Question

Design a PLC program that satisfies the following conditions (latched interlocks):

Momentarily depressing SW1 turns on light 1; light 1 will remain on even after SW1 is turned off. Momentarily pressing SW 2 turns on light 2; light 2 remains on even after SW2 is turned off. Light 2 can only be energized if light 1 is already on. Momentarily pressing SW 3 turns on light 3; light 3 remains on even after SW 3 is turned off. Light 3 can only be turned on if either light 1 or light 2 are already on. Momentarily pressing SW4 turns all lights off. After the circuit is turned off it is immediately ready to start again.

Answer 1

As shown in the Ladder Logic, SW1, SW2 and SW3 are NO Push Buttons. While SW4 is NC Push button. Lights are represented by L1, L2 and L3. A, B and C are latching relays used to hold up the latched interlocks or contacts (shown by NO contacts a, B and C) corresponding to A, B and C latches respectively. Negative is connected directly as shown.

When SW1 is pressed the Light L1 turns on and also the latch A, which in turn picks up the latched contact corresponding to A. Now even if push button SW1 is turned off the Light L1 is on as the +ve is connected to L1 through the contact A parallel to SW1.

Now if L1 is ON, contact A is in closed position, so if SW2 is pressed L2 will be turned ON. This also turns on the latch B, which in turn picks up the latched contact B. So even if SW2 is turned off the light L2 is ON.

Now if either L1 or L2 is ON we have either contact A or B in closed position. So we can turn ON L3 by pressing SW3. On turning ON L3 the latch C turns on and holds the contact C in closed position, so that even after releasing the switch SW3 the light L3 will be in ON condition.

Now if the switch SW4 is pressed, the +ve supply to all the lights L1, L2 and L3 and the latches A, B and C are cut off. So the contacts A, B and C are opened. So all lights L1, L2 and L3 are turned OFF and we have to start from the beginning again.