After decades of seeing projects
and circuits using ever increasingly
complex integrated circuits, I yearn
for simpler times. As a teenager,
I built fascinating and wondrous
circuits using just a few transistors.
My flashlight controlled relay could
control a buzzer; music from my
cassette tape player played on my
radio with a two transistor circuit;
my amplifier could drive a speaker.
These circuits were from dusty hobby
books found at my local library with
names like “ 29 transistor circuits”
or “electronic hobby circuits.”
BY KEITH BAYERN
To return to those glory days, I decided to build a
digital clock using only transistors as the active elements.
After a few years of “work” (it felt more like play), the final
parts count is 194 transistors, 566 diodes, 400 resistors,
and 87 capacitors. Check it out in Figure 1.
This article will explain the circuitry at both the logic
level and the transistor level. Time to get started ...
The Big Picture
Figure 2 shows the design at the functional block
level. The power supply (lower left) rectifies and filters
the incoming nine volts AC, converting it to nine volts
DC to operate the circuitry and a 60 Hz clock. Next, a
prescaler divides the 60 Hz by 10 and six, resulting in a
1 Hz clock. A 2 Hz clock is also routed to the time
The 1 Hz — which is also a one second per pulse
clock — drives a divide-by- 10 counter who’s output is
10 seconds per pulse. The four bits of the counter are
decoded into a one-of- 10 signal that drives a seven-
42 July 2009
■ FIGURE 1. Transistor Clock.
■ FIGURE 2. Top Level Block Diagram.