Donwill has brought this to us :

It is the tacho line - open collector arrangement I believe.

So feed it with a small current from a pullup resistor and you should get
pulses proportional to RPM.

There's a 4 wire fan too - the 4th wire controls fan speed.

There are some diagrams here that may be of interest:

http://www.bearblain.com/fan_speed_control.htm

Cheers

Tim

Many thanks Tim the link is very useful. The sensor housing is going to be
installed in a not yet built conservatory, I was going to drive the fan with
the aid of a 1Watt 12V solar panel which is going to be installed in the roof
of the conservatory.
I wondered if the yellow lead was some kind of speed indication and if so,
the output would be related in some way to solar radiation intensity and
could therefore be used to control an automatic watering system for when we
are on holiday; it would be more accurate than a timed system I think.
I will have to integrate the pulses by some, as yet undetermined means to
enable a timed valve opening for watering.

Time to get the old scope out I think. :-)


Poke a couple of stainless steel probes in the soil, connect to an
op-amp, then arrange the op-amp to turn a solenoid valve on when the
conductivity between the probes falls - simples.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk