Motorhome Electrical Systems continued --
I assume everybody is sufficiently familiar with switches and, to a lesser extent, relays so I'll just hit some of the aspects that tend to be confusing or less familiar.
(If you have questions regarding switches and relays, please PM me rather than asking via a thread post. I may already be planning to answer the question, may answer your question via a reply PM, or may quote your question and answer via a thread post if I think it is of interest to a wider audience.)
Switches and relays are categorized as normally open (N.O. or NO) and normally closed (N.C. or NC). This designation is especially important for momentary switches and most relays. If the designation is missing from the description, you can assume the switch or relay is N.O.
The meaning of the two terms is simple. If the designation is N.O., it means the switch or relay, when not activated (pressed or energized), "opens" the circuit and prevents electricity from flowing through the circuit. N.C. means the opposite, the switch or relay, when not activated, "closes" the circuit and allows electricity to flow through the circuit.
Switches and relays usually have another designation in the form of xPxT, with both x replaced with a S, D, or occasionally a number. (For example, SPDT, DPST, 3PST, etc.)
The xP refers to x poles, which is a fancy way of saying x circuits. SP is single pole, meaning the switch or relay controls only one circuit. In turn, DP is double pole, meaning the switch or relay can simultaneously control two separate circuits. (3P means simultaneous control of three separate circuits and so on.)
The xT refers to switch or relay positions. ST is single throw, meaning a simple ON-OFF switch like most of the light switches in your house. Just as with poles, DT is double throw, meaning an ON-ON switch or relay. (Be careful, ON-ON switches are sometimes labeled as ST and ON-OFF-ON switches are technically 3T switches but are often erroneously labeled as DT switches.)
Multiple position switches (3T, 4T, etc.) are common. However, almost all relays are either ST or DT, due to technically difficulties in making multi-position relays and a general lack of need for relays with multiple positions. (Furthermore, electronic circuits can easily be created that satisfies the need should it arise.)
So, momentary SPST N.O. is the complete technical description for a simple pushbutton switch.
Note: N.O. and N.C. is important in momentary switches, with the designation being meaningless in all other switches that are not "spring-loaded." On the other hand, the designation is important for relays, especially xPDT relays to identify which terminals are normally open and which are normally closed.
In addition to simply turning a circuit on or off, switches and relays can be used to create complex circuits involving a degree of logic, just like simple computer circuits. For example, I have a circuit using a relay to light up a bright red LED if the headlights are on when the engine's not running. The pseudo-code statement is: IF NOT (engine running) AND (headlights on) THEN (turn on LED). (Buzzers/chimes are annoying so I'm using a LED instead.)
