Sometimes, it is simpler (uses fewer gates) if a value within a circuit can be changed
to the other state, the item for this (Sk 5) is an inverter, with this a 1 input
results in a 0 output and visa versa.
The output from a logic chip is low power and as a result is unable to power such
items as a relay coil, contactor coil, solenoid valve or say an indicator lamp. In
this case the connection between the logic elements and such external devices would
be a buffer to provide the additional power, Sk 5.
As the buffer symbol shows only an input and an output you may ask, from where does
the extra power come? Actually, in addition to the logic inputs and outputs, all
logic gates are connected to the positive and negative power rails. These are normally
left off the diagrams to avoid overcomplicating them.
Whilst there are many other logic IC's, Counters, Oscillators, timers, etc. the gates
mentioned above form the basic building blocks.
To illustrate a system using logic gates I have chosen an application that will be
familiar to viewers, that is a control system for a simple drive motor. In this there
are the following external controls, Run, Jog and Stop push buttons, a guard limit
switch and a motor overload device.
Sketch 6 shows the circuit marked up with the binary numbers for four states, 1 initial,
2 Run pb. pressed, 3 Run pb. released and 4 Stop pb. pressed. With the Stop pb. released
it reverts to state 1.
Note that gate AG1 is controlled by the three items that will stop the motor, or
prevent it starting. However, being a four input “And” gate the spare input requires
to be connected to make it a permanent 1. See how the output from AG1 feeds both
“And”gates, AG2 and AG3, ensuring that the stop commands are master over both the
Run and Jog commands. An important part of the circuit is the maintaining loop between
the output of AG2 and the input of OG1. This ensures that the motor continues running
after the Run button is released.
I have included this loop to illustrate an interesting feature of logic sequences,
it does though have a serious weakness in this situation, that is, it does not fail
safe. Assuming there is an intermittent open connection to the coil of the motor
contactor, the logic would still maintain after the run button was pressed. At some
later time the intermittent connection may make, say due to vibration, and the motor
start of its own accord.
The answer to this is to have an auxiliary contact on the motor contactor fed back
into the logic to prove that the contactor has energised when the run button has
been pressed. This could be achieved with the contact placed at X. However, I prefer
external connections to be fed via the power rail, typically as the Run button. This
would require some additional logic gates, perhaps you may like to update the circuit
See my pages elsewhere on Safe Circuit Design.
Returning to a comment made earlier. All the gates shown would be connected to the
positive and negative power rails but adding these would have complicated the sketch
unnecessarily as they have no bearing on the logic sequence. It is normal practice
therefor to leave the power rails off of diagrams for logic and other electronic