Electro-mechanical signal processing is primarily undertaken by relays or contactors. The relay consists of the coil, the magnet core, the armature with the contacts. When a current flows through the coil, a magnetic field is produced and the armature picks up. The relay switches. A contactor works according to the same principle, but it is designed to switch power circuits. Back to the relay. How the relay processes the control signal depends on the type of contacts and how these are connected. Relays can be equipped with several contacts, normally change over contacts. This means that one control signal can cause several circuits to be opened or closed simultaneously. Here for example, the relay is connected in such a way that it has one normally open, one normally closed and one change over contact. Although the relay actually has three change over contacts. You can see that pressing the button results in three different simultaneous actions. In pneumatics, solenoids of 0.5 to 12 watts are used. In hydraulics, solenoids range from 12 to 50 watts. As a result, valves have to be controlled indirectly via relays. Special types of relays have other functions in addition to the circuit switching function, counter relays and time delay relays. This pickup delay relay does not switch until the control signal has been present for a preset time. This time is adjustable. The elevator door is an example. When it is closed and the door contact gives a signal, there is a pause before the elevator is set in motion. The dropout delay relay works the other way round. When the control voltage in the coil is interrupted, there is a pause before the relay drops out. Here is one in a car. The door contact interrupts the control circuit and the light goes out after a short pause. Counter relays switch circuits when a certain number of counter pulses has been reached. This number can of course be set. They are used for example when a step is to be repeated a number of times before the next step is started. The hydraulic cylinder is not to be actuated until the pneumatic cylinder has extended and retracted three times. Here the relay counts the pulses from the cylinder limit switch. The relay does not switch the solenoid valve for the other cylinder until the preset number of pulses has been reached. Electromagnetic signal processing by relays is part of the topic hard-wired programming, which is covered in a separate section.