EXPERIMENT #4
PHOTOCELL RELAY
NAME________________PER____
PURPOSE: To study the basic
PROCEDURE:
operation of a photocell relay circuit.
1.Connect the circuit as shown below.
NOTE: You will recall this circuit from
experiment #1.
SUMMARY OF MATERIAL:
The photocell relay has many
uses. One typical use is to control light
by the occurrence of darkness. Such a
circuit will turn on lights at dusk and
turn the lights back off at dawn. Such
units are frequently used in streetlights
to control the turning on and off of these
lights. Another use of the photocell
relay is the intrusion alarm, in which the
relay functions when a beam of light
between the source light and the
photocell is interrupted.
In this experiment you will
connect a simple photocell relay using a
light sensitive photo resistive element.
This photocell changes its resistance
with a change in illumination. The
resistance increases as the illumination
decreases, and the resistance decreases
as the illumination increases. The
resistance of the photocell determines
the amount of current that passes
through the coil of the relay. When
sufficient current flows through the relay
coil, the relay is energized and the
contacts will close.
MATERIALS NEEDED:
1. The Expermenter
2. Small Flashlight
2.Turn on the power switch. What does
the meter “read”? ________ma.
Shine a source of light on the photocell
(room light or flashlight). Does the
amount of current passing through the
photocell increase or decrease with an
increase in illumination on the
photocell?
3.Turn off the power switch. Remove
circuit you have been using and connect
the circuit shown below:
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#4/Page 4
NOTE: In the circuit you have just
connected, you are using the photocell to
control the amount of current that is
passing through the coil of the relay.
4.Turn on the power switch. Does the
left lamp or the right lamp light?
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The lamp indicates the relay is nonenergized.
5. Shine a source of light on the
photocell. Does the relay make any
sound? _____________How would you
describe the sound?
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6. With a source of light illuminating the
photocell, which lamp is lighted?
____________
This lamp indicates that the relay is
energized.
7. Pass your hand between the source of
light and the photocell. What happens to
the relay?
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What happens to the lamps?
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SUMMARY:
1.What is the function of a relay?
2.How could you use a photocell relay in
your home?
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