Components in Pneumatic Electrical Signal Control Units

Mechanical Signal Elements:

Mechanical signal elements, switches, have the effect of supplying power to a load (e.g. a solenoid valve) within the circuit or interrupting the power to it. Basically, three types of switches can be distinguished:

Momentary-contact switches, whereby the selected switch setting is maintained only as long as the switch is being actuated (example: a doorbell)
Latching switches, whereby the possible switch settings are latched mechanically so that one switch setting is maintained until the switch is reactivated in a new setting (example: living room light switch)
Limit switches (momentary-contact limit switch). a particular variation of the pushbutton switch

Momentary-Contact Switches:

The following graphics show a momentary-contact switch in the form of a normally open (NO) contactor with the relevant circuit symbol according to DIN EN 60617-7 (right - switch operated by manual pushbutton). In its non-actuated state (rest setting) the NO contactor of the circuit is open as normal. If the pushbutton is actuated, the circuit is closed. After releasing the button a spring restores the switch to its rest setting and the circuit is opened again. Bear in mind that even the numbering of the switch contacts have been standardised (to 3 and 4 for this type of switch ).


The following graphics show the design of a momentary-contact switch in the form of a normally closed (NC) contactor with the relevant circuit symbol. In the non-actuated state (rest position) the NC contactor of the circuit is closed as normal. If the pushbutton is activated, the switching contacts are separated thus opening the circuit. After releasing the pushbutton the spring restores the switch to the rest state and the circuit is closed again. NC contacts are used, for example, to ensure that the interior light in a car goes off when the doors of the vehicle are closed.


changeover contact represents a combination of NO and NC contactors (see the following graphic). When the momentary-contact switch is operated a circuit (from contact 1 to contact 2) is opened while at the same time another circuit (the one from contact 1 to 4) is closed. Normally when switching over, a break in the circuits occurs (if only momentarily). The right-hand circuit symbol shows the changeover contact in the actuated operating state, as symbolised by the double arrow.



Latching Switches:

In contrast to a momentary-contact switch, in a latching switch the switch setting assumed is also maintained after release. This type of switch can also be designed with NO, NC or changeover contactors. Furthermore, they come in various types of housing (e.g. as a pushbutton, rocker or rotary switch). The following graphic shows a example of such a switch in the form of a normally open pushbutton.


Limit Switches (momentary-contact limit switches):

Limit switches constitute a particular variation on the momentary-contact switch. A limit switch is actuated by a machine component or a tool assuming a particular position (e.g. a cylinder piston reaching its end position). As a rule this is performed by cams. Limit switches can also be designed with NO, NC or changeover contactors. The following graphics illustrate the design of a limit switch (left) as well as the circuit symbols for an NO contactor (centre) or an NC contactor (right). We can see that the limit switch is depicted with an inverted arrow in the shape of a right-angled triangle drawn directly onto the switch contacts of the circuit symbol. The limit switch symbols depicted here do not provide any information as to switch actuation, which, if needed may require additional depiction.



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