a) is connecting sensors in star formation ok, i guess it's a form of parallel, series would be like the old daisy chain? I guess parallel will allow for constant voltage so to speak.
b) i will give digital and analogue there own + but they must all share the same (common) ground?
c) If my input is 12v into the jack, that then also shares the same common ground as the sensors on the board, which also shares the same common ground as the atx psu, but one rail?!? will be 12v and the other 5v? Does this sound ok. I know i could give the arduino 5v, but want to keep that as 12v.
Regards
Before deciding about power, figure out which sensors you will be using.
Everything must have a common ground.
Cheers, yeah this is what i was hoping to use
1 x Light Intensity Sensor Module 5528 Photo Resistor
1 x RTC Real Time Clock DS3231 I2C AT24C32 Board Module
2 x DS18B20 Waterproof Digital Probe Temperature Sensor
1 x DHT11 Digital Humidity & Temperature Sensor
1 x Soil Hygrometer with a LM393 driver board
2 x 5V 2 Channel Relay Module,
Always put this kind of stuff in parallel. Look at Kirchhoff's law for voltage to know why.
Also, the DHT11 is ... suboptimal. The DHT22 is MUCH better.
Thanks Elcaron, where's the thank you button!! Yeah sub optimal should be ok, but i might look a little further into the accuracy.
Regards
It is the [add] link next to the karma on the left 
Changing the DHT11 for a DHT22 is just a matter of changing one line in the code. It is also a little bigger, physically. I remember using the DHT11 once when I started with this stuff, and I only remember that it was considerably off to a point that I found it unusable. And I don't think that was a high precision application. Regarding humidity, I think I remember deviation >10%.
Yey gave you some karma, best go back and give some more love!! Thanks 10% could be doable but for a few extra pounds i guess it's better to start the project as you mean to go on!!
Ok i looked at that Kirchhoff's law, only briefly as it was getting late, But i can see roughly where it makes sense. I guess with a star connection nothing has been taken into account one at a time if that makes sense, where in parallel it's almost like the voltage/current meets one load, then moves onto another rather than the whole loads being connected at one point. And i guess it's always connected through both polarities at the same given point. Maybe i have miss understood it. But thanks
In these regards, you can view electric current almost like water in pipes and reservoirs.
Voltage is then pressure of the water (e.g. generated by a pump, or reservoirs at different heights) and current is the amount of water that flows through the pipe. Resistance is the diameter of the pipe.
Then Ohms law R=U/I or in another form I=U/R then means:
The amount of water that flows through the pipe is proportional to the pressure and antiproportional to the diameter of the pipe. Makes sense, doesn't it?
The power law P=U*I means that you can either drive a turbine with a lot of low pressure water, just a little water very high pressure.
Kirchhoff's voltage law means that if you have small pipes in series, the pressure will drop after each small pipe and in the end, the sum of the pressure drops will be equal to the overall pressure difference between beginning and point. However, the same amount of water will flow through all of them (obviously).
Kirchhoff's current law means that if your pipes split, then the amount of water that flows towards the junction is equal to the amount of water that leaves the junction, distributed over the multiple pipes. However, they all get the full pressure.
Since you have multiple pipes, overall more water will flow through compared to just one pipe.
So in your case, if you put you sensors in series, they all just get a fraction of the voltage (voltage law). If you put them in parallel, they will all get the full voltage.
Regarding topologies: This is something completely different from series or parallel. For example, both the old daisy chained BNC network and the modern switched networks are parallel. The T pieces connect inner and outer conductor separately to the computer. This is merely how you run your cables, it has in first approximation no influence on your circuit.
Yes, how you explained ohms law is exactly how i use it in my head!! How you explained Kirchhoff's laws is bloody excellent. When i looked it up i saw a circuit with loops, yet each one had it's own power source, which i found confusing, so thank you for clearing that up.
Just to clarify a star connection one for each of the positive and the ground, i.e one wire divided into 8 for the positive and another for the ground, and say the same length, ran to the sensors or a load, does this make it series or parallel or something else? When i have looked into star connection i get 3 phase stuff pop up.
Regards and thank you very much a true star.