3v3 LCD

[Ron Marcus]

Hi,

There are various threads out there about 3v3 LCD options.
I have a circuit where evrything except for the LCD runs at 3.3v,
so it would be really useful not to need 5v for the display.

Any thoughts?

Ben

Hey Ben,
I had a project with exactly the same issue. The bnattery supply limited me to 3.3, and everything ran fine but the LCD
I used the HPWM output to pop the voltage up to 6 volts or so,and used a tiny regulator to bring it down to 5 volts. It used two diodes and two caps. I believe it would run without the regulator, but as the PS sags, it may create contrast issues. If you have a free HPWM line, this is the EASY way to go. I've got schems and software. The LCD drew just a couple of mils. Just for giggles, I did it also on a PIC without HPWM using interrupts. Timing is a little trickier especially if there are comms going on.

Ron

[barkerben]

That sounds great - in fact, I have managed to get my original circuit to work:

2 AA batteries ---> Step up to 5v to run RS232 level converter and LCD----> step down to 3v3 to run GPS and PIC

I had some problems, because the MAX233 ties its input lines to 5v. I externally tied them to a more helpful voltage, and now eveything works fine.

Out of interest, how can you up the voltage using PWM - did it use a voltage doubler capacitor circuit?

Cheers,

Ben

[Ron Marcus]

That sounds great - in fact, I have managed to get my original circuit to work:

2 AA batteries ---> Step up to 5v to run RS232 level converter and LCD----> step down to 3v3 to run GPS and PIC

Out of interest, how can you up the voltage using PWM - did it use a voltage doubler capacitor circuit?

Cheers,

Ben

You are better off, from a battery standpoint, to raise the voltage as little as possible. I would jump it to 3.3 and use the HPWM for the 5 volt line if you only need a few mils.
It is a capacitor doubler, I attached a .1 uF from the HPWM output pin to the anode of a shottky diode. This diode is then attached to the circuit to be powered. A second shottky diode feeds 3.3 volts to the junction of the cap and the diode. A second cap is attached to the output of the first diode and to ground. When the HPWM pin is at 0 volts, the first cap charges to 3.3 volts. When the pin goes high. the bottom of the cap goes to 3.3, so the top goes to 6.6 volts(less the diode drops). This goes through the second diode, and charges the cap. Pretty neat huh? You can feed the 3.3 volt supply from another PIC pin, so you can shut down the display when not in use. I needed to do this because my app needed to sit for a year on a set of batteries.

Ron

[barkerben]

I might try that - at the moment, I have no time to reetch the board - but there is an enourmous list of things I'd like to change when I get a chance, and your suggestion has made the list a tiny bit longer :-)

Thanks for your help,

Ben