As part of a larger project, I wanted to have an LCD display and a keypad for navigation of some simple menus. I will be using a lot of the I/O ports on the Arduino for other jobs, so I wanted an I2C interface for the LCD. So I bought some hardware, consisting of a 1602 LCD Keypad shield module from DFRobot and an anonymous I2C serial module for LCDs. I want to use these with an Arduino Nano.
Then I found there are some challenges to make these components work together - but it is possible. So I want to share my experience and maybe help some others.
This photo is of the working breadboard, which can display messages on the LCD and identify key presses. The LCD is controlled through the I2C interface, including the LCD backlight. The key presses are received by the Arduino on pin A0 (it can be any of the analogue pins, except for A4 and A5 which are tied up for the I2C interface).
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Step 1: Parts Description - LCD Keypad Shield Module
The LCD Keypad shield module consists of a commonly used 1602 LCD mounted on top of a circuit board that holds the keys, and which takes a subset of the LCD connections and makes them available to the header pins on the underside of the circuit board. I understand this board is intended to be mounted on top of an Arduino Uno or similar and provides the right pin layout for it to work in that environment. I bought this module on Ebay from a vendor in China. The header pins on the lower (keypad) side of the board are mostly labelled but the pins on the upper side, which is the LCD interface, are not labelled. However the pins on the LCD itself are labelled.
Step 2: Parts Description - I2C Serial Module
The serial module has the usual 4-pin header for I2C, and a set of unlabelled header pins which I understand are intended to plug directly into the underside of the LCD module. By studying the labels on the LCD, I was able to identify the functions of the pins on the serial module.
This module is based on the IC PCF8574T which terminates the I2C protocol, has 3 pins for address control (20 to 27) and has 8 digital input/output pins P0 to P7. According to the data sheet of the PCF8574T, each I/O pin has a FET to pull it to ground for the LOW state, and can sink at least 20ma. In the High state, it has a transient active pull-up and then a continuing pull-up current of about 0.1 mA.
On this module, all of the digital I/O pins, except for P3, are simply brought out to header pins (on the right in the photo). In the case of P3, it is connected to the base of a transistor (visible in the photo at the top right area just under the label "LED"). The emitter of that transistor is connected to Vss (ground) and the collector is connected to header pin 16, where it can be used to control the LCD backlight. Because of the transistor, the logic state is reversed relative to that assumed in the software library. That is, the LCD backlight is turned ON when the P3 pin is Low, and OFF when the P3 pin is High.
The label on the transistor says L6 which according to my research probably makes it a MMBC1623L6 which has a minimum current gain of 200. With 0.1 mA of base current, it should be able to maintain a LOW condition at its collector (module Pin 16) with at least 20mA collector current.
In addition this module has a 10K potentiometer connected between +5 and Ground, whose variable lead is brought out to pin 3 (third from the bottom in the photo). When connected directly to the LCD, this pot will control the contrast of the LCD. However, that function is provided by a separate similar pot on the LCD shield, so this pot on the serial module has no function.
I was unable to find any connection to the INT pin of the PCF8574T.
UPDATE 22 August 2019
As mentioned above, the PCF8574 has 3 address control pins. These appear to be brought out to pads on the backpack board where they are labelled A0, A1 and A2. They can be seen in the photo. I have not tested this, but it seems almost certain that by bridging one or more of these pins to the adjacent pads, the I2C address can be controlled over the range from 20 to 27. Furthermore, there is a second almost identical device, the PCF8574A which has identical functionality to the PCF8574 but covers the address range from 0x38 to 0x3F.
The address your device actually uses can be checked with an I2CScanner. There are several simple I2C scanners available from various sources. This one at https://github.com/farmerkeith/I2CScanner also identifies some of the devices found.
Step 3: Connections
Thanks to ChaitanyaM17 who provided the Fritzing diagram which depicts the connections, described below.
The LCD module has a pin on the lower side labelled "5.0V". To the right, adjacent to that are two unlabelled pins which are both ground.
Holding the serial module with the I2C interface at the left hand end, there are 16 pins at the lower edge. The first of these is ground, and the second of these is +5v. Another option is to use the lower two pins on the I2C interface for power, but I found it more convenient to use the pins as described above.
I2C interface. On the serial module, the top pin is SCL (clock) and it goes to the Arduino A5. The second pin down is SDA (data) and it goes to the Arduino A4.
LCD print interface. There are 6 connections between the serial module and the LCD Keypad shield, all of them between pins with no labels. I will identify them on the LCD module by counting from Right to Left, with the first pin as 1. There are 2 blocks of 8, so they go from 1 to 16. I identify them on the I2C serial module by counting from Left to Right, there are also 16 of these. In addition I give each wire a label, which is the equivalent pin on the Arduino that is normally associated with that function, in the case of a direct connection without the serial module.
So the 6 data connections are:
Arduino equivalent // Serial module pin // LCD Keypad module pin
D4 // 11 // 5
D5 // 12 // 6
D6 // 13 // 7
D7 // 14 // 8
D8 // 4 // 9
D9 // 6 // 10
LCD backlight control: This uses one more connection:
Arduino equivalent // Serial module pin // LCD Keypad module pin
D10 //16 // 11
Keypad interface: This uses a single wire from the LCD module pin on the lower side labelled "A0", to pin A0 on the Arduino. At least that was pretty easy!
Step 4: Making the RST Key Usable Similar to the Other 5 Keys
The RST can be connected directly to the RESTART input of the Arduino Nano.
However if you want the RST key to be usable in the software for other things, this can be done by connecting a 15K resistor between the RST pin and the A0 pin on the lower side of the LCD keypad shield.
This works as follows: There is a 2K resistor between +5V and the Right key. Then a chain of progressively larger resistors for each of the other keys (330R to the UP key, 620R to the DOWN key, 1K to the LEFT key, and 3K3 to the SELECT key. All the keys (including the RST key) connect to ground. When connected to a 10 bit A/D converter (as for Arduino Nano A0) they deliver approximately the following values:
Right = 0; Up = 100; Down = 260; Left = 410; Select = 640.
With the 15 k resistor to RST, it delivers about 850.
The software will use values around the mid points between these values to decide which key has been pressed.
Step 5: Software
Useful software is of course an exercise for the reader. However to get you started, you can have a look at my test software. I used the NewLiquidCrystal library, which contains support for the I2C interface. It all just worked, once I have the libraries correctly installed.
The main point was to reverse the polarity of the Backlight OFF and ON commands (due to the transistor on the I2C module as explained in the Parts Description section).
UPDATE 22 August 2019
If you have problems with the LCD display not working, please check the I2C address of your serial backpack using an I2C scanner. A suitable scanner is attached. Then if necessary adjust the first parameter in the statement
Step 6: Review and Discussion
As you can see I got the main functions working.
My next intention is to put this into a project box as part of another project. However after setting out on this path I learned that there is another difficulty that I had not anticipated.
The difficulty is that this LCD Keypad module is not set up to be installed in a box. Any sort of box. The buttons of the 6 keys are well below the level of the LCD screen, so that if the module is mounted in a box (eg in the lid) with the top of the LCD circuit board flush with the underside of the lid, the tops of the keys are about 7mm below the top of the lid.
Possible solutions are:
a) Put up with it. Drill access holes in the lid and use a tool (eg a cut down knitting needle of suitable diameter) to press the buttons.
b) Remove the LCD from the module circuit board and perform surgery on the keypad so that the two components can be fixed to the lid of the project box independently (I think there still may be a problem with the buttons being too short)
c) remove the existing buttons and replace them with taller buttons. The new buttons have to be about 13 mm high so that they can be operated through the lid of the project box). Replacement button switches are readily available in a range of heights, including 13mm.
d) discard the LCD keypad shield module and use separate LCD and keypad units (ie start over). There is a wide range of keypad units available, however I have not seen one with the same 6-key layout as on this module (ie Select, Left, Up, Down, Right, Restart). It may not be a big issue, but one of the reasons I started off with this module was that I thought this key layout was what I wanted.
I am planning to go with solution c) above, and see how I go.
Another snippet of information that may be of interest:
With the backlight ON, the current consumption of this project is:
Arduino Nano 21.5 ma; Serial module 3.6 ma; LCD module 27.5 mA; Total 52 mA.
With the backlight OFF, the current consumption of this project is:
Arduino Nano 21.5 ma; Serial module 4.6 ma; LCD module 9.8 mA; Total 36 mA.