Peter's electronic projects

Getting character LCDs to work at 3 volts

Generic character LCD modules contain an industry standard HD44780 compatible controller*, which can operate down to 3 volts. But the modules are usually only specified to work at 5 volts, unless you choose one designed to operate on 3V.
lcd at 5 volts - OK
There are two reasons why the modules won't work at 3 volts. First, the required voltage to drive LCD segments is between 3.8V and 4.4V. This driving voltage is measured between VDD (pin#2 on pictures) and VLCD (pin#3 on pictures). Creating this is not a problem when running on 5V - the VLCD voltage can be adjusted between 0.6V and 1.2V which translates to 3.8V to 4.4V for the LCD (between VDD and VLCD).
lcd at 3 volts - OK
However at 2.7V, 3V or 3.3V the LCD still needs a minimum of 3.8 to 4.4 Volts to show visible segments - this means a negative voltage is required. To get a suitable LCD driving voltage (min. 3.8V) between VDD and VLCD, the lower side of the adjustment potmeter needs to move to a generated negative voltage (-3V).

Second reason is that the RC oscillator of the controller runs slower at 3 volts, thus the Rosc resistor on the LCD module may need to be adjusted in case the program driving the LCD goes too fast for the LCD controller.
lcd at 3 volts - OK
To solve the first problem, the circuit needs a negative voltage to set the required voltage on the VLCD (pin#3) pin. This is easily accomplished with a small charge pump which converts the supply voltage of 2.7-3.3 volts to -2.7 to -3.3 volts.

PCB of the LCD bias circuit
This small, 13 x 9 mm (0.5 x 0.35 inch) charge pump PCB is designed by me, and it is easy to integrate to an existing 3V circuit.
You can download the Eagle 6 schematics and board layout from github or buy assembled and tested modules from My Tindie store for $3.

charge pump circuit on the PCB
You can add the charge pump to the circuit board to the LCD connector pins #1, #2, #3, and leave the original contrast trimmer footprint (TM1 on picture) empty.

charge pump circuit on the LCD module
If you don't want to modify the circuit board, you can convert the 5V LCD to a 3V LCD - just add the charge pump to the LCD module connector pins #1, #2, #3 and make sure that pin#3 does not connect back to the circuit board.

charge pump circuit on the LCD module

In either case, make sure that the header is soldered to the LCD or the circuit board with a small amount of solder in the holes before adding the bias circuit!

Finding and adjusting the LCD oscillator

For the solution of the LCD clock slowdown, see the next page >.

* generic HD44780 compatible controllers: Samsung KS0066, KS0076, KS0070, Sunplus SPLC780D, Epson SED1278, Sitronix ST7066U and others