Are there AC sensors?

Question

Electronics

Gentlemen, tell me, are there sensors that operate on alternating current or do they all need only constant current?
We are talking about hall sensors, smoke detection temperature sensors, etc. , which are powered by an alternating current network.
And yet, for example, a pressure sensor, if the limit is exceeded, is it designed for direct current by closing the contacts?

Answer 1

There may be sensors, but in order to process signals, you probably still need to use a constant voltage, because. logic (microcontrollers are the same) will ask for a constant from 3 to 15 volts (depending on what we use).
Although some thermostats will quite well close / open the 220V circuit themselves when a certain temperature is reached. :)
Usually, the mains voltage is rectified and lowered to the power levels of the entire logic, and if it must switch a powerful mains load, then it does this through relays / thyristors.
One of the cheapest options, if you do not touch the sensor with your hands and it is not connected to anything with wires - damper capacitors + diode bridge, but there is no galvanic isolation from the network ...
There are ready-made compact low-power modules 220AC-> 12DC and 220AC-> 5DC, like these . A fairly good and compact power supply, sealed, with decoupling from the mains.
Well, and another assortment of power supplies for every taste, color, voltage and power.
Regarding the pressure sensor - it is not one for all occasions, there are no psychics here. Name or photo is required. By themselves, pressure sensors only output it in analog or digital form, but do not switch anything, if it is paired with logic and with a relay, then you need to look at the characteristics of the relay, it can already pass through itself constant / variable in accordance with its characteristics.