Although the previous posts described the two wire, 4-20 mA transmitters’ internal workings, sometimes a simpler action is adequate.
The first and pretty obvious example is the temperature switch (which is actually a bimetal). It doesn’t need any power supply, it makes (or breaks) a contact if the temperature reaches a certain threshold.
Another example is the float switch used in water tanks. it also makes (or breaks) a contact if the water level reaches the given level.
If the power they can deliver is insufficent, then these “sensing devices” usually operate a relay, and the relay’s contact switches on the higher power mechanism.
For example a reed switch based float switch (used in a water tank) can only switch a maximum of 0.5 A, but the water pump (operated by this float switch) needs multiple amps for its operation. In this case the float switch operates a relay, and the pump’s multiple amps flow through the relay’s contacts.
On the other hand there are more complex issues to be solved, as simple as can be.
Let’s see the following situation: There is a garden with an automatic sprinkler system. This system has a rainwater storage tank, a water pump, sprinklers and a soil humidity sensor. If the soil moisture falls below a certain level then the sprinklers begin to operate, and if the moisture rises to an other certain level the water pump stops.
It is a simple hystheresis function with adjustable low (turn on) and high (turn off) threshold.