Funky 20x4 LCD with interesting circuitry

Other Hardware Displays
1

I just got a batch of 20x4 HD44780 LCDs in and I noticed that this has very different circuitry compared to my older 20x4 LCDs.
This one has a V1.3 marking on it, most of mine have V1.1.

Most interestingly, it looks like the design might’ve considered integrating an I2C GPIO expander on the board itself ( no backpack).

U7 ties to the SCL and SDA lines, but I’ve no idea what chip it was spec’d to use.

The U6 circuitry (I’m assuming this is for V0) is also different from my 1.1 boards. This has VR1, maybe for onboard contrast control?

Does anyone know where one could find a schematic for these? They’re a fairly generic part so I feel like something should be out there.

IMG_7691
IMG_76913024×4032 1.64 MB

2

Why not look at one of the most common I2C LCD backpacks, and see if it makes sense?

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I see SDA/SCL pins in th R edge, wire it up & see what happens.

4

U7 is surely a PCF8574.

6

Hi,
I would say it is the same old parallel model, but upgraded with I2C option.

Try it, although I would think it has not got I2C components fitted for this model you have bought.

You may have to run the I2C scanner first to see if it has an I2C address.

Can you please post a link to where you purchased the displays?

Thanks.. Tom.. :smiley: :+1: :coffee: :australia:

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Found it on aliexpress. The ad shows the old version in the pictures. YMMV.

https://a.aliexpress.com/_mPAVHdK

10

Alright, so I got some time to hit it with the meter.

Based off of the datasheet for the PCF8574, it seems like the IC would at least fit in the same spot, its GPIOs would go to the LCD's IO. PWR and GND are in the correct spots.

Going off the schematic of the I2C backpack from Amazon seller "HiLetgo", the pinout from U7 to the LCD is identical.

Pin 7 of the PCF8574 goes to the bottom pad of Q1 and R10. I'm assuming that its the gate, or base control pin for Q1. Q1 probably controls the backlight. It very much seems like they just copied the circuitry from the I2C backpack onto the back of the LCD.

V0 contrast, goes to the top pin of VR1 and R6. Interestingly VR1 isn't just a potentiometer with references to ground and VCC. The left pin of VR1 goes to pin 5 of U6 and the other side of R6.
I can't quite think of what U6 is. 555 timer? Pins 2 and 3 of U6 go to C4.

11

I don't see any ICS on the board that would hint that they've populated it yet. Wiring it up for I2C would probably just give the board power and thats it. I'll look into populating the board by getting an I2C backpack to donate its IC.

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12

Wouldn't it be easier to just buy the backpack and solder it to the 16 pin header instead of messing with the surface mount chip?

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Just TRY it. What do you have to lose?

14

That Ali Express pages says:

LCD2004 2004 20x4 2004A Blue/Yellow Green/White Screen SPLC780D Character LCD IIC I2C Serial Interface Adapter Module AIP31066

The SPLC780D and AIP31066 are hd44780 compatible chips that only support parallel mode. (controlling using DB0-DB7, RW, E, RS) i.e. no I2C

There are some LCDs that support I2C but they have chips like
PCF2116, PCF2119, or AIP31068

It should be easy to verify ALL the pin connections on the U7 pad(s) to see if they map the same way they would on a PCF8574 based backpack.

PCF8574 SCL/SDA pins (pins 14 and 15) should connect to the labeled pins on the side of the LCD PCB.

If P7 on the PCF8574 goes to transistor, then the pin mapping is one of the less common ones.
Likely this one:

PCF8574   LCD
---------------
P0      LCD DB4
P1      LCD DB5
P2      LCD DB6
P3      LCD DB7
P4      LCD RS
P5      LCD RW
P6      LCD E
P7      Transistor (active low)

This is the most common pin mapping used by backpacks, and most libraries hard code to this mapping:

PCF8574   LCD
---------------
P0      LCD RS
P1      LCD RW
P2      LCD E
P3      Transistor (active high)
P4      LCD DB4
P5      LCD DB5
P6      LCD DB6
P7      LCD DB7

The hd44780 library hd44780_I2Cexp i/o class can auto detect 6 of the most common pin mappings, and also supports a manual configuration.
So if that area is for a PCF8574 to add a on board "backpack" and you want to use it,
then you should be able to use the hd44780 library to talk to the PCF8574 to control the LCD.

--- bill

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