Just a quick question, because i want to know if its possible. My son asked me if a kind of light harvesting robot could be made. That roams around, finds light, then charges, then runs around again.

The intelligence side is fine for my but i was thinking about how to handle the energy harvesting.

I was thinking if the main bot is running at 5v running two very small motors for movement, light sensor to detect light etc.

for the power though, i was thinking could be provide the power from a capacitor? When we find light, the solar panels feed the capacitor. Am i able to use the capacitor to then power a ATtiny and two small 3v motors? Is there a way to measure the current charge in the capacitor? so if it drops below a certain point, it can sent a signal through a digital / analog pin on the ATtiny so we know we need to charge, and another when it reaches a high point so we know its done?

The motors will be operated using simple transistors to turn them on / off

  • \$\begingroup\$ Search for "solarbotics" ... encyclobeamia.solarbotics.net/articles/beam.html and beam-online.com/Robots/Circuits/1381.html and www.beam-online.com/Robots/Galleria_other/solarollers.html \$\endgroup\$
    – user16324
    Commented Jan 6, 2015 at 12:33
  • \$\begingroup\$ You could try a dc dc converter with variable input, boost to 4.2V and charge a li-ion battery. Capacitors are very unlikely to work. Self discharge, changing supply voltage, and other things will cause issues \$\endgroup\$
    – Mauvai
    Commented Jul 16, 2015 at 22:16

1 Answer 1


The biggest capacitor I saw was one at 300 Farads, similar to the one below: -

enter image description here

At full charge, the voltage is 2.7 volts. This means it can store an energy of: -

Energy = \$\dfrac{CV^2}{2}\$ - plug in 2.7 volts and 3000F and you get 10935 joules.

This is equivalent to consuming 10.9 watts for 1003 seconds. In reality this won't be achievable due to non-ideal power convertors. You might get half this time before things are pretty much spent and the robot won't move.

This is just an example calculation for you to plug in your own values.

  • \$\begingroup\$ Ah yes thank you, i only considered it may be possible because ive noticed there are many solar garden lights that dont use a battery, just a capacitor. Although, this being said, they only power a couple of LED's for a few hours at the most \$\endgroup\$ Commented Jan 6, 2015 at 10:18
  • 1
    \$\begingroup\$ Li-ion polymer batteries are excellent for this purpose, although they do need control circuitry. Some do have this incorporated though. They are very light and compact, efficient to charge do not suffer from the memory effect, and maintain a near constant voltage. \$\endgroup\$
    – Jodes
    Commented Apr 7, 2015 at 15:04

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