Charge Capacitor with Solar Panel

Is anyone aware of a simple circuit that will charge a capacitor with a solar panel (Small one from calculator) and then discharge to an LED via a transistor when the capacitor get's fully charged? The point is to make the LED flash. The frequency of the flashes would depend on ambient light conditions (less light, longer charge time). The closest thing I found is the circuit bellow. Would replacing the +12V with a solar panel still make it work (just take longer to charge)? Also what is the N.C. symbol at the transistors base ?(No Circuit or Normally Closed???)

I am aware of circuits that use a solar panel to charge a battery and use the batteries to make a flashing LED but I am trying to avoid the extra componenets. Thanks guys.


A common name for the

A common name for the function you’re after is a ‘solar engine’, with the MSE (Miller Solar Engine) being one of the most popular:

There are quite a few other simple circuits that accomplish the same task, many of them are also listed as ‘solar engines’.

N.C. often stands for ‘No Connection’.

Have you tried the sample
Have you tried the sample circuit as shown? You can replace the 12v as long as your solar panel is rated 12v, but you said it came from a calculator, so not sure about the rated volts, perhaps 1.5v? I’ve never tried this but I think a large enough capacitor is what you need. You shills be able to scale back the design with smaller resistors so your not geared for 12v but instead gear it for 2v - appropriate for an led. IgnobleGnome made some of these ( last year?) as Christmas tree ornaments.

FYI neither the

FYI neither the flasher circuit you posted nor any of the typical solar engine circuits is capable of generating an output voltage higher than what you can get out of the solar panel.
If your LED has a forward voltage greater than the solar panel’s output voltage you’ll never be able to get the LED to light, no matter how much energy you dump into a capacitor. That is, unless you have a part of your circuit that can boost the voltage to the LED.

I suggest you stick the solar panel out in the sun and measure the open circuit voltage, the short circuit current, and also find out the forward voltage of the LED you want to light up. From there we can find an appropriate circuit for lighting the LED.

Thank’s alot. Solar Engine I

Thank’s alot. Solar Engine I knew there was a name I was missing! That really helped me with my searches. Thanks for reminding me of the capacitor output, for some reason I forgot that output voltage will be the same as the voltage that charged it. Luckly I won’t have to bother with the calculator cell, I was able to get some 2V panels shipped to me with a fast priority shipping. I think I will just use the circuit I posted or this circuit (I have all the components laying around so I won’t have to order anything) and replace the Motor with an LED and resistor. After I breadboard them I will decide which I like more.


You were right about the calculator panel. It’s output was about 1V O.C. and 0.1A S.C. in standard room light, whereas the LEDs needed about 2V to work.

If you find the voltage

If you find the voltage output of the solar panels is still too low, even after tuning the solar engine, you can try putting 2 panels in series, or adding some additional circuitry to boost the output voltage.
There’s a popular little single transistor circuit you may have heard of called the ‘Joule Thief’, which acts as a tiny self-starting boost converter. It’s not exactly efficient, but it’s very easy to build and works well for lighting LEDs from a very low-voltage supply. With the right solar engine you can add a Joule Thief or similar circuit in place of the load, allowing the circuit to light your LED/s even when the charging voltage is less than the voltage required by the LED/s.

Thanks for the advice,

Thanks for the advice, helped alot :slight_smile: