interfacing to a raw lcd display

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springer

New Member
sorting through my 'box of bits' i came across a raw lcd display, essentially it's a piece of glass with 40 legs.
After much googling i came across some info on lcd displays, but nothing on driver circuits for them.
After experimenting I assume I've found the common pins, (1&40) and if i hold a 3v supply in one hand and touch the pins with a finger i get some sort of output, but now i need my finger for other things!
So... do i just treat LCD's like LED's?
Most google results show displays requiring power of 3 > 7 V for this 3 1/2 digit display.
Do i need pull up / down resistors? and what value ( i'm assuming 15pA per segment)
are they common anode / cathode ? or does this not apply.
Any help would be appreciated.
 
hippy

hippy

Ex-Staff (retired)
I recall raw LCD's need an AC signal, possibly a phased one : +Ve, zero, -Ve, zero, +Ve etc.

You may get away with DC or simply pulsing but that could damage the crystals. If it were easy no one would be buying LCD's with driver chips on them :)
 
B

BeanieBots

Moderator
A raw LCD MUST be driven with absolutley no net DC current or it will be damaged. The damage is a fairly slow process as the 'crystal' material breaks down and the atom thick metallic coating errodes away.

If you don't mind doing a destructive test, try it on DC overnight.
It won't work the next day.

You can get general purpose LCD segment driver chips but they would still need further chips to make a display that a PICAXE could drive.
Much cheaper and easier to buy a ready made mode.
 
R

Ralpht

New Member
Code: 
Much cheaper and easier to buy a ready made mode.
Agreed, much easier with a driver.

But if you so feel inclined and don't mind the boredom of wiring up a lot of chips, you can get yourself as many X-OR gates as you have segments.

You then need a clock source, approx 30 - 60 Hz, which can be a 555 or an '08 programmed to produce square waves at the right freq.

The clock output goes to ALL the backplane (common) pins of the LCD. As well, it goes to one input of a 2 input X-or gate. The other input of the x-or goes to whatever drives the segment. The output of the x-or gate goes to the appropriate segment itself on the LCD.

Repeat for each segment.

This will ensure that the segments and the backplane are 180% out of phase with each other, preventing DC from appearing on the LCD.

It will also ensure you go cross-eyed wiring all that stuff up - :D

I do this on 2 digit 7-segment LCD's that I have, that are used to replace standard 7-segment LEDS.

You probabaly could do this in software and dedicate a large picaxe to do what the clock source and x-or's do.
Depends on whether you are more comfortable with software or hardware wiring.

Note: Resistors not needed.
 
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R

Ralpht

New Member
BB.

These LCD's are from Jaycar, part # QP5514. They are a dual 7-segment display. The datasheet doesn't specify an operating voltage so I use standard 5V nominal.

Not too bad at about AU $4.00 each.

I have a handfull that have been working for a year or so in a couple of projects and are still going strong.

Only disadvantage is there is no backlight, but the flipside is that they draw bugger all current and are perfect for battery operated circuits.
 
B

BeanieBots

Moderator
Interesting. The reason I asked is because I was also unable to find any voltage specs. I guess they don't expect anyone to drive them direct.
 
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Ralpht

New Member
No, I guess they don't expect anyone to actually try to drive them the hard way. The datasheet specifies what driver chips to use but I found them to be hard (ie: a pain in the butt ) to get so didn't bother.

I had the time to do the wiring by hand, so did it more as an experiment to see how well it would work in reality. Turned out to be very good and am happy with the results. Don't know if I am over-driving the LCD's but they show no ill effect after nearly 2 years of use.
 
N

nbw

Senior Member
Agreed with others: LCD and DC, not good bedfellows. Get yerself some AC, they need to be kept on their toes with pos and neg voltage swings.
 
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