Indoor Light Charged Power Supply

Open source, easy-to-build Power supply charged by ambient indoor light to power low power micro-controller projects and an external device.

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Open source, easy-to-build, low cost power supply that is continuously charged by ambient indoor light with two outputs to power a low power micro-controller with a gold capacitor and an external device such as a motor, transmitter or other device with a fast charging electrolytic capacitor for short cycles.

Energy harvesting is a rapidly growing industry.  With micro-controllers now requiring ever decreasing levels of power, window sensors, temperature sensors and a wide variety of remote sensors are now available.  Energy harvesting is starting to show up everywhere, creating many opportunities for individuals to develop products, start businesses and work toward financial independence.


The energy collected from energy harvesting is typical very small.  Voltages are often so small that they require a charge pump to supply a usable voltage.  The current is typically low enough that it takes a long period of time to build up enough power to be usable, often just enough to pulse a transmitter or an LED.  If enough power could be collected to turn on a motor, even with short pulses over a period of time and also power a micro-controller, then all new applications would be possible, bringing with it new opportunities.  Although it would be very slow, energy harvesting devices could gain the ability to move.  The energy harvesting technique also needs to be low cost, enough to be in everyone’s reach and simple enough that even an individual just starting in electronics can build.


One of the most abundant energy sources available is light. Outdoors, the sun provides a clean energy source to harvest.  Indoors, ambient light is also available but at considerably lower levels.  The Indoor Light Charged Power Supply is a power supply designed to harness this available, but low power resource and stores it in both a gold capacitor to power a micro-controller and an electrolytic capacitor to provide pulses of power to an external device, such as a motor.  Keeping the Indoor Light Charged Power Supply both simple and functional is all about selecting the right components.

Below is an initial schematic of the Indoor Light Charged Power Supply.

The solar cells used to collect light need to be cheap, available and able to provide the power needed to charge the capacitors. Fortunately, the solar industry has developed amorphous solar cells commonly used in calculators and other indoor devices. The cells on The Indoor Light Charged Power Supply provide 2.5 volts in typical room level light at a very low current. Since the voltage from the cells is at a usable level, there is no need for a charge pump. To provide more current, five cells with schottky diodes feed a power bus in parallel. Schottky diode have a forward bias of .3 volts verses .7 volts for a silicon diode.  Less voltage drop at the diodes means more voltage for the load.

From the power bus, a schottky diode feeds a gold capacitor. Despite the name “gold,” gold capacitors are very economical, around a few dollars for a 4-farad capacitor. Gold capacitors have a slow charge/discharge time, which makes them ideal for powering a low power micro-controller, but initially will take a while to charge.

Also from the power bus, a schottky diode also feeds an electrolytic capacitor. The electrolytic capacitor can charge/discharge much faster than a gold capacitor, which makes it a good storage tank to provide a short powerful pulse for a motor and then recharge more quickly.

 The Indoor Light Charged Power Supply can be adapted and fitted directly into a project verses a standalone power supply. If more power is needed, additional solar cells with diodes can be added to increase the power. If the project needs a smaller power supply, fewer solar cells can be used. This will allow the Indoor Light Charged Power Supply to be used in many different projects of many different designs and applications.

First power test, powering a quartz movement clock.

The Indoor Light Charged Power Supply powering...

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  • 5 × amorphous silicon solar cell 2.5 volt ASC-1950-4B
  • 1 × 4 farad super capacitor PAS1020LA3R0405
  • 1 × 3000uF 6 Volt capacitor
  • 1 × Terminal block 4 terminal

View all 6 components

  • Powering a project.

    Dennis07/13/2018 at 15:44 0 comments

    As part of the Energy Harvesting Competition, the challenge is to “Build a module that harvests ambient power for various applications and show your module powering a project.” In this update, the Indoor Light Charged Power Supply will be powering a project and also demonstrating the Indoor Light Charged Power Supply in use as a bench top power supply for general use on low power applications. 

     The project is a CMOS oscillator based on a CD4001 quad nor gate built on a solderless breadboard.

    CMOS chips are ideal candidates for energy harvesting projects due to their low power requirements. After wiring the above schematic on a solderless breadboard and hooking up the power, the circuit began to oscillate.  It is always nice when things work. :) Check out the short video below. The meter is oscillating between 2.17 volts and 0 volts.

  • First power test of the Indoor Light Charged Power Supply.

    Dennis07/12/2018 at 22:31 0 comments

    After hours of sitting in indoor light to charge the super capacitor for the first initial charge, the Indoor Light Charged Power Supply is ready to test. I tested the power supply on a clock from my work bench. As expected, the output from the electrolytic capacitor powered the clock as it discharged, but when the load was removed, the charge started to build up.

    With the clock connected to the super capacitor’s output, the clock was also powered but with very little drop in the super capacitor’s voltage. Check out the video.

    Now, to power some micro-controller projects.

  • Assembled Circuit

    Dennis07/11/2018 at 02:15 0 comments

    Here is an assembled circuit on a perfboard.  The circuit is a very simple design to make it easy for anyone to copy in their own projects.  Seven schottky diodes, a 4 Farad Super capacitor and a 3000 uF capacitor, along with the solar cells, is all that is needed.

    Below is a picture of the assembled unit.  Not too pretty, but good enough to test the concept.

    Next, set the Indoor Light Charged Power supply aside and let it charge. The initial charge time will take a while, but after charging it should be able to power a low power micro-controller easily, and the 3000 uF capacitor should be able to recharge in a few seconds.  After several hours of ambient room light, the Indoor Light Charged Power supply's super capacitor is up to 880 mV, it should build up to 2 volts before use. This will take a while.

  • Parts arrived from Digi-Key!

    Dennis07/07/2018 at 20:38 0 comments

    It’s always great when a Digi-Key box comes! Now I can build a circuit with the Schottky Diodes instead of the Germanium Diodes.

  • Repository storage

    Dennis07/04/2018 at 15:27 0 comments

    Repository storage has been added to hold files and data sheets. Also the creative commons license 4.0 details.

  • Germanium Diodes vs Schottky Diodes

    Dennis07/04/2018 at 14:22 0 comments

    In the comments, Martin suggested using Schottky Diodes vs Germanium.  They both have a low forward bias, and both would work.  After searching for both types of diodes, the Schottky Diodes are much more widely available and at a lower price.  The main goals of the project is to keep it as low priced as possible and make it as easy as possible for anyone to build and use in their own project, so I think it is a great idea to switch the design to use Schottky Diodes.  The details and diagram have both been updated to use Schottky Diodes.  

View all 6 project logs

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Daniel wrote 04/01/2023 at 08:44 point

would it be possible to combine the output of the two capacitors such that the faster diode could compensate for the short comings of the larger?

Some loads seem to have a higher draw just at start up then settle once in operation.

  Are you sure? yes | no

kwhiteusc wrote 07/11/2018 at 18:00 point

Do you think this would be useful in a shady outdoor context?  I'm struggling with an esp8266 sensor project that needs to almost be portable, but will be in the same 10 foot outdoor but typically leaf/tree shaded area for its lifetime.

  Are you sure? yes | no

Dennis wrote 07/12/2018 at 00:42 point

Hi kwhiteusc, It should work well in the shade. I’ve been using
it in fairly low light indoors. It will need to be weather resistant and if you
need more power, more cells can be added.

  Are you sure? yes | no

Robert Mateja wrote 07/07/2018 at 14:41 point

Can you provide part number for cell? Thanks.

  Are you sure? yes | no

Dennis wrote 07/07/2018 at 20:27 point

Hi Robert, The solar cells are ASC-1950-4B. I bought them a
while back from a surplus company. They were made to power calculators. These cells work well but I’m having a hard time finding more. I’m looking at other more
common cells and I’ll post information on new the cells as soon.

  Are you sure? yes | no

Robert Mateja wrote 07/07/2018 at 21:21 point

Thanks again. It seems that best way to obtain indoor cell is to buy $0.60 pocket calc from ali ;) 

  Are you sure? yes | no

Dennis wrote 07/07/2018 at 21:48 point

 Hi Robert, You’re

right, it is easier to find low prices cells in cheap products then buying them

as components.

  Are you sure? yes | no

Martin wrote 07/04/2018 at 07:58 point

I would eliminate the didoes D1 to D5. Your solar cells are small and mounted very close together, so they are always exposed to the same lighting conditions. I don't see partial shadowing as a real issue.

I also would think about using Schottky diodes instead of the old fashioned Ge diodes.

  Are you sure? yes | no

Dennis wrote 07/04/2018 at 13:05 point

Hi Martin, Thanks good suggestions. I was considering schottky
diodes since they also have a low forward bias like germaniums. I started the
first prototype with germanium because I have a lot of them in my parts bin and
the low forward bias.  Thanks for the suggestions!

  Are you sure? yes | no

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