STATIC CONTROL OF MACHINES 259
The second statement is: Statement 1 will be true only if LS2 and T2 are open . The graphical picture for second statement is obtained by ORing inputs from LS2 and T2 followed by a NOT element as shown in Fig . 11 . 19 ( b ).
The next step is to combine the circuits for the above two statements . This is done as shown in Fig . 11 . 19 ( c ) . Further , next step is to simplify the circuit , if possible . In this case it is
done by using a NOR gate instead of an OR gate followed by a NOT gate . The last step is to analyse the circuit whether it works according to the specifications . According to Fig . 11 . 19 ( d ),
when PB1 is pressed , it sends a signal to OR gate through its signal converter . There is an output from OR gate , which is fed through an amplifier for energisation of contactor M .
If LS1 and T1 are both closed , there are two inputs at the AND gate . The third input is provided by the inputs from LS2 and T2 . If these two are open , there is no input to the NOR
element , and therefore it will have an output which is the third input to the AND gate . When all the three inputs are present at the AND element it will have an output . The output of AND
gate provides an input to the OR element and thus there is an output for energising contactor M . WITH REGARD
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