PROJECTS: REACTION TIMER
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Fig. G: This test circuit, laid out as you would be likely to
place it on a breadboard, allows you to trigger a counter
manually to verify that the display increments from 000
upward to 999.
Component values:
All resistors are 1K.
S1, S2, S3: SPST tactile switches, normally open
IC1, IC2, IC3: 4026 decade counter chips
IC4: Kingbright 3-digit common-cathode display
C1: 100 μF (minimum) smoothing capacitor
Wire the output pins on IC1, IC2, and IC3 to the pins on
IC4, according to the numbers preceded by arrows. The
actual wires have been omitted for clarity. Check for the
pinouts of IC4.
102 Make: Volume 21
makezine.com/21/reaction_timer
breadboard immediately above IC1. Don’t leave any
gap between the chips. Disconnect S3 and R3 and
connect a wire directly between pin 3 (output) of
IC5 and pin 1 (clock) of IC1, the topmost counter.
Power up again, and you should see the digits
advancing rapidly in a smooth, regular fashion.
Press S1, and while you hold it, the count should
freeze. Release S1 and the count will resume. Press
S2 and the counter should reset, even if you are
pressing S1 at the same time.
Refinements
Now it’s time to remember that what we really want
this circuit to do is test a person’s reflexes. When
the user starts it, we want an initial delay, followed
by a signal — probably an LED that comes on. The
user responds to the signal by pressing a button
as quickly as possible. During the time it takes for
the person to respond, the counter will count milliseconds. When the person presses the button, the
counter will stop. The display then remains frozen
indefinitely, displaying the number of pulses that
were counted before the person was able to react.
How to arrange this? I think we need a flip-flop.
When the flip-flop gets a signal, it starts the counter
running — and keeps it running. When the flip-flop
gets another signal (from the user pressing a button),
it stops the counter running, and keeps it stopped.
How do we build this flip-flop? Believe it or not,
we can use yet another 555 timer, in a new manner
known as bistable mode.
In bistable mode, the 555 has turned into one
big flip-flop. To avoid any uncertainty, we keep pins
2 and 4 normally positive via pull-up resistors, but
negative pulses on those pins can overwhelm them
when we want to flip the 555 into its opposite state.
The schematic for running a 555 timer in bistable
mode, controlled by two pushbuttons, is shown
in Figure J, page 105. You can add this above your
existing circuit. Because you’re going to attach the
output from IC6 to pin 2 of IC1, the topmost counter,
you can disconnect S1 and R1 from that pin. See
Figure K, page 106.
Now, power up the circuit again. You should
find that it counts in the same way as before, but
when you press S4, it freezes. This is because your
bistable 555 timer is sending its positive output to
the “clock disable” pin on the counter. The counter
is still receiving a stream of pulses from the astable
555 timer, but as long as pin 2 is positive on the
