Hello everyone, Im new in Arduino programming. Can someone tell me a source for DC/DC converter.
I have to convert DC from Solar panel to 3,5V DC. 
Sunwell:
Hello everyone, Im new in Arduino programming. Can someone tell me a source for DC/DC converter.
I have to convert DC from Solar panel to 3,5V DC.
What's the input voltage range?
What's your budget?
What's your size requirement?
What's your output current maximum requirement?
Hi
Welcome to the Forum.
You will need some sort of storage device on the PV side of the DC-DC converter, because a PV is not a source of voltage, it is a current source.
As well as Power_Broker questions, what power are you expecting to get from your system?
Thanks.. Tom.. 
I will power the second arduino with the first arduino. So I need 3,3 V Thats why i need DC/DC convertor. The voltage range from the panel is till 9V.
I'm not sure WTF you're doing with the rest of that design. That diagram is less than clear.
But, sourcing dc/dc converters, that I can help with:
Ebay/aliexpress (search for dc dc converter, and either "step up", "step down" or "step up down" - the sellers put both "buck" and "boost" in the listing titles regardless of which it is) - this is the place to go for cheap dc-dc converters. They're not fancy, and they exaggerate maximum current spec, and the build quality is at the low end, but they're generally functional, and CHEAP.
Pololu - Pololu - Regulators and Power Supplies - significantly more expensive, but not crazy. Better build/design quality, higher specs, and they actually meet those specs.
Digikey - Digikey sells an impressive array of board-mountable dc-dc converters for integrating into other designs. For small ones, they have ones not much more expensive than the above sources, but they aren't mounted on a PCB or anything - they're meant to be used as a component on your PCB - which may or may not be desirable. For larger units, the prices can get steep though. I'm impressed with what's available now - last time I looked, prices were higher and selection was meagre (be sure to look at the package drawing before you buy - some of them are not hand solderable! One batch of affordable ones has a BGA-like package...)
Hi,
OPs diagram.
What are you trying to do?
Please read the first post in any forum entitled how to use this forum.
http://forum.arduino.cc/index.php/topic,148850.0.html
Thanks.. Tom...