| Quick access links | |
|---|---|
| Carrier board v1.0 schematic (PDF) | Doxygen code documentation of individual files and methods |
| Wiring diagram v1.0 (PDF) | Doxygen TODO's |
This battery-powered logging volt/ampmeter was made with the intention to easily get .CSV files filled with voltage/current/power measurements on a SD-card to plot and use this data further. It is supposed to have the same functionality as much more expensive dataloggers, with the added benefit of the ability to change the firmware to add more functions.
The device consists of two Chinese (but very accurate and highly functional) meters (BY56W 50V and BY56W 5A) with independent power-supplies so no common-mode errors can occur. They can be individually powered on or off with toggle switches left of these meters. On the right of them are combination banana- and screw-terminals to easily connect measurement cables. The black and red one are the negative and positive terminal of the voltmeter, the blue and yellow one are the negative and positive terminal of the current meter. In series with the latter plugs and the current meter are a 5A 5x10mm fuse (fitted in a holder on right right side of the logger), along with an 5V internal NC-relay (with a 5V boostconverter and protection-diode) to open the current connection in certain scenarios (over-voltage, over-current, under-voltage, ...). Currently no code is written to use this relay functionality.
On the bottom a third module with the same shell as the voltage and current meters (I asked the Chinese store if they could sell me some) is situated with two 1.3" 128x64 I2C OLED-displays and a STM32 Blue-Pill board (with a genuine (!) ST STM32F103C8T6 microcontroller). This functions as the brains of the whole unit. This bottom module can also be switched on or off with the toggle switch to the left. The display-module asks the two meters for measurement values and does additional conversions and calculations with them, drives the displays, keeps the time with its internal RTC and 3V coincell backup-battery, reads the battery voltage, outputs the measurements to the UART port on top, ... Next to the bottom module an up/down monostable toggle switch and pressable rotary encoder can be used to start or stop logging, view measurements, change settings, ...
This module however does not write the data to a SD-card directly. For this a Sparkfun OpenLog module has to be plugged in to the external UART port on top. Because of the way the firmware is developed, a new .TXT file is created each time a new measurement-session is started. The resulting file can be easily renamed to a .CSV file to facilitate importing in Excel or OpenOffice Calc. The first line of the file contains a header which explains what data goes where in the following measurement-lines. The last two lines of the file contain another header with corresponding data like date, total runtime, capacity and maximum/minimum values.
The whole thing is powered by two (parallel) 18650 batteries in a battery holder. A charge and battery-protection PCB facilitates charging at 1A using USB Type-C on the left side of the meter. I used a breakout-board with two 5.1k resistors on the CC1 and CC2 pins to activate the 5V output on USB-PD compliant chargers. Above this Type-C port charging and done LED's are also fitted. Right of this the programming-port and two external GPIO-pins with a 3.3V and GND terminal on Dupont-connectors are situated. These two external GPIO-pins however currently have no function. Internally there also is a buzzer which can be used to signal various states, but no code is written to use this functionality as well. Some buck-boost converters (after the toggle switches) are also placed on a prototype-PCB to convert the battery voltage to 5V (with these boostconverters) for the meters and 3.3V (with this buck/boostconverter) for the bottom STM32-display-module.
The picture below depicts the internal wiring-diagram, standalone modules/boards are surrounded in blue frames.
The up/down monostable toggle-switch and pressable rotary encoder on the right of the bottom display-module are used to switch between display-pages and to set various settings.
The left display always indicates the power on the top. If the display is selected (selection-line drawn on the bottom of the display) the rotary encoder can be used to switch between the different pages to display additional information: max/min voltage, max/min current and calculated capacity in Ah and Wh. Upon an encoder-press the displayed information can be reset (if the logger is not saving data to a SD-card). Below these three available pages on the left display are depicted.
A down-press on the monostable toggle-switch changes the selected display. The selection-line on the bottom of the display will be moved to the newly selected display.
The right display always indicates the internal battery voltage and state of the logger (STOP/START/RUN/SAVE) on the top. If this display is selected (selection-line drawn on the bottom of the display) the rotary encoder can again be used to switch between the different pages to display additional information. The first page displays the current date and time, and pressing the encoder enables the user to change this information. Values are changed by rotating the encoder, the value (for example the day) is saved on an encoder-press, and the next item (for example the month) can now be edited. The second page displays the period between measurements saved to the logfile with the external logger. Pressing the encoder activates the ability to change the period by rotating the encoder, pressing it again saves the value. The third page displays the runtime if the logger is saving data to the external logger. Below these three available pages on the right display are depicted.
An up-press on the monostable toggle switch (if the logger is in STOP-mode) powers-up the external logger (START) and writes the header, along with a first measurement-line to the .TXT file (SAVE). Then the logger waits (RUN) until the measurement-period runs out and writes another measurement to the file (SAVE). Another up-press on the monostable toggle switch stops the logging and writes two more lines to the end of the file including data as the date, total runtime, capacity, max/min values, ... An example of the resulting data in such a .TXT file is depicted below.
Time [hour:min:sec],Runtime [hour:min:sec],Voltage [V],Current [A],Power [W]
17:46:21,00:00:01,1.291,0.0000,0.000
17:46:23,00:00:03,1.291,0.0000,0.000
17:46:25,00:00:05,1.291,0.0000,0.000
17:46:27,00:00:07,1.291,0.0000,0.000
17:46:29,00:00:09,0.875,0.0000,0.000
17:46:31,00:00:11,1.092,0.0000,0.000
17:46:33,00:00:13,1.290,0.0000,0.000
Date [day/month/year],Runtime [hour:min:sec],Capacity [Ah],Capacity [Wh],Max Volt [V],Min Volt [V],Max Curr [A],Min Curr [A]
17/08/2021,00:00:13,00.000,00.000,01.291,00.000,0.0000,0.0000
To easily convert the .txt/.csv files from this meter and get plots in PDF-form, a bash-script using gnuplot and ps2pdf/imagemagick was created. To create a PDF-plot of a file (LOG00110.TXT) with the title "Title" and subtitle "Subtitle", and to make sure the capacity and max/min voltage and current values are displayed above the graph, one can use the following command:
logplot -f "LOG00110.TXT" -t "Title" -st "Subtitle" -avBelow is a list of all of the possible commands, which can also be shown with the command logplot -h. The bash-script itself can be found here
To simplify the firmware for the rotary encoder logic some pins were swapped around on header J9 and J11. This is highlighted by bold text on the wiring-diagram on the STM32F103C8T6 board.
Unfortunately a wrong MOSFET was used for the function to power the external Openlog UART datalogger on or off. This means that MCU pin UART1-ENout/PB8 is unused and a PMOS bodge PCB was constructed. This can be seen on the wiring-diagram. MCU pin PB14 is used instead to power the logger.
The firmware is constructed with a lot of extra variables to facilitate the functionality to only update changed values on the displays. This speeds up the code but does however create a lot of overhead. In the future this can certainly be optimized.
As of yet not all of the hardware is used, like for example the relay and buzzer. All TODO's can be found in the Doxygen TODO's.