Project: Build an LCD Learning Device without a Microcontroller

2022-06-02 | By Maker.io Staff

License: See Original Project

Standard 16x2 character liquid crystal displays (LCDs) are commonly found in a wide variety of ‎DIY projects, consumer products, and professional equipment. Usually, it’s a microcontroller’s ‎task to communicate with the display and to transmit character data that the LCD controller can ‎show on the screen. A recent article took a deep dive into the HD44780 LCD controller’s ‎communication protocol. There’s no reason you couldn’t send commands to the display ‎controller manually without using a microcontroller (MCU). Therefore, this article discusses a ‎simple-to-build project that lets users control a standard character LCD with a few physical ‎switches, a button, and a potentiometer.‎

project_1

This image shows the finished device. I used five buttons to demonstrate how the display ‎functions in its more complicated 4-bit mode.‎

BOM

Part/Qty

‎16x2 Character LCD - ‎1‎
‎10K Potentiometer‎ - ‎1
SPDT Switches - ‎5
SPST Button - ‎1
‎1K Resistor‎ - ‎1
‎10K Resistor‎ - ‎1
‎100uF Polarized Capacitor - ‎1
Project Box - ‎1

Designing the Circuit‎

This project uses the HD44780’s 4-bit communication mode. Implementing the circuit in this way ‎requires fewer components and space. Further, it also demonstrates how the more complicated ‎model works and how you can switch the display from accepting a single 8-bit command to ‎expecting two consecutive 4-bit commands instead. I’ll discuss the process of switching modes ‎in more detail below. However, let’s focus on the schematic diagram for now:‎

This image contains the schematic diagram of this project.

Even though the schematic might look complicated at first, you should quickly see that it is very ‎similar to a project that would contain an MCU to control the display. ‎

First, this project uses a standard potentiometer so that users can tweak the display’s contrast. ‎Then, five switches replace the MCU input lines. Note that the four data switches interface the ‎display’s four most significant bit inputs. Connect the other inputs (D4-D0) to ground (GND) to ‎permanently pull them low. ‎

Note: I used single-pole double-throw (SPDT) switches to make the project easier to assemble. ‎Using these switches, you can directly connect one input to GND and the other to the +5V line. If ‎you decide to use single-pole single-throw (SPST) switches, you’ll have to include a pull-down ‎resistor on each input line. ‎

Lastly, a simple SPST push-button replaces the MCU’s enable signal that tells the display to latch ‎the input data. However, since this is an SPST switch, you’ll have to add a resistor that pulls the ‎display’s EN input low when the button is not pushed. The two resistors and the capacitor ensure ‎that the input remains low until a user presses the button. However, these components ‎simultaneously act as a simple hardware debounce measure so that the display receives a single ‎enable signal when the user pushes the button. Without these components, a single button push ‎could trigger the EN line to quickly alternate between LOW and HIGH multiple times due to the ‎button’s contacts bouncing, which would cause unwanted behavior, such as repeated characters.‎

Assembling the Project‎

Start by cutting out a hole that matches the display size. You can either use a sharp knife to ‎carefully create a cutout or a rotary tool, for example, a Dremel, for a cleaner-looking result. ‎Next, use some masking tape and map out the positions of your input switches, the button, and ‎the potentiometer:‎

case_2

This image shows the build process. I have already created a cutout for the display. Then, I ‎used masking tape and a pen to lay out the inputs. Note that I used rectangular switches in my ‎build. However, creating rectangular cutouts for each switch takes a long time, and you may be ‎better off using round switches that only require you to drill some holes.‎

As you can see in the image above, I chose rectangular switches for this build. Creating five ‎rectangular cutouts requires more effort than drilling five holes for the input switches. Therefore, ‎I recommend you use simple round rocker switches instead or design and 3D print a custom ‎enclosure. Either way, make sure to drill a small hole in the back of the enclosure so you can ‎route a wire through the hole to supply the circuit with power. Also, don’t forget to drill four screw ‎holes to mount the display.‎

Once you’re done creating the necessary holes and cutouts, it’s time to add the components to ‎the project box. Use the nuts and washers that come with the switches to mount them to the ‎enclosure. In my case, the switches just snap into place:‎

hardware_3

Use the supplied mounting hardware to attach the components to the case. Don’t forget to use ‎four 1/10 inch (2.5mm) screws to secure the display.‎

Lastly, run wires from the switches to the appropriate inputs of the display. Pay close attention to ‎the order in which you connect the switches. Remember that the device is flipped over while ‎you’re attaching the wires, which makes it easy to connect the switches in the wrong order by ‎accident.‎

Setting up the Device

‎As mentioned above, a previous article discussed the HD44780 communication protocol and ‎commands in great detail, so I won’t go over the details in this article. Instead, let’s look at how to ‎get the device to operate in 4-bit mode and how to send character data to the display.‎

Start by flipping all the switches to their off position before supplying the device with power. It’s ‎normal that the screen displays all pixels when you first turn on the device because you need to ‎run through the initialization steps that let the controller know which mode and font to use and ‎how large the display is. Start by setting the display to 4-bit mode by sending the following ‎command:‎

switches_4

Set the switches as shown in this image to activate the LCD controller’s 4-bit mode.‎

Remember that at this point, the display still operates in 8-bit mode, which means that you’re ‎sending the following command to the LCD controller:‎

Copy Code

0010 0000‎

Next, you need to set the font and the display size, which can be done with the following input ‎sequence:‎

Copy Code

0010 1000‎

However, at this point, the display already operates in 4-bit mode. Therefore, you need to send ‎the eight bits in two steps. First, set the switches to send the four most significant bits (0010). ‎Then, press the EN button once. Next, set the controls to reproduce the next four bits (1000) and ‎press the EN button again. Make sure that you leave the RS line low during this procedure. ‎When done correctly, the previously displayed characters should disappear from the screen.‎

Next, activate the display, turn on the cursor, and make it blink by setting the following sequence ‎like before:‎

Copy Code

0000 1111‎‎

Remember that you can refer to the previous article to learn more about the commands and ‎possible parameters. Either way, the display should show a blinking cursor in the top-left corner ‎once you're done. You can send the following command to clear the display should it show any ‎garbage characters after this step:‎

Copy Code

0000 0000‎

Sending Character Data to the Display‎

When the RS switch is low, the display expects to receive control commands, such as the ones ‎for clearing the display or setting the font. Flick the RS switch to HIGH to enter character data. ‎Then, use the ASCII to binary conversion table showing printable characters at the end of this ‎article to find the binary representation of a character you want to enter and input the bits in two ‎‎4-bit nibbles, as demonstrated above. For example, the binary representation of an upper-case D ‎is as follows:‎

Copy Code

0100 0100‎

Repeat this procedure for all the characters you want to enter. Eventually, you’ll be able to spell ‎out words:‎

display_5

Use the switches to input the binary values of the ASCII characters you want to send to the ‎display.‎

Summary‎

This simple project allows you to directly interact with a standard 16x2 character LCD as it’s ‎commonly used in DIY projects as well as commercial products. The schematic is quite simple ‎and using SPDT switches allows you to further simplify the build process. Four of the five ‎switches act as data inputs to the display, and the fifth switch selects the LCD controller’s ‎register. A simple debounce measure ensures that the display controller doesn’t misinterpret a ‎single button press as multiple consecutive presses.‎

Assembling the project is also not too complicated when working with suitable components. I ‎recommend choosing round switches, as creating five rectangular cutouts for rocker switches ‎can become quite time-consuming. You can use a Dremel or a similar rotary tool to create a ‎cleaner cutout for the display. Lastly, remember to pay close attention when connecting the ‎switches to the LCD to attach them in the correct order.‎

You have to repeat a simple setup procedure whenever you connect the device to power. This ‎procedure sets the display to operate in 4-bit mode and then initializes it by selecting the display ‎size and font. Next, activate the display and clear it before you can transmit character data. ‎Finally, use the below ASCII to binary conversion table to find the binary values of standard ‎printable characters.‎

table