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Crafting a battery-powered heart with ATiny85

2022-02-14

You may be reading this post because of two reasons:

With both A and B you should be satisfied - but A may leave you with unanswered questions, because I’ll be talking mainly about my specific use-case (described below).

“Option-A-choosers” should also know that this isn’t a typical “scroll down to copy-paste the answer”. You should probably read the whole thing 😉

What I’ll be crafting

Main idea is - a pretty, flashy heart that’s ment as a gift :)

Besides being pretty and flashy, it would be awesome if it would also turn out ✨useful✨

Final look of the heart - turned off Final look of the heart - pink light

What it will do

I want it to work like a standard light/torch - you have a single button that you click to enable it, then it changes modes, then it turns off. From user perspective, this is blatantly simple. But form electrical/programming side, it may be a challenge.

The modes/functions:

Parts

After laying everything out, this is how it initially looks:

Initial look of the heart - front Initial look of the heart - back

Making it low-power

…this is the part for all “option-A-choosers”

Preamble: I am not a professional electronics engineer, and I didn’t precisely measure every single step. I messed with this for 3 full days straight, and I’m gonna just tell you what I did 😌

Lifecycle we want to achieve:

Cutting away unnecessary parts

First off - grab yourself a digispark schematic

Things from Digispark board that we can throw straight away - they waste power all the time:

There also is a 1KOhm pull-up resistor (R3 on schema) that keeps wasting power… but it’s kinda required for USB communication… Solution? Solder it directly to USB-5V instead of VCC - this way it will only work with USB connected. Guy in this tutorial showed it nicely: https://www.instructables.com/Reducing-Battery-Power-Consumption-for-Digispark-A/ - but he suggests messing with a knife to cut the SMD resistor off (??) - I went ahead, de-soldered it altogether and used my own big ass THT one 😌

Digispark ATTiny85 board with some modifications

1 was the R3 resistor. 2 was the diode that connected 5V from usb to VCC of the board. I de-soldered it too, and connected 5V to input of the charger and my own pull-up

Actually, there turned out to pe a very cool side effect of this - now we can easily read if board is connected to usb with a simple digitalRead(3) ( https://github.com/TheLastGimbus/mklove/commit/bd1b86d3 )

Ps. If you’re evaluating if zener diodes for USB draw power or smth - don’t. I tried de-soldering them, and it didn’t make a µA difference 👌

Other hardware surprises

This may not apply to you, but it turned out that WS2812’s also drain power while off 😳

Quite a lot of it, actually…

WS2812 LEDs draining power while turned off

This ☝️ is 19-LEDs strip, draining 5.7mA (that’s 0.3mA per LED) while off! My heart has 10 of those, so - consuming 0.3*10=3mA - with a 150mAh battery - it would go from 100% to 0 in 150/3/24 ~= 2 days - while off

That’s why I had to add small mosfet transistor to cut the power from LEDs:

MOSFET transistor for disabling LEDs

Playing with bootloader

While playing with deep sleep (more on that later), I discovered that ~50% wakes from it end up with reboot rather than back to my code - meaning, ATtiny does it’s “6-second initial delay” thing

We do not want that - we want user to press the button and have the light instantly - besides, it wastes power too 🙄

So - turns out that Digisparks are running this neat thing called micronucleus - it’s a bootloader with soft-USB out-of-the-box! Digistump Wiki says that if you want to remove “the 6-second delay”, you should install alternative flavor of that bootloader, where it waits only if P0 is pulled down on boot.

Sounds great! Thing is, the links are old, and nowadays-version of micronucleus does not include that flavor compiled in “releases” folder. Luckily, I’ve successfully managed to compile it myself!

Here is my fork: https://github.com/TheLastGimbus/micronucleus/tree/87346a2d/firmware/upgrades - I added “ upgrade-t85_jumper.hex” and “upgrade-t85_jumper_bod_disabled.hex” versions - bod_disabled version should use even less power :)

Note: If you’re curious, BOD is some ’thing’ that watches for voltage drops - you probably don’t need this

Once you download .hex file of your choice, download the micronucleus-cli tool itself from their releases: https://github.com/micronucleus/micronucleus/releases

Then, open the terminal, and just run it: ./micronucleus upgrade-t85_jumper_bod_disabled.hex - similarly to uploading your Arduino code, you first run it, and then plug the USB in 👀

…in my case, it didn’t really work at first try :face_with_monocle: - try a mix of uploading bootloader, then the code, then bootloader again until it works ❤️

Note: this seems scary, and sounds like something you could brick your device with! Well… probably yes, but I’ve done it few times with few boards, and didn’t brick anything yet!!

But, as a precaution, you probably should try with some backup board first 😉

Tada 🎉 boots instantly 💯

Saving power with code

Disabling ADC

ADC draws quite a lot of power (few mA’s) just by sitting there - you will find quite a few different ways to disable it 0_o - many of them didn’t work for me - in particular, the nice one disable_adc() from #include <avr/power.h> didn’t :(

Here are ones I made:

void disableAdc() { ADCSRA &= ~_BV(ADEN); }

void enableAdc() { ADCSRA |= _BV(ADEN); }

With these, you can just disable ADC at startup, and dynamically enable it for few ms when using it 👍 - make sure to disable before for deep sleep tho!

Going deep sleep

Here are my sleep functions:

// Will be called when button pressed
void wake() {
    detachInterrupt(0);  // Disable the interrupt - will not be called anymore
    sleep_disable();  // Precaution
}

// Some additional setup *before* sleep itself
void _preSleep() {
    disableAdc();
    digitalWrite(LED_BUILTIN, LOW);
    pinMode(PIN_LEDS, INPUT_PULLUP);
    digitalWrite(PIN_MOSFET, LOW);
}

/// Go to deep sleep with interrupt wakeup
void sleepNow() {
    _preSleep();

    attachInterrupt(0, wake, LOW);
    noInterrupts();  // This should (?) make it more reliable ??
    set_sleep_mode(SLEEP_MODE_PWR_DOWN);
    sleep_enable();
    interrupts();
    sleep_cpu();  // This is *the* moment where it sleeps
    sleep_disable();  // ...and this is when it wakes up
    detachInterrupt(0);
}

Results 🎉

After all of those modifications, my Digispark board uses 3.5uA of current while in deep sleep, and about 13mA when on (with LEDs disabled of course) :ice: 😎

With a 400mAh battery that I use, it will last:

This is nice!

Coding it!

After all of this mess, actual programming was pretty straightforward… well, except…

Flash size issues

Turns out ATTiny85 is… well… tiny. While I was casually adding library for the LEDs, for buttons, and started implementing some crazy animations, I was surprised with PlatformIO telling me I don’t have any space left!!

After some optimizations and trade-offs, I realized I have a new bootloader. Which, conveniently, takes less space. So, now, I can tell PlatformIO that, “hey, I DO have space!”:

; PlatformIO Project Configuration File

[env:digispark-tiny]
platform = atmelavr
board = digispark-tiny
framework = arduino
; ### HERE IS THE THING ###
; I don't want to get you into a hell of calculating this magic value. Here is micronucleus' "guide" if you really need it: https://github.com/micronucleus/micronucleus/blob/c2c7fa096de55303f324ec7d650d71d10cc29d1a/firmware/configuration/t85_default/Makefile.inc#L20
; But - if you just uploaded *my* bootloader just as I did above, you can freely pase this:
board_upload.maximum_size = 6586  
lib_deps =
    fastled/FastLED@^3.5.0
    git+git://github.com/TheLastGimbus/EasyButton#attiny85-hack

Code itself

Whole code is available on my GitHub: https://github.com/TheLastGimbus/mklove/

Conclusions