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Digital integrated circuits I
It’s not a good idea to leave an input unconnected (what is
called ‘floating’), as it may not be at the logic level you expect
it to be, so we’ve added a fairly high-valued resistor to the
input of the inverter to connect it directly to the positive battery
supply. This means that, under normal circumstances
— that is, with no other input connected — we know that the
inverter’s input is held constantly at logic 1. To connect the
inverter input to logic 0, it’s a simple matter of connecting a
short link between the input and the negative battery supply.
This isn’t shown in the breadboard layout, but all you need to
do is add it when you are ready to perform the experiment.
Measuring the output of the inverter is just as easy, and we
take advantage of the fact that the output can be used to
light up a light-emitting diode (LED). So, when the inverter’s
output is at logic 1, the LED is lit: when the inverter’s output
is at logic 0, the LED is unlit.
Once the circuit’s built, it’s a simple matter of carrying out
the experiment to note the output as its input changes, and
completing its truth table. Figure 10.8 is a blank truth table
for you to fill in. Just note down the circuit’s output, whatever
its input state is. Of course, your results should tally exactly
with the truth table in Figure 10.5 — if it doesn’t, you’ve gone
wrong, because as I’ve said before in this book — I can’t be
wrong, can I?
Figure 10.8 Empty truth table for your results of the experiment in Figure 10.6
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