Due to the lack of replies, im guessing the answer must be no. Cause I think the Arduino cant take 21v 5w input and no-one wants the headache of transistors.
I wouldn't draw that conclusion at all. You've thrown together a daunting list of questions, suggestions, and requirements there.
Firstly it wouldn't be ideal to put 21V into the Arduino certainly. It's higher than the top voltage specified on the on-board regulator, but even if you drop it slightly to get it into spec, that's potentially a huge amount of power wasted generating heat for no return. And ideally you'll be chasing efficiency so you want to avoid using the Arduino's onboard linear regulator in favour of something less wasteful.. So to power the Arduino you want that to be off a lower voltage, point 1. Point 2 I don't understand, but then maybe I've never suffered one of these transistorised headaches of which you speak
Can I use transistors to switch on and off the charge either when the battery is full or when the solar pannel isnt producing enough power? Been trying to find out about 1N60 Zenner diodes. Is the lm 393 any help for compairing against a known voltage or something? I have switches and leds too, maybe good for a shunt or something??? Got about 150 1N4148's
You can certainly use a transistor to act as this switch. A lower power method would probably be found using a MOSFET, however if you have a stack of components on hand it's best you read up the datasheet on each of the items you have to see which would be appropriate.
Since you're asking your Arduino to be solar powered there are lots of sources online that talk about how that's done, and do read Nick Gammon's excellent article on running an Arduino with the least possible power
so your control circuit spends as much time asleep sipping as little power as it can, so you get the most of what comes off your PV.
ps... Would be good if I could just add aditional solar pannels without having to change the design.
Really need something I can throw together in about 2 mins.
That sounds like a dream, and is possible within limits. PV panels in series add to the total possible voltage, and in parallel add to the total possible current, just like cells in a battery. If you keep within the spec of your system you can add to your PV array for sure however once you achieve the input limit of your charger on a bright sunny day, the output clips so power is simply lost. Adding more PV at that point would be theoretically useful for low light days when panels aren't at full output, but in reality it's the law of diminishing returns and greater gains will be found for less money seeking efficiency in the consumption side to stretch the output further.
Good luck with your project,