A Programmable Logic Controller (PLC) could also be
used to control the traffic light bulbs. The PLC has the
capability of receiving inputs to control the outputs to the
The problem becomes a little more complex when
you design the circuit to operate like a real traffic light
rather than to control low current LEDs. Real traffic lights
have either eight inch lenses or 12 inch lenses, with 150
watt incandescent lamps inside. This size bulb takes about
1.3 amps with a surge of about nine amps when first
energized, so relays would normally be required to
operate the lamps reliably. You could use a Silicon
Controlled Rectifier (SCR) to control the higher current,
but SCRs have a habit of being unreliable because of the
large surge current required to
illuminate incandescent lamps.
A former co-worker of mine
has 110 real full-size traffic lights
that have no control circuits. So,
they cannot work as regular traffic
lights do. He approached me
because he wants to sell them, and
he wants to be able to offer the
buyers control circuits for the lights
if they wanted them. Concerned
about the cost of such control
circuits, he asked if I could come
up with a circuit that would cost
less than $20 to build.
I first designed an Arduino
circuit to control the traffic light.
The main problem with building
the Arduino circuit was that it
$25; the drive transistors, relays,
and power supply would cost
more than $20. It would be a
versatile multi-function controller, but the cost was beyond
my co-worker’s budget.
I then designed and tested a circuit that used two 555
IC timers that worked well, until I added the relays and
driver transistors to the circuit. The inductive kickback of
the relays — even with protective diodes — caused the 555
circuits to trigger at the wrong time, so the circuit would
skip a lamp periodically. Also, the sequence would
sometimes be yellow-green-red rather than yellow-red-green.
The second digital circuit I tried used a 555 timer, a
4017 decade counter, 13 diodes, three drive transistors,
and three relays. With these parts, I was able to arrive at a
suitable operational circuit. This design, however, had a
March 2015 31
■ FIGURE 1. Digital circuit using a 4017 decade counter.
What is Project Lead The Way?
Project Lead The Way (PLTW; www.pltw.org) classes
are now held in more than 6,500 K- 12 schools in the US.
These classes help students develop skills for today’s
world by aligning themselves with Science, Technology,
Engineering, and Math (STEM) programs. PLTW classes
are typically taught by instructors that have been trained
in a professional development PLTW class.
There are many different genres of PLTW classes
available. Here is a brief list of some of the classes being
taught in schools:
Introduction to Engineering Design
Principles of Engineering
Civil Engineering and Architecture
Computer Integrated Manufacturing
Computer Science and Software Engineering
Computer Science Applications
Engineering Design and Development
Principles of Biomedical Science
Human Body Systems
Digital Electronics is just one of the choices of
possible classes offered. Each school decides which
PLTW class to offer, based on instructor availability and
what classes the students might desire to take. As an
example, one of the assignments in the Digital
Electronics class at Fishers High School in Fishers, IN is
to build a traffic light controller.