Lesson 9 Relay and Ladder diagram

Relay: There are 2 switches and 1 coil in a relay. The switch may be turned on by

the magnetic force of the coil. Each switch have 3 pins, named COM, NO and NC

Do the following projects on bread board.

(1) Connect relay’s coil with 4.5V power. Listen to the sound of relay。

(2) Add a 300Ω resistor and a LED. Turn on the LED with the switch in relay

What is the difference between these 2 circuits?

(3) AND circuit. Press switch 1 AND switch 2. then the LED shines(use 2 relays)

(4) OR circuit. Press switch 1 OR switch 2, then the LED shines (use 2 relays)

(5) Self keep circuit. Press the switch and loosen, the LED keep shining

(6) LOCK circuit

There are 2 LEDS. If Red LED is turned on, then the green LED cann’t be turned on

until the red LED is turned off..

The upper circuit can only lock green LED. Haw can the circuit lock each other?

Try to draw the ladder graph before doing the exercise..

(7) What is the result of the circuit? Try to finish the circuit.

(8) Erasable self keep circuit

Press switch 1 and loosen, the LED keep shining until press switch 2..

Exercises

Draw ladder diagram at first, then draw components and wires on the relays below.

Then do the experiment on breadboard.