Make Electronics

Chips, Ahoy! 177

Experiment 18: Reaction Timer

In bistable mode, the 555 has turned into one big flip-flop. To avoid any un-
certainty, 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 4-40. 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 4-41.

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 counter, the counter simply ignores the
pulses.

Now press S5, which flips your bistable 555 back to delivering a negative out-
put, 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.

To Pin 2

of 4026

Counter

IC1

1

2

3

4 5

6

7

8

IC6

S4

R9

R10

S5

Figure 4-40. Adding a bistable 555 timer to the reflex tester will stop the counter with a
touch of a button, and keep it stopped.

R9, R10: 1K
IC6: 555 timer