Introduction

• Electronic load. Isolated DAC for current control, ADC for current and voltage measurement. USB
  
• 18b DAC precision voltage source
• DIY-SMU precision Source-Measure unit

• Arduino Leonardo compatible processor
• ATMEGA32U4, 5V, 16MHz
  
• 32K Flash, 4K RAM, 1K EEPROM
  
• Proper, fast, on-chip USB
• Serial emulation
• HID capable
• Single chip
• Bootstrap programming 6 pin header
  
• Encoder and switches for controls
• Enhanced measurement and control options
  
• 16 bit 4 channel ADC
• +/- 0.256V, 0.512V, 1.024V, 2.048V ranges
• 12/14/16 bit resolution. Less bits are faster
  
• Input resistor dividers to allow any V ranges
• 10 pin 1 row .1" connector (J3)
• Dual 12 bit DACs
• Isolated with > 100V isolation
• 5V to 5V DC-DC for isolated power
• Isolated I2C connections
  
• Basic Arduino I/O, non-isolated
  
• 10b ADC, 6 pins, which can be used as digital pins (J7)
  
• Digital and timer I/O (J8)
  
• I2C expansion
  
• OLED display
• 128 x 64, 1.3"
  
• Graphics and multi-line display
• u8g2 graphics library
  
• Lots of fonts
• Graphics primitives
• SPI for fast updates
• White, Blue or Yellow/Blue OLED options
• Small and cheap: 
  
• ~$15 parts baseline
• ~$15 more for DAC, ADC, Isolation
• Power Options
• USB
  
• External +5V, non-isolated
• External +5V, isolated
• Isolation
• 16b ADC, DAC are 200V isolated
• 5V DC-DC is reversible: feed power from USB or isolated side
  
• Basic Arduino (10b ADC) is not isolated

Design

Previously I built a panel meter with a small 8x2 character LCD. It worked, but was very limited. The LED backlight made it tall, and the 8x2 character display was very limited. 16x2 displays would have made it too large. OLEDs allowed flexible, low cost and bright displays. No backlight, low profile, small size and low cost.

Here is LeoLed design schematic. On the left is the processor, USB, programming and OLED connectors. J8 is the I2C and digital expansion connector. J9 is the analog expansion. I use J9 it for SPI expansion and use bit-bang code to control my 18b DAC and DIY-SMU projects. 

The middle is the encoder with switch, and the 5 push-buttons. These use built-in pull-up resistors on the processor.

On the  right is the 5V power isolation DC-DC, and the I2C isolation U2. Note that the 5V power DC-DC is reversible: power can either come from J3 (isolated) or non-isolated from USB, or from an external +5V connector on any of J7, J8 or J5. The DC-DC can be bypassed to save $6 if isolation is not needed. 

U1 is a 4 channel, 16 bit Delta-Sigma ADC from Microchip, MCP3426. Easy to use and low cost.  U5 is a dual 12 bit DAC. The resistors between J3 and U1 are optional voltage dividers. For bipolar voltages, plan to use the low voltage +/- .256V range. With this range the ADC input can be as much as -.3V below ground, allowing accurate bipolar measurements. the 16 bits are bipolar, so it's really 15 bits plus sign. If you need more bits, the Microchip 18b part MCP3424 can be substituted. I haven't done this.
 




Here are 3D models of the board, and in the 18b DAC enclosure. Board 3D models are from DipTrace. Box 3D model is from FreeCad.
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This is a LeoLed controlling my E-load project. Before LeoLed, I used a 10-turn pot to set the current and an external DMM to read the current. And another DMM to read the voltage. This is so much better. It uses the 16b ADC to read current and voltage, the DAC sets the current.