counter, the counter simply ignores the pulses.
Now press S5, which flips your bistable 555 back
to delivering a negative output, at which point the
count resumes. We’re getting close to a final working circuit here. We can reset the count to zero (with
S3), start the count (with S5), and wait for the user
to stop the count (with S4). The only thing missing
is a way to start the count unexpectedly.
Suppose we set up yet another 555 in mono-stable
mode. Trigger its pin 2 with a negative pulse, and the
timer delivers a positive output that lasts for, say,
4 seconds. At the end of that time, its output goes
back to being negative. Maybe we can hook that
positive-to-negative transition to pin 4 of IC6. We
can use this instead of switch S5, which you were
pressing previously to start the count.
Check the new schematic in Figure K, which adds
another 555 timer, IC7 above IC6. When the output
from IC7 goes from positive to negative, it will trigger the reset of IC6, flipping its output negative,
which allows the count to begin. So IC7 has taken
the place of the start switch, S4. You can get rid of
S4, but keep the pull-up resistor, R9, so that the
reset of IC6 remains positive the rest of the time.
This arrangement works because I have used a
capacitor, C4, to connect the output of IC7 to the
reset of IC6. The capacitor communicates the
sudden change from positive to negative, but the
rest of the time it blocks the steady voltage from
IC7 so that it won’t interfere with IC6.
The final schematic in Figure K shows the three
555 timers all linked together, as you should insert
them above the topmost counter, IC1. I also added
an LED to signal the user. Figure L (on page 107) is
a photograph of my working model of the circuit.
Because this circuit is complicated, I’ll summarize
the sequence of events when it’s working. Refer to
Figure K while following these steps:
1. User presses Start Delay button S4, which triggers IC7.
2. IC7 output goes high for a few seconds while C5
3. IC7 output drops back low.
4. IC7 communicates a pulse of low voltage through
C4 to IC6, pin 4.
5. IC6 output flips to low and flops there.
6. Low output from IC6 sinks current through an
LED and lights it.
Fig. H: A basic astable circuit to drive the decade counter
in the previous schematic. Output is approximately 4
pulses per second.
C2: 68 μF
C3: 0.1 μF
IC5: 555 timer
When you hit S3, I think
you’ll find that the count
sometimes increases by
more than 1. This does
not mean that there’s
something wrong with
your circuit or your
components; you’re just
observing a phenomenon known as “switch
On a microscopic level,
the contacts inside a
pushbutton switch do
not close smoothly,
firmly, and decisively.
They vibrate for a few
settling; the counter
chip detects this vibration as a series of pulses,
not just one.
Various circuits are
available to “debounce”
a switch. The simplest
option is to put a small
capacitor in parallel with
the switch, to absorb the
fluctuations; but this is
less than ideal. Switch
bounce is not a concern
in this circuit, because
we’re about to get rid
of S3 and substitute a
555 timer that generates
nice clean bounceless