Simple Fan control

Often there is the need to control the on / off cycle of heathsink's cooling fans in order to obtain a silent operation and avoid premature mechanical stress
on the fans. Recently I have bought a chinese dual power supply for my lab, its a cheap but robust linear design but it has two very noisy fans always on.

While these days its very simple to obtain such a function using an Arduino nano and a few components, operating in a RF sensitive enviroment I wanted
something that couldn't create any RF noise. I searched my home lab's drawers for inspiration and came out with the following simple design very '80s style.

The LM35DZ its a well known precision temperature sensor, delivering 10mV / C  (eg. 25C = 250mV). The LM311 is a precision comparator, but here
I think any OP could be used with simple modifications. When the voltage from the LM35 surpasses the reference voltage (Fan ON threshold)  its output activates
the NE555 in monostable configuration. The "ON" time   is given  T= R + C *1,1 d, values given are for a T abt 1 minute.

Board placement inside the power supply:

30 nov.2016