need temperature based circuit

Sorry Ween - I thought it was webpager that posted that circuit. webpager - no, it can;t be done with plain relays because you are comparing two resistances - ie, voltage levels; specifically when one is higher (or lower) than the other. But the circuitry should be simple - 2 resistors & thermistors as + & - inputs, 1/2 a LM393 dual or 1/4 of a quad LM339 comparator, and a MOSFET output (with 2 resistors) to turn on the fan or whatever load. It can also be done with CPUs etc - eg, PICAXE 08M2 or AT-tiny chips. Someone on mp3car supplies cheap ATtiny (or whatever they're called) PCBs... There may be commercially available units - you'll just have to search - but they'll probably be (more) expensive.