Sensor is not needed. Just install Mac's Fan Control and let it start with the mac.
http://www.crystalidea.com/macs-fan-control
Already covered in post 24 - not something the knowledgeable user tends to employ. You have your view, and that's all it is. If you are content to use a software solution then that has to be your decision. I would however caution against encouraging others to use this more risky procedure.