SWITCHES (includes relay contacts):
Q: Does a closed switch close and stop the electrical flow?
A: No, it refers to the two internal metal contacts touching together.
As soon as contacts touch, switch is "closed" and current flows.
When contacts open, no flow.
An analogy is a drawbridge, where closed is down so traffic can pass, and open is up - no pass.
Switches/relays come in various forms, depending on application required.
Examples are push-buttons, toggle switches, rotary selectors, knife switches (mostly obsolete), reed relays, tactile switches.
The contact configuration varies greatly, such as:
PTM = Push-to-make refers to a normally-open push-button or tactile switch which may be latching or non-latching type.
SPST = Single-pole single-throw is one set of contacts. 1 in 1 out. As on a standard single-light switch.
SPDT = Single-pole double-throw is two contacts joined at a commmon point. 1 in 2 out. Directs voltage IN to to A.out or B.out.
In reverse, it selects either one of 2 inputs A or B and sends it to common OUT.
DPST = Double-pole single throw. 2 independant contacts controlled in sync by one operator, ie button or relay coil. 2 in 2 out. Think geyser isolator for L+N.
DPDT = Double-pole single throw. Two sets of DPST contacts each joined at own commmon points. 2 in 4 out. Often used for polarity reversal.
Multipole selector = A common point connects to one or more selected outputs via rotary or slider switch. Voltage selection, speaker select, etc.
Switch tricks:
By combining various switches or relays, just about anything can be controlled remotely.
At each end of a corridor or staircase, connect 2 SPDT face-to face with two wires between them, ie dual out to dual in.
Feed live to switch A's common IN, take live from switch B's common OUT to lamp, then from lamp to neutral return to complete the circuit.
When both switches A&B synchronize in either direction, ie 1-1 or 0-0, light "Q" is ON.
This is actually a logic XNOR circuit; Exclusive Nor; aka Not Xor.
Truth table:
A.1100
B.1010
Q.1001
12,3,3,9,
