To effectively collaborate, a plan is needed to share tasks. This way different bits can be completed simultaneously.

Initial:

• Setup software toolchains
• Using Arduino IDE for now.
• Add in libraries for HX711 and HT1621.
• Ideally: set up software to work with a regular arduino for faster debugging.
• Scales reverse engineering:
• Measure idle and active current draw.
• Measure timing for wake up and sleep.

MCU:

• Test programming works using a blink sketch.
• Test out sleep state entry.
• Test out pin change interrupt on piezo pin to wake.
• Potentially add handler for listening to piezo tap. remember it's likely to ring for 100 ms.
• Measure working and sleep mode current (vs voltage)

Piezo:

• Measure piezo across 1 M resistor with scope- check it's high enough voltage.
• Clamp with some zener diodes at 0-5 V if necessary.
• Get MCU working with a Piezo to wake - from a gentle tap on the scales.
• If piezo alone won't do it then some op-amp is needed!

Display:

• Connect and test to Ardunio then ATtiny85, showing some demo counter.
• Test working voltage range (ebay lists only 5V).
• Measure working and sleep mode current (vs voltage)
• Consider contrast changes as voltage changes. (note will need to monitor voltage)

ADC module:

• Connect and test to Arduino then ATtiny85, outputting to display, serial, or simpler display.
• Use variable resistor in place of strain gauge
• Attach to strain gauge, choose appropriate gain (128, 64, or 32) based on e.g. 1 kg weight.
• Quantify noise level when on battery. Consider adding metal shield.
• Measure working and sleep mode current (vs voltage)a

Optimising:

Minimise Active power consumption

• Reduce clock frequency of ATtiny85 (disbenefit if it takes too long wasting power on serial comms)
• Disable peripherals (virtually everything)
• Add in a crystal oscillator to Display to reduce power, setup with software.

Minimise Idle power consumption

• Ensure all devices are sleeping correctly
• Ensure all IO pins are high or low, not floating, or fighting pullups. Consider built in pullups state.
• Ensure voltages are all ~5V or less. Adjust diode if necessary. Keep in mind battery voltage.

Extra features:

• Monitor own battery voltage roughly:
• https://wp.josh.com/2014/11/06/battery-fuel-guage-with-zero-parts-and-zero-pins-on-avr/
• Need to consider diode drop too here.
• and then display on boot.
• Display negative signs
• Tare when e.g. triple tap.
• Use WDT to do delays
• Add backlight feature somehow. (maybe on reset pin)
• Consider 18650 cell supply - no diodes needed. (But more expensive and self discharge more of a problem)
• Consider integration with external circuits - e.g. esp8266 to wifi.