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

Ex-Staff (retired)
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

Ex-Staff (retired)
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

Ex-Staff (retired)
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

Ex-Staff (retired)
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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