I'd like to power a microcontroller with a small solar cell. The idea is for the solar cell to charge a capacitor, and when a given voltage is reached, do some clever things to discharge the capacitor through a voltage regulator, and power the microcontroller. When the microcontroller is finished with its task (updating an e-ink display with sensor data), it switch the power back off, and the capacitor is back to charging mode again.

I was thinking about a zener diode triggering some kind of transistor latch, and the microcontroller being able to reset that latch, but I need some guidance !

  • \$\begingroup\$ No need for a uC to do this, but a rechargeable cell would be far better. They also can make Li-Ion tiny i.stack.imgur.com/8HlFq.png \$\endgroup\$ Mar 10 '19 at 16:51
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    \$\begingroup\$ @SunnyskyguyEE75 No need for a uC to update an e-ink display? Really? \$\endgroup\$ Mar 10 '19 at 16:59
  • \$\begingroup\$ I recommend that you try to find similar projects and see how they do that. A (super) capacitor might not be the best choice as it's voltage will fluctuate with charge level. That makes things complex, often too complex for a beginner. A small NiMh battery will make things much less complex. Start with MCU + display and make it so low power that it can run for months from a set of AA cells. You can then later change to NiMh AA cells and add a solar panel to charge them. \$\endgroup\$ Mar 10 '19 at 16:59
  • \$\begingroup\$ You probably wouldn't run the power system of this in a pulse mode, but you might have the processor briefly come out of an ultra-low-power sleep state. This is going to be a very challenging project however, you might do better targeting the more modest goal of a coin cell. And realize that any little mistake will kill your power budget - so you'll have to spend a lot of time hunting down every little oops. \$\endgroup\$ Mar 10 '19 at 17:01
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    \$\begingroup\$ No need for a uC to make a solar charged regulator !! Although I still have a ambient light powered Sharp Caclulator that probably uses a cap. It never dies. and is over 20yrs old \$\endgroup\$ Mar 10 '19 at 17:20

Basically, this could work, but it's going to need a lot of components just to have that threshold voltage, then to activate the microcontroller, and then to disconnect it once the voltage is below another, lower, threshold voltage.

The problem is that all these design components will need to be extremely low in power consumption, or things won't work out, and they need to work without an external power source, so coming up with solutions is harder.

However, integrated solutions for your problem do exist: TI, for example, has a supercapacitor solar charger demo board, http://www.ti.com/tool/TIDA-00242.

It features the BQ25570 as central component that takes care of getting the most out of your solar cells (you want to operate solar cells at a specific point in their I/V curve for sensible efficiency), controlling capacitor and system voltage.

Now, be very sure that spending your space and cost and complexity budget on a solar cell is wise – a simple LiIon battery, as used in hearing aids, pocket calculators, or e-Ink store price tags, would probably suffice for more than the life time of your device.


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