Look at the post after this one first!
Thanks for the pic it helps alooot. There is a few ways you can do this. The switches on the top of the biocube are not rated for 120v or 240 if your in UK so do NOT wire the switches directly in the cord going from receptacle to power supply. Also please let me know if you are in UK or US or w/e because if you are in uk you need to look on the power supply and there should be a little red switch receded in the casing that reads 120v and 230 or 240 or w/e. If your in UK it needs to show 230 or 240 or w/e it is. If your in US make sure it says 120 or 110 or w/e they say. If you plug it in before you check it and its wrong it will destroy the power supply.
Ok one option you can do is if you want to operate on 2 different switches you can take the 2 black wires coming from the potentiometer and wire each one to a different switch, HOWEVER it is preferred to wire the hot wire or red to a switch, so if there is only one red wire for each circuit then use it instead. Please use a high temp nut and use electrical tape. you can get both from most hardware stores. Make sure there high temp ceramic nuts. We call them baby blues. If you cant find the nuts you can wrap electrical type really tight and well and that will be good enough. As long as no bare wire is showing. Wire it this way will allow the fan to stay on 24/7 and control the circuits separately.
If you want both circuits to be operated by the same switch witch should also allow the fan to run 24/7 then you can take both the red or if you have to use negative, both of them from the potentiometer and tie them together to the switch. It should not increase voltage, unless there on different phases witch is really unlikely, however it will increase the amp load on those tiny wires witch will produce more heat and is not recommended, to much heat can cause overloading in switches or wire and either trip something or cause fire. If you only have 2 circuits it should not make that much of a difference. Its the same way even if you use the negative wires.
The third way you can do it is by using the wires coming from the power supply to the circuit board. this way the fan will shut off as well. you can do it either one or two switches like I mentioned before preferably the red wires. this will also shut off the circuit board witch increases life time.
The fourth way is to buy a timer or a 120v rated or 240v if your in UK, switch, and wire it to the plug end. This will turn everything including power supply, on and off, you can even buy a cheap slandered light switch and a box to mount on the wall or in w/e you want. If you want to do it this way let me know and ill give instructions.
