Luke - Open Framework Multimeter/Logger

Auto-range is feature to select suitable measurement range automatically. This function helps you when target signals have wide dynamic range. Especially, it works for software engineers who aren't good at measurement. For example, they sometimes fail to measure current because system reset happens due to voltage drop caused by impedance selected measurement range has.

The important thing to avoid this issue is to change current measurement range to suitable one automatically and quickly. Luke has a tricky solution using a critical alert pin of the current monitor IC, INA3221. When ADC result is higher than threshold set by a register, it asserts a critical alert pin, and then turns on FET that bypasses shunt resistors. As result, impedance becomes lower because there is only high current sense resistor on the path.

However, if you are measuring negative current, that is ADC result is a negative value, this auto-range function doesn't work at all. This is a limitation of this hardware solution using a critical alert function of INA3221.

I decided to change a power monitor IC from INA226 to INA3221, that is a 3ch current monitor, based on the discussions with a talented engineer on this page. Although ADC native resolution is downgraded from 16bit to 13bit, measurement range of voltage is expanded and auto-range select is supported for both voltage and current measurement. Auto-range function will be realized by alert pins of INA3221 which can notify system when the measured value exceeds the programmed threshold. These alert signals directly control FETs to select the optimum measuring range. Now I uploaded the latest schematics and modified the specifications of Luke. New artwork has been almost completed. I managed to put all components on the board which is the same size as 1st prototype.

Thank you for being interested in my work and following this page. Now I'm pretty sure that the project is going in the right direction. I wrote introduction to clarify the project's purpose and then submitted it to The Hackaday Prize 2016!!

I started this project by myself couple of months ago.

First prototype was built in February 2016 and then I studied how USB communication between PIC and host PC works by using HIDAPI, this is awesome library released by Signal 11 software.

After the examination of hardware, I decided to change schematics to isolate signals and power between processor and measurement circuits. Now I'm making second prototype PCB.

My short-term goal is to release hardware and software until this summer. Please keep in touch on hackaday!!