Make Electronics

Chips, Ahoy! 175

Experiment 18: Reaction Timer

Pulse Generation

A 555 timer is ideal for driving a counter chip. You’ve already seen how to wire
a 555 to create a stream of pulses that made noise through a loudspeaker. I’m
reproducing the same circuit in Figure 4-38 in simplified form, using the posi-
tive and negative supply configuration in the current project. Also I’m show-
ing the connection between pins 2 and 6 in the way that you’re most likely to
make it, via a wire that loops over the top of the chip.

For the current experiment, I’m suggesting initial component values that will
generate only four pulses per second. Any faster than that, and you won’t be
able to verify that your counters are counting properly.

Install IC5 and its associated components on your 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.

1

2

3

4 5

6

7

8

IC5

R7

R8

C3 C2

To Pin 1

of 4026

Counter

IC1

Figure 4-38. A basic astable circuit to drive the decade counter in the previous schematic.
Output is approximately 4 pulses per second.

R7: 1K
R8: 2K2
C2: 68 μF
C3: 0.1 μF
IC5: 555 timer

No Floating Pins!

A CMOS chip is hypersensitive. Any

pin that is not wired either to the

supply voltage or to ground is said

to be “floating” and may act like an

antenna, sensitive to the smallest

fluctuations in the world around it.

The 4026 counter chip has a pin

labeled “clock disable.” The manu-

facturer’s data sheet helpfully tells

you that if you give this pin a positive

voltage, the chip stops counting and

freezes its display. As you don’t want

to do that, you may just ignore that

pin and leave it unconnected, at least

while you test the chip. This is a very

bad idea!

What the data sheet doesn’t bother

to tell you (presumably because

“everyone knows” such things) is that

if you want the clock to run normally,

the clock-disable feature itself has to

be disabled, by wiring it to negative

(ground) voltage. If you leave the pin

floating (and I speak from experi-

ence), the chip will behave erratically

and uselessly.

All input pins must be either

positively or negatively wired, unless

otherwise specified.