measuring voltage using attiny13/attiny85
Recently I bought a BMS to use in my future project (PicoWeatherStationV3 foreshadowing) to protect its spare power supply, this cheap 1S boards have one critical flaw, battery low voltage cutoff is 2.5V (lowest safe voltage for 18650 lipol battery is around 3.1V), without way to change it (at least I don't know any).
I thought to myself I can build simple one myself, all I need is Mosfet transistor, some fuses and a way to control Mosfet basing on current battery voltage. In the past I built voltmeter using RaspberryPi Pico + LM4040, but for this application this setup is too expensive + takes too much space.
My final choice was Attiny13/Attiny85 as it is relative cheap, small and works in following voltage ranges (2.7-5.5) so it can be powered directly from battery without using voltage stabilizer or buck/boost converter.
I got myself USBasp programmer + Attiny13 and Attiny85 (to test which one fits better), wired prototype board and I got to work.
In order to upload code to Attiny I gonna use USBasp ISP programmer and Arduino IDE 2.3.2.
Despite not having dedicated TX/RX pins both microcontrollers support SoftwareSerial. All what is needed is a USB <-> UART converter, for example PL2303.
If you encounter permission error while using USBasp programmer:
- Create rule file.
touch /etc/udev/rules.d/99-usbasp.rules
- Put following text into it.
SUBSYSTEM=="usb", ATTRS{idVendor}=="16c0", ATTRS{idProduct}=="05dc", MODE="0666"
- Relog (or reboot just in case).
Before first use (or after changing properties in Tools tab, I'm not sure about that) don't forget to set fuses by using Burn Bootloader.
For this little buddy I used following hardware package.
https://github.com/MCUdude/MicroCore (version 2.4.0)
Installed by putting
https://mcudude.github.io/MicroCore/package_MCUdude_MicroCore_index.json
to Additional Boards Manager URLs in the IDE settings.
Package by MCUdude comes with built-in support for UART operations all we need is USB <-> UART converter and proper connection.
| Converter | Attiny13 |
|---|---|
| RX | PB0 |
| TX | PB1 |
For this project I'm using 1.2 MHz internal oscillator for which proper serial baundrate is 19200.
Alright with everything in place we can verify is everything is working as correctly.
- Put following code into sketch.
void setup() {}
void loop() {
while (true) {
Serial.println("Hello world");
delay(2000);
}
}
- Upload using Programmer.
Ctrl + Shift + U
And if you are the lucky one, you should see Hello World printing every 2 seconds.
For Attiny85 I used following hardware package.
https://github.com/SpenceKonde/ATTinyCore (version 1.5.2)
Installed by putting
https://drazzy.com/package_drazzy.com_index.json
to Additional Boards Manager URLs in the IDE settings.
Package by SpenceKonde comes with built-in support for uart operations all we need is USB <-> UART converter and proper connection.
| Converter | Attiny85 |
|---|---|
| RX | PB0 |
| TX | A1 / PB1 |
For this project I'm using 1 MHz internal oscillator for which proper serial baundrate is 9600.
Alright with everything in place we can verify is everything is working as correctly.
- Put following code into sketch.
void setup() {
Serial.begin(9600);
}
void loop() {
while (true) {
Serial.println("Hello world");
delay(2000);
}
}
- Upload using Programmer.
Ctrl + Shift + U
You should see Hello World printing every 2 seconds.
I created following circuit with voltage divider in ratio 1:4.3 (1V on ADC pin = 4.3V of source voltage).
I hooked Attiny to UART converter (powering it from the same converter), switched ADC reference to internal 1.1V, set PSU to 4V and took some readings.
It is quite noisy, isn't it ? Let's check if it can be tweaked with some sampling.
Yup It helped, now it looks much better.
Attiny13 has 1Kb of internal space, too little for using ADC and Serial libs at the same time, so I had to figure out a way to test it.
I modified previous circuit, added LED to indicate if voltage is below certain threshold + powered Attiny directly from battery.
This way I can test if variable supply voltage can make readings worse. I uploaded following sketches and tested it.
After some tests it looks like variable supply voltage doesn't affect readings much, it still works great.
Attiny does a good job as a voltmeter, and it will be core part (Attiny85, because of bigger flash size) of simple kind of BMS I talked about.