3 volt PIC and 5 volt LCD

temtronic · Posted: Wed Apr 13, 2016 5:16 pm

As most of you are aware there are a LOT of 5 volt LCD modules at great prices. The problem is trying to run them with a 3 volt PIC. Seems every other add-on peripheral these days IS 3 volts (GPS, RFID, etc.)
So if you can spare a PWM output, even at 4KHz I've come up with a solution. Right now I've got an 18F46K22 running on 3V attached to a 4x20 LCD. I'm using the 'flex_LCD_420' driver. The only hardware mod is to add 2 diodes and 2 caps to form a 'Dickson doubler'. The PIC supplies a free running, 50% duty cycle square wave to the doubler which produces a good 5 volts to power the LCD.
I did this to test a new 4x20 LCD driver, that displays 4 characters, each 4 pixels high. Once I clean up the code, I'll post it in the code library.

Jay

ezflyr · Joined: 25 Oct 2010 Posts: 1019 Location: Tewksbury, MA

Hi Jay,

Hmmm, that seems to address the supply to the +5V display, but what about the I/O? Are +5V displays happy with +3.3V I/O signals?

I've got a lot of mixed voltage designs. +12V in is regulated down to +5V and +3.3v with individual regulators. Seems easier and cleaner to me??

It seems rare that you're starting with only +3.3V anyway, almost always something higher, right?
_________________
John

If it's worth doing, it's worth doing in real hardware!

temtronic · Posted: Thu Apr 14, 2016 9:45 am

The PIC's being powered through TTL<>USB which has 3V output. The LCD seems quite happy with 3V on the data+cmd pins. Almost a week 24/7. ZERO missed data!
This all comes about with trying to get some nRF24L01+ to talk to each other for the wife's greenhouse.
I've yet to find economical, SIMPLE RF 2way transceivers (serial<>RF). Zigbee is too powerful (overkill), Bluetooth can't go the distance. RF Digital have them but $$$. sigh....

Jay

gpsmikey · Joined: 16 Nov 2010 Posts: 588 Location: Kirkland, WA

I liked the level shifter that Sparkfun has for sale ( https://www.sparkfun.com/products/12009 ) and was planning on using that system with my next little project. (you still need Jay's voltage step up for the supply though).

mikey
_________________
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

asmboy · Joined: 20 Nov 2007 Posts: 2128 Location: albany ny

This is a nifty idea --
i tried it - this morning with a devel board on hand
already set up with an LED illuminated LCD.

as somebody who has a LOT of analog circuitry in my pic work -there is an unsavory aspect of this means of voltage generation.
i was drawing 55ma with a 5.1 ohm series I limiter -series with the led at doubler output -

If you build this circuit - add a small sense resistor
Look at the input current waveform to the doubler to see what i mean

i used .47 ohms I-sense resistor in series with Vdd 3.3v and
a 330uf/10v ( $$ ) tantalum cap , which HELPS but ..........

Gabriel · Joined: 03 Aug 2009 Posts: 1067 Location: Panama

For low cost, reliable data, "good" range, go for the ESP8266!... and guess what! they are 3.3V... who says you cant transmit small bits of data via WIFI.

as for the Power generation, i posted a "driver" for an LM35 a looong time ago using a charge pump (voltage doubler, or other fancy names) ... same solution as you.

Just dont read data from the LCD!

Its incredibly simple.... its good to know it can drive a whole LCD!

G.
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CCS PCM 5.078 & CCS PCH 5.093

temtronic · Posted: Fri Apr 15, 2016 2:27 pm

Just a 'point of clarification'...
The 3>5 doubler feeds the LCD 'VCC' only

The regular 3 volts powers the backlighting LEDS.

I didn't even look with a scope, just grabbed what was lying on the bench and it worked! I suppose I should properly engineer it, heck i didn't even put a resistor in the 'I'm alive ' flashing green test LED.
To do it 'right' I'll have to get the magnifying glass out, can't read nothing on little parts any more, sigh.......

Jay