TRUTH TABLES AND EXERCISES
Answers to example 1.
The safety signal must be received for greater than 10 minutes before it will pass
through the time delay. If the safety signal is removed before 10 minutes has elapsed
no signal will be passed to Gate 2.
Gate 2 will turn on when the hand-switch is in the AUTO position and a safety signal
has been received for greater than 10 minutes.
If flow switch (FS) 30-38 senses less than 20,000 cfm, 45 seconds after the fan has
started, or the same condition exists on the 1B-B fan, the alarm will sound.
AND Gate 2 prevents the on signal from passing until a safety signal is also received
Ten minutes after receiving the safety signal, the fan started. At 12 minutes, removing
the safety signal only removes the continuous start signal to the fan. The fan will
continue to run until the hand switch is placed in the stop position. Further, with the
removal of the safety signal, the fan will remain stopped when the hand switch spring
returns to the AUTO position. Note that if the hand switch is placed in the stop
position while the safety signal is present, the fan will stop, but will restart as soon as
the switch spring returns to the AUTO position.
It can be started by two signals - START and AUTO plus a safety signal.
It can be stopped by one signal - STOP (but will only remain stopped if no safety signal
is present or the switch is held in the stopped position).
Answers to example number 2.
a. 1 & 4
Right input is - on - this is because the hand control switch is in the AUTO position, and
the AUTO switch contacts are made up, resulting in an on signal. Therefore the hand-
switch CLOSE position contacts are open, resulting in an off signal. The off signal is
reversed in the NOT gate and becomes an on signal.