Found a circuit on the net that should do exactily what I want (control a cooling fan) but it is 'on' all the time. Not sure if there is an error with the schematic or if there is something else I've missed.
If the thermistor is 'cold' the fan should be off. As it heats the fan should come on. At the moment the fan is always on. I've double checked my wiring etc and am sure I have it as per pic. I have substituted R4 for a 10K trimmer to allow temp trigger adjustment.
Here is the circuit diagram:
UPDATE: Made a simulation (using Qucs) to see how the circuit should behave. I used the resistor's actual values I measured with the multimeter (see discussions below). Here is a screenshot:
(note: I could not find a fan in the parts bin so I inserted a diode for effect)
Could there be a terminal problem with the op-amp that is messing up the voltage levels? It is brand new, but it is not to say it hasn't been static zapped.
ANOTHER UPDATE: Decided to use Qucs to see what the circuit might do if the thermistor was 'heated'. Picking a value for R1 at random, it came up with this: This simulation shows the op-amp bias changing to produce a 'low' output, however, Q1's base is still high and causing approx 2.4V drop on the fan. For those following the conversation with @vicatcu below, this suggests that there may be a design floor in the circuit. Anyone know what else could be holding Q1 in the 'ON' position?
741 OP-AMP datasheet
UPDATE #3: Using some of the pointers given, I managed to make a working simulation of the circuit.
The top circuit is with the thermistor 'cold' and other than the leakage current, the fan is practically 'OFF'! The bottom circuit shows the thermistor 'hot' with a comfy 11.4V driving it. The trick now is how to achieve this using a single power source! I intended to use a single 12V power pack to drive the circuit. These circuits have dual supplies. I tried simulating with a voltage divider to split the voltage from a single source, however, when the thermistor drops when 'hot' it drags the voltage across the circuit to about 2V and the fan gets about 0.8V. Not exactly 'ON'. I do have some spare 9V power packs, so can use a 12V and a 9V pack to power the circuit in the above configuration, but if I can get away with a single source, that would be ideal. Especially if people in the future wish to build the circuit themselves.
UPDATE #4: Here is a rough plot of the thermistors resistance as temperature changes (in degrees celcius)