What is the lowest voltage the LCD needs to operate?

Put a diode in series with the LCD 0V to raise it about 0.6V above
0V. Connect LCD Vlc to real 0V which will give about -0.6V for
contrast. However the digital signals to the LCD will be at -0.6V to
the LCD instead of 0V when low. I don't know if this will work, just
a thought.

Best regards, Ruben

> Hello list,
>
> I have several battery run applications using PICs and standard ASCII
> LCD modules. The devices run on 4.8V nominal (NiCd) and the LCD
> contrast is not good enough. I am going to try to make a small charge
> pump to make a Vlc of -0.8 to -5V to fix this. I have made charge
> punps before, and I would not like to use a chip for this, therefore
> please do not suggest one (I don't want a 7660 in it). The following
> ideas come to my mind:
>
> 1. Voltage doubler charge pump driven from pic pin using 0.01 uF + 220
> R to drive a SOT32 double diode as rectifier with 0.1 uF filter cap to
> contrast potentiometer. This should make -3.2V or so. I cannot afford
> an interrupt so the pic will output pulses on that pin whenever not
> busy doing something else (bursts of 10's of fast pulses several tens
> of times per second).
>
> 2. A simple booster converter circuit with one PNP bipolar transistor
> and a transformer, controlled by a PIC pin (on/off). This can make as
> many volts as I want but it contains a coil that needs to be built.
> The output needs to be regulated (or clamped) somehow (or not ?).
>
> 3. A simple booster using a PNP bipolar and a standard inductor,
> driven by the PIC with short pulses from time to time (as in 1).
>
> 4. Give in and use a proper micropower switching supply for the whole
> system(s).
>
> I like #1 and this is what I am going to try out soon. What would you
> pic(k) ? #1 adds only four standard parts to my design. All the other
> options are expensive.
>
> tia,
>
> Peter
>
> PS: The Vlc potentiometer is 10K and the LCD is standard (Vlc 0..5V).
> I suspect that -0.6V of Vlc is all I am going to get with these LCDs
> but I have to try. Those 0.6V are the 0.6V that are missing to make a
> proper contrast display imho. The other option is to add another cell
> to the battery and use some sort of low loss voltage dropping scheme
> (not a regulator) when the battery is full.
>
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==============================
Ruben Jvnsson
AB Liros Elektronik
Box 9124, 200 39 Malmv, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
==============================

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