Datasheet-1

Project 69 Relay
Build the circuit and turn on the slide switch (S1); the lamp
(L4) is on. Now push the press switch (S2) to activate the
relay (S3); you hear a click as the relay switches power
from the lamp to the white LED (D6).
You can replace the white LED with the motor (M1) and fan
if you like.
With the press switch off, the relay is
off, so the 1 and 2 contacts are on
the relay are connected, and current
flows through the lamp.
With the press switch on, current
flows through a coil in the relay,
which magnetically switches the
relay’s contacts. Now the relay’s 1
and 3 contacts are connected, and
current instead flows through the
white LED.
The current through the press switch activates the coil in the relay, which
changes the relay connection from contacts 1-2 to contacts 1-3. There is no elec-
trical connection between the relay’s coil and 1-2-3 contacts, so those circuits do
not affect each other, and can operate at different voltages. In this way a low-
voltage circuit can control a high-voltage or high-current circuit.
Most industrial machinery and home appliances operate at voltages of 120V or
higher. However, the circuits used to control them (either automatically or by in-
terfacing with people) operate at low voltages. These voltages are usually less
than 6V and very rarely higher than 50V. Relays allow these low voltage devices
to control high voltage machinery and appliances.
List some devices in your home that use relays.
What is the main reason to use a relay?
Relays use magnetism to open or
close a mechanical switch. Look at
the relay symbol in the drawing.
The relay contains a coil, and a set
of contacts that are switched when
the coil is activated.
Coil
Contacts 1-3:
Coil is off: 1-2 connected
Coil is on: 1-3 connected
Relays are electronically controlled switches, which allow a low-voltage circuit
to control a high-voltage or high-current circuit.
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