Make cheap fan control from old thermostat p.1

If you are not skilled to make changes to mains powered equipment do NOT attempt this. In a shed you need to have a qualified person check your work before connection. Fortunately I made this change well before the new regulations came into force.

I wanted a temperature control for my shed workshop to switch on a fan when the temperature got too hot, like 30%, even when the shed was not in use.
Such a "Fan Controller" would be rather expensive - the sort of thing you might fit in a greenhouse. Not being one to throw money away, I was looking for a cheaper alternative, and there are plenty of old central heating thermostats to be had in Car Boot Sales (Yard sales, flea markets...).

There is one small snag - they turn the power OFF when the room gets hot. I wanted one that turned the power ON!

If you are lucky, manufacturers of thermostats generally fit universal microswitches, and they tend to have a double switch - when one switches on the other switches off. If you are unlucky you get one that works like Form B left which simply breaks current on actuation. However I picked up three thermostats from a Norfolk Boot Sale and 2 of the 3 were of Form C with a Make switch.
This Drayton RTS1 unit has an electronic sensor and a good Form C microswitch with a 2amp capacity - quite enouigh for my 240 Volt fan unit. It cost me 25pence.
If you turn the circuit board round you can see that although the switch has make and break contacts, only the break contact has a pin soldered to it.
Similarly the casing with the mains terminals only has contacts for the break contacts.