# Small Solar Rechargeable Led Flash Light With No Solar Panel/cell. (uses the Led to Charge!)

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We see leds every where. what do they do? they light up. well yeah thats their main purpose but not a lot of people know that if you expose them to sunlight it can
produce a small amount of voltage. So in a way it can act as a solar panel. The flashlight I made uses one 10mm white led. The led charges the battery and with a flick of
a switch it allows the led to emit light. It is such a small circuit you can fit it into two pop bottle caps. For this small build I used a 10mm white led, small nickel battery
(20mah size of a super cap. good runtime for one led) a blocking diode and a spdt switch.
I will explain how all the componets go hand in hand in the 1st step.
if you want to see a video of this working please copy and paste this ;link into your browser. I didnt feel like uploading it to youtube just to emmbed it. its such a short
video so I hope you dont mind. here is the link https://www.facebook.com/photo.php?v=10200431925699995¬if_t=video_processed

In the comments section you will see there was a bit of confusion. I take full blame for that confusion. When I first measured the voltage output of the led it read as 1.2 volts. That was the foundation of the confusion. However I was measuring the voltage output when the sun was setting and being blocked by clouds and trees. I meaured it again today in absolute sunlight. And it read as high as 2.7 volts. The battery charged to 2.7. Now that is enough to charge the battery and enough to run the led, however it doesnt live up to its full potential. The led works best at 3 volts. I am sorry for the confusion and I thank everyone who commented and got me interested in taking it apart and re-testing the voltage. I will be making a newer better one in the future. The more leds you use in series the higher voltage output you will get. I plan on making a new instructable in a few days. Sorry for the confusion!

## Step 1: How It Works/quick Brief

This is a small brief on the circuit.
First I recommend using a white led as they produce the most voltage (in my studies) when in sunlight.
The white led I used outputs 2.7 volts when in absolute direct sunlight which is perfect for a slow charge. that being said, the type of battery I used is a very small nickel
based battery. when i first got it I thought it was a super cap. the battery is rated at 20mah and 3.6 volts. once its fully charged it can run one led for a decent amount of
time. it charges best when its in direct sunlight.
in the next step you will see the schematic and even if youre new to electronics you will see its very easy. It revolves around the switch and diode, in one position the
switch allows the led to charge the battery and in the other position it allows the battery to run through the led and emit light. The charge can also be an off position.

## Step 2: Schematic and Parts

The blocking diode serves a great purpose. It allows the led to pass the charge its getting from sunlight to the battery it also stops any current from the battery to the led
(stops the battery from discharging to the led. lighting it) . so its a one way street, now in the other position of the spdt you can see that its joined right after the diode, this
position allows for the battery to discharge into the led (light!).
very very simple. I also included a picture of the parts.

## Step 3: Assembly

the assembly is very easy. as easy as the schematic looks. i used two pop bottle caps as the enclosure. The flatness of the second cap comes in handy when charging.
pictures are in order from start to finish. click on them for additional info
step1 drill a hole through the top of one cap and glue the led to it. run the leads through the cap
step2 solder the positive lead to the center leg of the spdt switch
step3 solder the blocking diode from one of the outter legs og the switch to the battery 9positive)
step4 solder the last leg of the switch after the diode.
step5 solder the leds negative lead to negative battery terminal
step6 mount the switch and hotglue the battery in place. glue the second cap in place and done
check out last step to see how bright it is after a few minutes under a heat lamp

## Step 4: Testing... 123..

here is how bright it is. The one picture shows a picture i have on my wall. once the lights are off the flash light illuminates a good sized portion of the picture. i turned off
flash on my camera but the red led on the camera still lit up the background. once it dies set it in the sun and let it charge. I used a heat lamp to test it because I made
this at night :p thanks for your time I hope you enjoyed this!

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## 26 Discussions

that's great I never knew that. Thanks!

After reading this, all I could say was, "Woah". This is amazing! Thank you!

i did not know it. I tried something like that with zener diodes. But I only got 0.3V, have tou ever done with zener's?

How long does it work? And how long does it take to charge?

hi,,,can you help me to make a joule thief with solar charger with night detector...so it will turn on at night time,,thanks

i want to use it in the farm..thank you

I agree with him. U cant possible be charging this battery. This thing wouldnt charge if i put a 300 watt light bulb up to it. U said that 1.5 volts charges a 3.6 volt battery. Its impossible math and science says so. And i dont think u know what "trickle" charge means. A trickle charge still has to be above the battery voltage to charge. 3.8 or 3.7 would be a trickle charge. 1.5 volts - 0.5 volts from you diode = volt. Not even a 3rd of what you would need. No matter how you put it that led isnt charging that battery. If that battery is getting "charged" then there is other factors at work here.

7 replies

hey. i just found out the problem as to why you think it doesnt work. when i tested the voltage output of the led the sun was setting giving me 1.2 volts, i just tested the led in absolute direct light and got 2.7 volts. Heres why i got 1.2, I was standing in my door way testing the led through glass (some times makes a difference) and the sun was setting in the distance. trees and a few clouds were messing with the results. Sorry for the misunder standing! The battery charges to 2.7-2.8 volts and still powers the led (not to its fullest potential). I will be making a better one of these using a double a battery and a few inductors and what not. Again sorry for the misunderstanding and thank you for your feedback.

Well 2.8 i wouldnt call a full or a good charge but i suppose it couple power an led for a few minutes. And i dont see the point of an inductor in a new design.

The new desing uses a 1.2 volt battery. Which by itself cant drive an led. Adding an inductor or a circuit around an inductor will drive that led. Its used everywhere. Solar yard lamps have one double a battery and an inductor.

I think rather than the yard light design simply make a joule theif. Its 1 transistor, a 1k resistor, and a toroid. It can driv an led on voltages down to 0.6 volts. Look it up.

I already began the design for using the inductor. I made a few jouel theifs before. I just want to try the inductor. Could make both http://www.youtube.com/watch?v=fcTwhqfUkxk&feature=c4-overview&list=UU4CMvhUs1LWVNhkUQmbldWg

That's more reasonable. However, if you have a meter handy, check the current flow into the battery during charging. If you use a 20mah battery, and you put back in about 2 to 3 hundred micro amps, you are talking about a VERY long time to charge back up. Days upon days in direct sunlight. And that's getting you to just barely the conduction voltage of that led (some conduct as low as 2.5v). You are only probably getting a 25% charge at its best.

I would recommend using at least two in series. Get a small double pole double throw to replace that single switch. Wire it so one pole is charging, while the other has the leds in parallel. Then you'll have something.

I didnt bother measuring the current because I knew it would be very little. The idea behind this is to leave it in a sunny, weatherproof area until you need it. I already have a new one in the works. i have 3 leds in series in parallel with 3 more. The voltage out for those are close to 5 volts. im making it operate on one battery with the help of a few inductors

Unfortunately, I would have to see you perform this. You can't charge greater than the output voltage of the LED, which you measured out to be far less than the conduction voltage necessary to light up . Current will not flow into a voltage source higher than its own.

You would need at least two or three in series to charge the battery, then switch to them to parallel in order to work at the batteries voltage.

2 replies

hey. i just found out the problem as to why you think it doesnt work. when i tested the voltage output of the led the sun was setting giving me 1.2 volts, i just tested the led in absolute direct light and got 2.7 volts. Heres why i got 1.2, I was standing in my door way testing the led through glass (some times makes a difference) and the sun was setting in the distance. trees and a few clouds were messing with the results. Sorry for the misunder standing! The battery charges to 2.7-2.8 volts and still powers the led (not to its fullest potential). I will be making a better one of these using a double a battery and a few inductors and what not. Again sorry for the misunderstanding and thank you for your feedback.

hey. i just found out the problem as to why you think it doesnt work. when i tested the voltage output of the led the sun was setting giving me 1.2 volts, i just tested the led in absolute direct light and got 2.7 volts. Heres why i got 1.2, I was standing in my door way testing the led through glass (some times makes a difference) and the sun was setting in the distance. trees and a few clouds were messing with the results. Sorry for the misunder standing! The battery charges to 2.7-2.8 volts and still powers the led (not to its fullest potential). I will be making a better one of these using a double a battery and a few inductors and what not. Again sorry for the misunderstanding and thank you for your feedback.

Also, you are witnessing the battery voltage climbing up due to an effect called hysteresis. The battery voltage potential has a plus or minus factor. When discharged, it will lower. But if you meter the battery after unloading, the voltage will climb as if it were being recharged. You keep doing this enough, and the battery voltage will fall rapidly under load, to the point of failure.