Arduino(TM) LeOLED  front panel and panel meter project

I built a basic panel meter and front panel with a 2 line LCD. Pretty boring, eh? But add a new OLED display and this thing pops! LeOLED is a project that addresses the need for a simple and low cost, Arduino based front panel and panel meter. The basic features are those of an Arduino Leonardo or Micro board, plus a small graphics OLED display, encoder and buttons, and is designed to mount to a front panel. It is a big step above the usual character based LCD displays on a Shield board in terms of size, cost, and functionality.

The optional panel meter (PM) functions provide a higher resolution 12-16 bit ADC with 4 channels, and a precision 2 channel DAC. These can be electrically isolated to provide electrical safety or to eliminate danger to the PC USB ground.


leoled

LeOLED Basic Features

LEOLED PM Additional Features

History Lesson

I tried to design and use virtual instruments with no front panel controls, and was reasonably happy with their GUI based controls on a PC. But when you have a simple test to do, it can be a pain to get a PC, install the software, connect up the instrument, run the software.... I found that I prefer knobs and displays.

My first microprocessor projects used 2 and 4 line character LCDs and a few buttons. Back in the 80s and early 90s we were glad to have character LCDs. But nowadays, they are just so uninspiring. In addition to text based displays for menus and data, I generally needed some status LEDs. One example is a Power supply, Is the output ON? Its it in constant Voltage or constant current.  Graphics displays are bright, and way more fun to look at. They allow you to display any size text as well as simple or complicated plots, graphs, and symbols.

But monochrome LCDs are also pretty uninspiring as well as fairly big and complicated. Full color TFTs use a lot of data, and so are even more complicated. While they are OK for a larger 32 bit processor with more RAM and Flash, they require too much RAM and processing for a little 8 bit Arduino.

Along came OLEDS, Organic LED graphic displays. My intro to OLEDs was with the excellent Ornament and Crime synthesizer module.  I was impressed by the many types of displays they use as well as the nice menu structure. In Spring of '17 I began playing with Midi on Teensy and other Arduinos, and with OLED graphic displays. 

While I have built and used character and monochrome graphic LCDs for many years, I often did the graphics myself, including developing FPGAs to drive dumb graphics LCD panels. What a delight to have an easy to use panel that is cheap and has great software support. Here is some test code with a 128x64 1.3" OLED displaying current and voltage plus status. I plan to adapt it to my PS-Load system. The processor is a little 8-bit Arduino Mega32u8 board. Very small and cool.

Electronic Load Project
This project has evolved over many years. It began when I pulled some high power Toshiba IGBTs and large heat sinks out of the trash at Analogic in about 2001. With a simple opamp circuit and a 0.05 ohm 25W resistor, I was able to cobble together a nice 50V, 10A 100 Watt adjustable electronic load. For a control, it used a 10-turn precision pot to adjust the reference voltage. It used a separate multimeter to set the current, which was a problem because you needed to actually load a supply to set the current. This worked, but ideally you would set the current and then turn the load ON. Additions over the years consist of a fan and fan controller, and the first Arduino Panel Meter to read voltage and current on a 2 line LCD. I recently updated it with a LEOLED panel meter. The 0.05 ohm shunt resistor can handle 20A of current but 20A at 0.5 ohms is 20W of power. The fan and heat sink are needed to keep the resistor at a reasonable temperature. I would operate at low duty cycles if I wanted to run at 20A for long periods of time. And also at lower voltages like 3-10V.  At 100V I won't run it much over 1A or 100W. If I get brave and have a 100V supply that is beefy enough, I'l try it at 2A. At 200W things get prety hot.

I use two of the 4 ADC inputs, one for current and one for voltage. 20A at 0.05 ohms is 1.0V. The ADC has a 1.024V range so this is the default. The ADC is 16 bits bipolar or 15 bits unipolar. 15 bits is 32,768 or about 0.61mA of resolution on a 20A range. I display to 1mA, so no missing codes. If I want more meaurement resolution, the 0.256V range will provide 1/4 the range or 5A max, and at .15mA resolution, not quite one more digit of display resolution. This will require auto range software and calibration of the 2 different ranges.

The voltage ranging is similar. To get 100V range, I use a 1Meg / 2.5K divider and the  0.256V range. at 100V the ADC resolution is about 3mV.

How to get started with OLEDs:

Pick up an OLED display. Buy a couple since they are cheap and you are going to want to add them to all your projects. There are a few choices you'll have to make:
   
Install the SW library, either u8g2 or Adafruit. Un-comment the initialization line closest to the resolution and control chip that you have. Hardware SPI requires SCK and MOSI from your processor, sometimes on a separate 6 pin connector.  SW SPI can use any pins but is slower to update the display.  With u8g2 and hardware SPI I usethe following command:

U8G2_SH1106_128X64_NONAME_F_4W_HW_SPI u8g2(U8G2_R2, /*cs*/ 10, /*dc*/ 9, /*res*/ 8);

Wire up the VCC and GND, the clock, data, and 3 control signals (CS, DC and RES) from the initialization line you chose.
Compile and load the example code. Even without a display connected, with a scope you should see the clock and data signals output from your Arduino or Teensy. Once the display is connected properly and working, you should see the demo running on your display!

How much CPU time is used?
I measured the time to transfer the 1K buffer from the CPU to the OLED on a Teensy and on a Mega32u4. Both use Hardware SPI with an 8MHz clock speed.  On Teensy 3.2 it took 1.6mS and on '32u4 it tool 4mS. Not bad.

Why do I like OLEDs?

They are small.
I like the 1.3" 64x128s. These are only 1.4" wide overall, but highly visible. They are also available 0.96", and also half height 64x128. If they are too small for you, they are available in larger sizes for more $$$. In the past I used 8x2 LCDs to keep the size down, but they are very limited in what they can display. People have made custom fonts for symbols on character LCDs, but ugh.

They are bright and very visible.
Both Blue and White are very bright and visible. You can use tiny fonts to display lots of stuff, or large fonts so you can see things at a distance. The high brightness bery dark OFF state provide a high contrast ratio that beats all but the brightest LCDs. ????? Are they sunligh radable?

They are thin.
When building a front panel for a synth or any other project, you need all the front panel components to be about the same height so the controls stick through and the displays mount behind the front panel just the right amount.   This applies to displays, pots, encoders, switches, buttons, jacks. LEDs, etc. Add a thick back-lit LCD to the mix and you probably blow the height budget. OLEDs are only a few mm thick. You can mount them on any length pins or connectors to get the height you want.

They are cheap.
Like $4-9 for a 128x64 1.3".  0.96" are a bit cheaper.

They are high resolution.
What? You call 128x64 hi-res? Yes because the pixels are tiny, like 6-8 mils. Character LCD pixels are about 15 mils and so look blocky. 8 mil pixels are so nice. And at that small pixel size, you don't really need anti-aliasing.

You can add graphics and labels.
For my PS-Load project, I found myself adding LEDs as indicators for my Character LCD projects. Not needed for OLEDs, as long as your indicators can be all one color. There are some specialized OLEDs with a row of yellow pixels above the blue main screen. The intention of these is to have your annunciators in a different color. I haven't played with these.

Flexible display.
Any size characters, tons of fonts, lines, icons, simple graphics, small waveforms. Anywhere from 8 lines of tiny text to 2 lines of big test. You are only limited by your imagination and code and display space.

Flexible interface.
Either I2C (fewer pins and wires, but lower performance) or SPI (a few more pins, but fast). I2C ad SPI can be shared with other displays or other devices. This is better than the 6 dedicated wires on a Character LCD.

No Backlight needed
Character LCD backlights require a trimpot to adjust the backlight and display contrast. Plus the added thickness to spread the LED light evenly to the display. Yuk. OLEDs can be dimmed in u8g2 with the setContrast(x) command.

Flexible Power
Monochrome OLEDs in either 0.96" or 1.3" sizes can operate from 3.3V or 5V.

Great SW support
The graphics libraries are cool, easy to use and simple. There are two that are widely available: Adafruit and u8g2. Both are great. I use u8g2 since it supports tons of panels, and am very happy with it. Just a few lines of code are needed to display many items. The libraries are small, bug-free and efficient. To get an OLED running, just run the example code, select the correct panel,  and hook up a few wires.

Supporting multiple displays in ether I2C or SPI is pretty simple. Just initialize 2 (or more), then write to one, then the other. 2 or more can share a buffer, saving RAM.

Reasonable code size
32 bit processors like Teensy 3.x have gobs of code space. But even 8 bit processors like Mega328 and Mega32U4 have enough code space and RAM for simple apps. Watch your font selection though: large fonts use up your code space. u8g2 has numeric-only, upper-case-only and no-glyph versions of some fonts to save space, so use them.

Small RAM size
The best way to use 128x64 displays (OLEDs and LCDs) is to write your graphics to a memory buffer (only  128 x 64 / 8 = 1KB) and then update the display by transferring the memory buffer to the display. Mega328 and Mega32u4 only have 4K RAM, so keep this in mind. A 320x240 TFT color LCD display with 16 bits per pixel needs 320x240x16/8 = 153KB of frame buffer. Everything takes longer and uses more memory.


OLED, meet Synth: The CV to Midi Project

Here is another OLED project, a CV to Midi converter. This accepts 1V/ octave Control voltage plus a Gate signal, and outputs Midi data to any Midi instrument. Currently it outputs Midi to the lower left connector. It also sends Midi to a Synthesizer-on-a-chip, the VS1053B, on a board by Adafruit, in the upper left corner. The display shows the Note code being output as well as the instrument being played. The two push-buttons select the instrument. I love this thing. It allows my various analog sequencers to play any midi synthesizer or other instruments.

My plan it to make a real PC board for Eurorack with a handful of trigger and CV inputs. General Midi synthesizers have plenty of drum sounds, so I plan to use trigger inputs to control drums. Additional CV inputs could affect the drum pitch or act as velocity inputs to affect the the instrument velocity.

The module on the lower left is a Teensy 3.2 processor. To process the control voltage is one op-amp. The Teensy 10 bit on-board ADC digitizes the signal and converts it to a note. A single transistor (to the right of the buttons) accepts the trigger input. The DIP near the OLED is an op-amp to process the CV input.   The DIP on the right is an opto isolator for the MIDI receiver, not used with this design. The upper right stuff is just leftovers from the VCA project.  I plan to post the schematic, board files and code....

OLED 2 CV to Midi project




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This page was last updated 10/18/2017