I see 3.3v 1602 displays on sale: example
I also see I2c backpacks: example
The backpacks are always (or at least the ones I can find) rated at 5 volts. However, the main chip (usually pcf8574 or similar) is rated down to 2.5 volts.
So the question is can I use this (or similar) combination of backpack and LCD at 3.3 volts without reliability problems ? Any concrete experience ?
I can at last answer my own question here.
This combination has worked in my case and is nicer for using it with an ESP8266 (or other 3.3volt mcu) because there is no need for an I2C level shifter.
Interestingly, I have noticed than on the 3.3 volt 1602 LCD, some of the footprints, which are normally empty on a 5 volt device, are populated with an ICL 7660 chargepump and two 10uF capacitors.
And the three jumpers are changed to match.
I am somewhat puzzled. The ICL7660 generates a negative voltage. The HD44780 chip operates just fine on the 3.3 V logic levels, it is the LCD panel itself which requires the boosted 4.5 V so I am not sure how Vo - an external pin - is going to be supplied from that negative voltage.
Most ESP8266 or ESP32 have 5V available. It provides 3.3V to the CPU and all other electronics.
If your backpack is connected to 5V, the contrast will work fine.
Remove any onboard pullups to 5V. Use proper external pullups to 3.3V
Yes, you can buy 3.3V 16x2 modules. They are more expensive than the 5V modules.
Using 3.3V pullups is simple and straightforward.
Using external level-shifter chip might be easier for you if you do not know how to identify or remove SMD pullups from the backpack.
David.
david_prentice:
. . .Using external level-shifter chip might be easier for you if you do not know how to identify or remove SMD pullups from the backpack.
David.
Judging from his username and avatar I suspect he may be more comfortable dealing with 2 watt carbon resistors.
Don
The 6V6GT was a lot neater package than the 6V6G.
OK. For me the problem is solved and the price difference between the models sold as 3.3 volt compatible and those as 5volt is negligible. But the idea of using 3.3volt logic on a 5 volt device is also an interesting solution. For those who choose this route, the schematic which also identifies the I2C pull-up resistors to be removed is here:
As for the jibe about the 2 watt resistors, I remember the days of electronics when men were men and if you needed 250 VDC you simply rectified mains voltage. If your mains connection was a 2 pin reversible plug the chassis would be live 50% of the time. You simply learned to live with a bit of risk. No one in those days would have been too concerned about the difference between 3.3 volts and 5 volts.
And my automatic interjection - while you are removing the pull-up resistors or even if not, see if it is practical to remove the connection between the potentiometer and Vcc without disturbing other connections.
