empty pcb pads in 1602 display modules, why? - Page 1
Forum section: electronics / beginners

author: minhaj.sixbyte
author_rank: Newbie
timestamp: January 22, 2023, 08:46:51 pm
content: Hello, this is my first post here! (Signed up couple of minutes ago)I've used these 1602 LCD modules in many of my projects. Always wondered what these empty pcb pads are doing there. Sorry I couldn't find proper datasheet for these cheap chinese modules that I collect from local shop. When I search for these, I always get the Hitachi controller chip datasheet (which I am familiar with). Maybe I can't figure out the correct string to search. Looks like there are some solder jumpers and a place for some kind of chip, as well as pads for two capacitors.If anyone knows their functionality, please let me know. The module works fine, I am just curious.Thanks!!

author: mariush
author_rank: Super Contributor
timestamp: January 22, 2023, 09:06:26 pm
content: It's probably room for a voltage inverter / doubler like ICL7660, to get negative voltage.See for example circuit on page 7 :https://www.renesas.com/eu/en/document/dst/icl7660-datasheetpins 2 and 4 go to C1 footprint as in schematic, pin 3 goes to ground through that J1 jumper link with solder, pin 5 goes to output and that C2 and out would be negative voltage.The other options would be a current driver for bigger led backlights (if you use multiple leds in series you'd have forward voltage higher than 5v) or maybe option to put an i2c / serial to parallel converter chip to use i2c/spi instead of at least 6 wires.

author: Gyro
author_rank: Super Contributor
timestamp: January 22, 2023, 09:58:46 pm
content: It's to allow the display to work on a 3.3V power supply rather than a 5V one (yes the IC is a 7660)...https://www.eevblog.com/forum/projects/character-lcd-at-3-3v-instead-of-5v/https://www.codrey.com/electronic-circuits/hack-your-16x2-lcd/

author: jpanhalt
author_rank: Super Contributor
timestamp: January 23, 2023, 12:21:05 am
content: Judging from the fact that those displays have a backpack for serial communication, I agree with the others that they are/were possibly for generating Vee. Of course, any MCU such as used in the backpacks can do that easily. Slightly related, The SparkFun 4x20 character display had pads for the MCU on the display PCB, but there are not enough traces to those pads to make me think they are for an MCU to allow serial communication.

author: floobydust
author_rank: Super Contributor
timestamp: January 23, 2023, 03:28:21 am
content: LCD character displays with low temperature fluid require higher voltage drive, which is usually a -5V rail as well as the usual +5V rail.We use them in Canada to -30°C (-22°F) and if colder a heater is used.The extra pads are for an ICL7660 voltage inverter option.

author: minhaj.sixbyte
author_rank: Newbie
timestamp: January 23, 2023, 04:20:08 am
content: floobydust, jpanhalt, gyro, mariush, everyone thanks for your replies. specially links provided by gyro and mariush really helped!

author: tooki
author_rank: Super Contributor
timestamp: January 23, 2023, 07:52:50 am
content: Quote from: jpanhalt on January 23, 2023, 12:21:05 amThat’s an I2C backpack, it doesn’t need negative voltages.The negative voltage is simply as negative bias for the LCD. As others have said, extended-temperature models, as well as large-digit models, need a negative bias voltage.