadn't realised that what you were seeing was entirely different to what I was seeing due to caching and the source having later redirected the image from what it was.

There's a moral there, but all fixed now.
 
geezer88

geezer88

Senior Member
P

pete20r2

Senior Member
That document is amazing, it's like all electronics wisdom combined into I comprehensive win book, they could be selling that.
 
hippy

hippy

Technical Support
Staff member
I was sure we'd discussed charge pumps previously and found these earlier posts which shows it works ...

http://www.picaxeforum.co.uk/showthread.php?t=10040
http://www.picaxeforum.co.uk/showthread.php?t=6976
http://www.picaxeforum.co.uk/showthread.php?t=6125

ASCII Art in those doesn't work so well since we changed forums and I think most deal with positive voltage doublers but the principle and results seem sound enough.

As all it needs is a couple of caps and diodes and a PICAXE it should be easy enough to experiment on breadboard if anyone's inclined.
 
geezer88

geezer88

Senior Member
hippy said:
I was sure we'd discussed charge pumps previously and found these earlier posts which shows it works ...

http://www.picaxeforum.co.uk/showthread.php?t=10040
http://www.picaxeforum.co.uk/showthread.php?t=6976
http://www.picaxeforum.co.uk/showthread.php?t=6125

ASCII Art in those doesn't work so well since we changed forums and I think most deal with positive voltage doublers but the principle and results seem sound enough.

As all it needs is a couple of caps and diodes and a PICAXE it should be easy enough to experiment on breadboard if anyone's inclined.
Click to expand...
Thanks for digging that up. I spent more time than I'd like to admit searching the forum, but did not use charge pump as a search term. After I found other references to it on google, I should have gone back to searching this forum.

Later today I hope to breadboard something quick to test out a charge pump, and I'll report back. My particular display only requires 1/2 Ma from the negative supply, so it should be pretty easy.

tom
 
geezer88

geezer88

Senior Member
Got a few minutes, so I wired up two electrolytic caps and two small signal diodes as shown in this diagram:

http://www.electronicecircuits.com/electronic-circuits/555-negative-voltage-power-supply-circuit/

Instead of the 555 producing the square wave, I used one of the PWM outputs on the picaxe and set it for 50% and 10Khz.

Worked great. I think I will go the 555 route though, because that can be glued to the display, making it self contained.

Thanks all for your suggestions and comments.

tom
 
hippy

hippy

Technical Support
Staff member
geezer88 said:
Worked great. I think I will go the 555 route though, because that can be glued to the display, making it self contained.
Click to expand...
Or go the PICAXE-08M route. Same size and fewer components, probably slightly higher cost, but you do get to tweak everything in software.
 
geezer88

geezer88

Senior Member
hippy said:
Or go the PICAXE-08M route. Same size and fewer components, probably slightly higher cost, but you do get to tweak everything in software.
Click to expand...
I actually already thought of that. The 08M may become the new "555" for a more flexible and controllable timer / oscillator / simple logic component. A 555 at RS is about $2 ( I know that's no bargain ) and the 08M isn't much more, but will do so much more.

tom
 
T

tiscando

Senior Member
http://www.picaxeforum.co.uk/showthread.php?t=6125
Click to expand...
ASCII art fix on Dreamweaver:
Code: 
pinx -----o-->|----o----o +9.5V @ 2mA
50:50PWM  |        |      +8.7V @ 4mA
          |       --- 10uF
          |       ---
          |        |
0V -------)--------o
          |        |
          |       --- 10uF
          |       ---
          |        |
          '--|<----o----o 0V
voltage doubler circuit: "Supply generation using PWMOUT"
Code: 
GND------o------------.
         |            |
        --- 22uF      |
        ---+          |
         |            |
+5V------o-----.      |
               |      |
            V --- 10uF
          + - ---+
          ||   |      |
pinx -----||---o-->|--o--- +9.5V @ 2mA
50:50PWM  ||               +8.7V @ 4mA
          10uF
Or:
Code: 
GND------o------------.
         |            |
        --- 22uF      |
        ---+          |
         |            |
+5V------o-----.      |
               |      |
               V     --- 10uF
            +  -     ---+
          ||   |      |
pinx -----||---o-->|--o--- +9.5V @ 2mA
50:50PWM  ||               +8.7V @ 4mA
          10uF
diodes: ex. 1N4002
 
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