Street
lights can be made to turn ON automatically when night time comes. Similarity
they will turn OFF automatically when day comes. This system can be done by
electrical control which utilizes a photo electric control device illustrated
in figure below (A). Figure below (B-B1) shows how simple schematic diagram of
the photo electric control.
(A) Actual
photo electric device.
Control
operation
(B) Simple schematic diagram of a photo electric device.
At night
time when sunlight does not strikes the photo electric control device, the
resistance of the photo resistor PR inside the photo electric device is high,
see figure above (B). The current I that will pass through the thermal device T
will be low. This low current I will not sufficiently heat and make the thermal
device T to expand. At this condition, the normally closed contact B which is
coupled to the thermal device T will remain close to energize contactor C. with
contactor C energized, contacts C (1-2,3-4,5-6) will remain close to energize
(turn-On) the street lights.
(B1)
Simple schematic diagram of a photo
electric device.
At day
time, when sunlight strikes the photo electric control device, the resistance
of the PR is reduce. The current I that will pass through the thermal device I
increase and cause the temperature of the thermal device I to rise up. With
rise up temperature, the thermal device T will expand and push the normally
closed contact B to open. With contact B open, the contactor C is de-energize.
Contact C (1-2,3-4,5-6) will open to de energize (turn OFF) the street lights.
(C)
Control circuit of a photo electric device with street lights loads connected
to 3-phase power source.
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