AND
NOT
AND
NOT
AND AND
can be obtained by passing 9V or 12V through an
LM7805 or L7805, as in Figure
E
. For this project,
a 12V AC-DC converter is ideal, rated for at least
2A, as you’ll need it to power an audio amplifier.
Low-cost 12V supplies are easily found online.
Apply power, and the LED should flash in an
unpredictable pattern. The next step will be to
use this to create an intermittent audio output;
but first, how does the circuit work?
Figure
F
reminds you about logic-gate inputs
and outputs, although if you've had the wonderful
experience of reading my book Make: Electronics,
you may already know this. Figure
G
shows what
happens if two oscillators are running at slightly
different speeds, and they provide inputs A and B
to an AND gate, which (as you might imagine) will
AND them.
As for the NOT gates, how do they work?
Turn back to Figure B, look at the first NOT, and
imagine you have just applied power. The input
starts low, so the output is high, because a NOT
output is always opposite to its input. Current
flows through the resistor and gradually charges
the capacitor. The NOT monitors this, and when
the voltage rises to 3V, this is close enough to
logic-high, so the NOT changes its output to logic-
low. The low state will sink current, so now the
capacitor discharges back through the resistor
into the bottom end of the NOT gate, and the
cycle repeats at a speed set by the values of the
resistor and the capacitor. For the test circuit,
I used relatively low resistor values to create a
quick result that is easy to see.
Each input of a 74HC14 has a Schmitt trigger,
which creates a gap between the logic-high level
and the logic-low level. In Figure
H
, you can see
how the voltage oscillates between these limits.
The red oscilloscope trace shows the voltage
on the capacitor, while the green trace shows
the NOT output. Figure
I
shows the second
oscillator, which runs a bit more slowly, and Figure
J
shows the oscillator outputs ANDed together.
For the final version of the circuit, you’ll want
it to run more slowly, to give weary rodents time
to nod off before being woken up again. Using a
resistor value of 470K in one of the oscillators,
I obtained the result in Figure
K
. The capacitor
now takes much longer to charge than discharge,
because the NOT input steals a tiny amount of
9V or 12V DC
0.1µF
0.47µF
or L7805
LM7805
C D
Input A
Input B
Output
A AND B
0
0
1
2
3
4
5
2 4 6 8 10 12 14 16
seconds
volts
0
0
1
2
3
4
5
2 4 6 8 10 12 14 16
seconds
volts
0
0
1
2
3
4
5
2 4 6 8 10 12 14 16
seconds
volts
E
F
G
H
I
J
PROJECTS
Electronics Fun & Fundamentals
AND
NOT
AND
NOT
AND AND
94 makezine.com
M85_092-95_EFF_F1.indd 94M85_092-95_EFF_F1.indd 94 4/10/23 1:48 PM4/10/23 1:48 PM
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