Minor updates: Zero-power battery voltage monitoring, battery holder

A project log for Slow Movie Player 7C

Yet another "Slow Movie Player" implementation with 7-color E-Paper module and ESP32.

likeabloblikeablob 08/18/2022 at 10:422 Comments
in Spongebob voice

After 5 months, the prototype is still running fine w/o replacing batteries. Under 30 min. refresh interval, it will survive the next 3+ months with the three 2000 mAh NiMH batteries.

Also I pushed some minor updates to the repo.

1. Zero-power battery voltage monitoring

When measuring battery voltage with a uC, placing a simple two register voltage divider in between ADC and V_BAT is one of the most common way to scale the voltage. However, since the divider are just two resisters between V_BAT and GND. it consumes current constantly at uA~nA scale. And that leak current is HUGE when we want to keep the device running over months or years.

Instead of adding two or more P/N FETs or a 8-pin OP-amp to the circuit, [JEELABS] shows a clever trick to reduce the current to (literally) zero with a single N-ch MOSFET.

In the above schematic, +BATT to be measured is about 4V and it's divided by 3k/1k registers. So ADC (BAT_V_MON) senses 1V (=4V/4) typically. To turn the MOSFET on, we have to apply a voltage to its Gate (1) several volts higher than the Drain (3). The condition is satisfied since BUS_POWER is at 3.3V regulated when the device is awake. Of course the MOSFET is turned off when BUS_POWER is kept at 0V during the device sleeping. (see details)

The firmware has also been updated to monitor the battery voltage and show a low battery notification when the voltage goes under 3.5V.

2. Battery holder improvement

Added some overhangs for holding batteries better.

And... one more thing. 

Simplest discrete battery holder ™


DJDevon3 wrote 08/22/2022 at 11:40 point

Nice design. That's a large display to go 6 months. When I saw the picture I didn't realize it was e-ink at first. Well done!

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Richard Hogben wrote 08/18/2022 at 19:44 point

Nice update

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