The sensors may have, only now, whatever signal processing you probably will still need to use a constant voltage, since the logic (the same microcontroller) will ask for a tenure of 3 to 15 Volts(depending on what use).
Although some thermostats
are quite good themselves will close/disconnect from 220V when reaching a certain temperature. :)
Typically, the network voltage is rectified and reduced to levels of power all the logic, and if she should commute a powerful network load, it does it already via the relay/thyristor.
One of the cheapest options if you do not touch the sensor with your hands and it is not connected with any wires - damping capacitors+diode bridge, but no galvanic isolation from the mains...
There are ready-made modules compact, low-power 220AC->and 12DC to 220AC->5DC, tipo such
. Quite good and compact power adapter, sealed, isolated from the network.
Well, other range of power supplies for every taste, color, voltage and power.
About the pressure sensor - it is not one at all occasions, psychics are not here. The name or the photo should be. By themselves, the pressure sensors only give it in analog or digital form, but do not commute, if it is combined with logic and relays, it is necessary to look at the relay characteristics, it can pass through itself already postoyanku/recess, in accordance with its characteristics.