Question

Draw a compact ladder logic diagram for a “push-on/push-off” program using only contacts and coils.

Answer 1

Please refer the Ladder logic diagram attached below. It labelled as follows

the input of push button as - I 0.0

Output (Bulb as indication) - Q 0.0

Q 1.5 and Q1.6 - Represent signal status

OPERATION :

PUSH ON MODE:

When the push is pressed,it turns the NC (Normally Close Relay) and the relay Q 1.5 become energized,however other another Flag relay Q 1.6 ,does not energized since out coil contact Q 0.0 is N-O status (Normally Open Status). Hence it cut the ladder logic circuit at Q 1.6 flag. Once Q 1.5 flag relay is energized, the contact Q1.5 turns to N-O(Normally Open) and output contact Q 0.0 become energized and it turn on the bulb as indication

PUSH OFF MODE:

When the push button is released by pressing it again, if it is in the ON position, then coil Q 0.0 still in energized mode since latching used as latching contact Q 0.0 (Normally open) as shown logic ladder diagram last rung ,Hence , while switch off the power goes to the bulb again ,push button has to press again, that closes N-O(Normally Open ) contact of output coil of Q 0.0, as a result it energize the flag coil Q 1.6, hence it turns the N-C contact of Q 1.6 ( in last rung of logic ladder diagram (please refer attachment) ) to N-O(Normally Open) contact.Consequently the last rung false signal and Q 0.0,the output coil is de-energized and finally bulb turns to OFF position