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Hello. I hope this is in the right group. I have created a solar charger based off of different diagrams on the web. Everything functions properly, but what I would like to do is add an indicator light to the design for when the charger reaches the proper voltage.

The reason I want the light is due to the variability of solar power. The voltage indicated on the following diagram is the peak voltage of each cell. I would prefer to know that the charger is operating properly before I plug my phone into it... I welcome your suggestions.

I've created diagram here to give you an idea how I have everything wired. https://www.circuitlab.com/circuit/6z4vbw/solar/

The diagram is designed to show the wiring and doesn't conform to standard notation. Also, I'm used to thinking in conventional theory vs electron. So if this causes any confusion I apologize.

(If you are curious as to why there are so many cells it is so the charger will operate properly in indirect/low light.)

The usual approach with solar cells is to use a switched mode converter to waste as little as possible of what solar power you can get. Using a large number of PV cells and a linear regulator is very inefficient.

If you need to know when the charger is producing output then the simplest way is to have a series R, series zener and a series LED to so that the zener plus the LED forward drop is just about where the linear regulator starts to operate (for a 7805 that'll be something like 7V to 8V: the dropout voltage of the old 7805 type regulators is quite high). As the input voltage exceeds about 7V, the LED starts to glow.

A better way is to use an R, and a zener in series with the base of an npn transistor and put the led in series with the collector and another current limiting R. Then the LED turns on harder once Vz + Vbe has been exceeded.

The best way is to use a comparator to sharply turn the LED on above a threshold set by a reference set by a simple zener diode.

You may either need a diode in series with the regulator output or a regulator with a reverse bias protected output in case the sun goes in before you unplug the battery.

Thank you for your response. My knowledge of EE is rather limited (ie: non-existent) and what I built was from different sources and improvised upon. It is also built from salvaged parts. Hence the general lack of components in the diagram.

Can you recommend a switched mode converter plan that will accommodate the varying voltage that the cells will provide?

The cells I have I pulled out of cheap solar lights from wal-mart. The idea is a low profile, portable device.