Schematic Drawing of Module

The module makes use of modern technology, yet is quite simple. IC1C is a voltage stabilizer. This voltage is presented to pin12, via RV1.

Another arm of the voltage goes to the sensor and is presented to pin 13. With two equal voltages at pins 12 & 13, this causes IC1D to switch the supply voltage to the base of T1, thereby holding the switching transistor off. {After Transistor Change}

When the Sensor NTC 10K0 'Sees' heat the Negative Temperature Co-efficient resistor reduces it's resistance and sends a greater voltage to pin13. This causes an imbalance of voltages on Pins 12 & 13, Pin 13 going greater than pin12. this in turn causes IC1D to turn off at pin 14.

In my installation, SK3 of the relay is used to switch the fan on.

 *My only concern with this module is that the relay it is turned permanently.  This takes more current in the quiescent state, and is not a condition I have found in my experience.

    I have changed T1 for an PNP transistor, which now turns on the relay when the module 'Sees' the fridge heat. This arrangement reduces the current used by the module at rest by 60%. All current saved in a Motorhome is a bonus.

For those not keen on modifying the module, it can be left in it's original state as it is quite mean with current used.

While in Portugal this year, [2005] we found the adjustment for the fan on/off could not be controlled, it was too sensitive. I have changed R6 for a 100Kohm variable resistor and R5 for 56kOhm. The adjustment is now in the half way position.