Tuesday, 24 January 2012

How to make a (really) simple DIY solar USB charger for your phone or mp3 player

How to make a (really) simple DIY solar USB charger for your phone or mp3 player

I am going on a trip in next few days where I will be without power, hence I decided to do some solar charging for my phone.  Here is one rudimentary circuit  you can make easily.

Parts you need :

Solar cells
Schottky diode 1N5819
Electrolytic capacitor 10uF
0.1uF capacitor
9V rechargeable battery
USB female port 
Perfboad or donut board or PCB
Soldering accessories

In this circuit , I use four 5V solar cells. Two are in parallel and other two in series, so that  the total output  would be 10V and 100mA.  I used it because it is the best alternative I have locally. You might choose better alternative like this one. Schottky diode prevents the back current to solar cells. Otherwise, solar cell will draw the current in the dark.  
The rest is ordinary LM7805 circuit. This IC will regulate any voltage between 7.5V and 30V to 5V. Capacitor C1 at the input prevents source oscillation while capacitor C2 maintains the output stable.
9V rechargeable battery will act as buffer power source when solar cells are not working.
The simple charging devices (like many phones and mp3, mp4) only use two port of USB ; pin 1 for power and pin 4 for ground. This circuit cannot charge the iPhones , iPods  because these devices use other two at specific voltage levels to be able to recognise as charger. See detail explanation about it at Ladyada.net Minty Boost USB charger. It is also not working with my ASUS transformer tablet. But my Nexus S phone is successfully charging.

This is very simple and inefficient design. I have some improved designs in mind. I wanted to use the more efficient step up transformer as in Minty Boost circuit. I am also thinking about using transistors to amplify current.  but the problem is I can't buy components I need because of Chinese New Year holidays here. And I really need to finish this solar charger before my trip. Can't really blame it, isn't it engineering is all about ? , to create something with limited resources we have (whether it is time or material). 
When my trip is over , I will be improving this circuit once again. Here are photos of my solar charger.