Schematic explanation

[bryan]

Thanks MrAl, very helpful.

I will only be using it between a range of 30-50 degrees Celsius so. Having a read of the web site again, and I see no mention of it being designed for a automotive application, although perhaps at one time it was. I did note in the web site that it can be powered by any 12v AC and DC source?? so perhaps the zener circuitry acts as some type of rectifier for AC.

 

[MikeMl]

Bryan, are you married to using the LM35 as the sensor? If all you want to do is turn on a fan at ~35C and turn it off at 30C, a thermistor is much simpler to use. The LM35 is good in an application where you need apriori calibration of x mV at y temp, but if you can calibrate the turn-on trip-point by measuring the temperature independently using a thermometer, and then setting a pot to where your thermostat turns on, a thermistor might be easier...

 

[KeepItSimpleStupid]

OK, my take: R6, C2 and ZD1 is basically a Zener diode shunt regulator.

D4 is for reverse polarity detection.

D2 and the one in series with it is to give you an ersatz -1.2 V supply on A1.

 

[bryan]

Bryan, are you married to using the LM35 as the sensor? If all you want to do is turn on a fan at ~35C and turn it off at 30C, a thermistor is much simpler to use. The LM35 is good in an application where you need apriori calibration of x mV at y temp, but if you can calibrate the turn-on trip-point by measuring the temperature independently using a thermometer, and then setting a pot to where your thermostat turns on, a thermistor might be easier...

I just happen to have most of the components, so thought it would be a easy project. Will breadboard it up and see if it works. One advantage of the circuit is it can be easily adjusted by measuring the voltage at the test point 400mv equals 40 Celsius.

 

[bryan]

Breadboarded it last night and the circuit works as described in the link. So good to go.