I welcome you to third part of my series as part of Upcycle IoT Design Challenge. In previous blogs, I described earbuds and shown their schematics which I reverse engineer. For practical reusing of earbuds, I want to use them with Curiosity Nano board and base board which has its own battery charger and voltage regulator circuits. It do not make very sense have two batteries in the design. So, I will disconnect battery and connect alternate source from curiosity nano base board instead.
Available Voltages on Curiosity Nano Base Board
There are two voltages available by default:
- 5V which comes from boost regulator boosting voltage from battery or USB when USB charger is connected
- 3V which powers the MCU
The problem is that for emulating battery I need voltage in the range of lithium-ion cell. I need voltage between 3.7 to 4.2V. Sadly, 3.3V is too low and 5V is too much.
Using Linear Regulator
Instead, I used 4V LDO. But because I do not have any, I used adjustable version of 1117 linear regulator.
Schematics
With following values output voltage should be 3.95V in theory:
Circuit
And here it is. I soldered wires to test points which battery were originally soldered to, connected them to 4V LDO and powered earbuds on:
Results
Earbuds do not worry about different type of source. I added series resistor for current limit in case of some failure but it was not needed. Over Bluetooth earbuds report battery state of charge as 80%, but I guess it will never discharge to 79% 
Plan
In next blog I will start integrating them to Curiosity Nano Base. I plan to desolder broken earbuds and solder wires to jack connector which will allow me to use them as wireless smart speaker with standalone external speaker. Curiosity Nano I will use for controlling buttons of the earbuds. I plan add display and show information from temperature sensor which I received as part of competition for free, so in next blog I will show my library for reading humidity and temperature from sensor.
Next blog: Blog #4: HDC1080 with SAM E51