Boolean logic or relay logic can be used in a simple sequential control. For example, let’s say we want to turn (turn on) a pilot light if either a fan or a fan of B work. To detect the status of the fan mounted differential pressure switch on the fan. The switch contacts will open if the pressure is low and the fan will shut down if the high-pressure fan. To make the pilot light turns on when the fan is on, then the fan is mounted parallel contact as shown in Figure 2.15.
If we want to turn the pilot light when the fan A fan B and active, then both the switch contact must be installed in series.
Figure 2.15 OR Logic relay (A or B) and AND (A and B)
With the notation of Boolean logic, if the pilot light interruption event labeled C, then the next two sequential events can be written sebgai follows:
If (A or B) then C (a parallel circuit – row 1)
If (A and B) then C (series circuit – line 2)
Say, we want the pilot light is lit when one of the fan is working, but when the second fan will not turn on the lights work. For this purpose single-pole switches needed double-through (SPDT) as shown in Figure 2.16.Terlihat in the picture, how to contact the double-through can be used to turn on lights when a fan works. But when the fan works even light goes out. This logic can be expressed as follows:
If (A or B) and NOT (A and B) then C
SPDT contacts are widely applied in the two-position thermostat, and differential pressure switch.
Figure 2.16 Symbol Contactors