Session 3 Digital

Now we have completed our Analog test exam its time to move onto something a little more challenging. You may refer back to the theory in the first two sessions a few times but essentially the same techniques apply.

When you complete your exam using this circuit you will have completed P2, P4, P6 and P8. We will be using the same format, two weeks of teaching, 4 weeks of investigation and then exams. Make sure you use your time wisely.

The circuit we will be investigating is shown below. It is a little more complex than the analog circuit and contains elements of analog and digital (its not strictly digital). It is also, I would mention here, certainly not the best way of achieving this function. There are better ways to do this and in M1 you will have a chance to investigate some of them and compare their performance with this circuit.

So here is the circuit we will be using.


The start of this circuit features 2 LM311 comparators. They are operational amplifiers but specifically manufactured to act as comparators (fast slew rate). In the next diagram I have attempted to convey as much information as I can condense into a few short lines. Fear not I will explain, but first the diagram.


A comparator is usually made out of an operational amplifier. An op amp has five inputs of interest (at this stage of our studies anyway). Two differential inputs called the non inverting (+) and the inverting input (-).

The operational amplifier will amplify any voltage difference between the inputs (however small). Therefore if we have a fixed voltage connected to one input (called the threshold voltage) and the other input is connected to a voltage which varies as a function of say for example temperature we have a circuit which will obey the following code.

+>- = ON

This simply says that if the voltage at the non inverting terminal is greater than the voltage and the inverting terminal then the comparator will switch on and its output will = the supply voltage

In the other cases – >+ the comparator will switch off and its output will equal 0 volts.

Therefore we can arrange for the comparator to switch on and off at a certain temperature.

A problem however arises. If the temperature is close to the threshold point the comparator could begin to switch on and off very rapidly. Not a huge problem for the comparator itself but certainly a potential problem for any electrical load it is switching (for example an inductive load (motor)).

However if we have two comparators with two different thresholds, an upper and a lower threshold. Then we have a situation where the motor can switch on at a higher temperature and off at a lower temperature. We have in effect widened the threshold and in theory we have pretty much absolute control over how wide the threshold will be.

All that is needed now is a way to feed both comparator outputs to a circuit which will combine them and give one output which obeys our intention for the circuit. Enter the flip flop circuit. Shown below.

Now this rather excellent video which somehow managed to attract a dislike, shows clearly how a flip flop operates. A diagram below shows the connections and truth tables which govern its behaviour.


The flip flop is made out of two NOR gates (in its simplest form). There are other techniques for making flip flops from other gates but lets not complicate matters.

If you follow the video and study the truth table you will have a good appreciation of how a flip flop works so take a little time to work it out for yourself as we will be studying this in class.

In summary the flip flop has two outputs ( Q or Q’) we could choose either but in this case we will choose Q. The comparator outputs feed one terminal each for both NOR gates, the other input terminal for both gates is connected to the output of the other gate.

The comparators are wired in such a way to ensure that the outputs are never both on at the same time. Can you see why from the flip flop truth table?

Once we have worked our way through this circuit we will be building, testing and investigating it. Next session we will look at the final component functions.