parts cost over $20 and would have required a printed
circuit board (PCB) to be designed and stuffed with parts,
plus a 12 volt power supply to operate the relays. Again,
the parts cost was too high (Figure 1).
For the least expensive circuit, I used limit switches.
With limit switches and a mechanical design, I built an
inexpensive control circuit for a real traffic light. In this
case, a simple mechanical design was less costly.
The limit switches can directly handle 120 VAC and
the nine amp surge current. These limit switches needed
to be mechanically operated. If I used a gear motor
running at one revolution per minute, I could design a
cam to activate the limit switches. If I used three limit
switches located 60 degrees apart
— activated by a dual-pronged cam
that was 60 degrees wide — each
light would be on for about 10
I needed a Computer Aided
Drafting (CAD) program to lay out
the switches with different cam
positions to see if the parts would
mesh correctly. When I taught
Project Lead the Way Engineering
and Drafting, I used Autodesk
Revit and was able to lay out parts
like this. However, I do not
presently have access to that program. So, I purchased
TurboCad 18, which is a great program for a reasonable
price ($39.99 at Amazon.com; Resource 1).
I found that TurboCad was able to lay out the
switches in a variety of ways, and I could visually verify
how the cam and switches would interact. Using the first
layout, the prototype caused the gear motor to bog down
since the cam was pushing on the switch directly instead
of pushing on the lever arm to provide some mechanical
fulcrum leverage. Modifying the TurboCad drawings was
easy, and it was fun to have a nice drawing program to
If I asked the program to print out the drawing on my
Brother MFC-7360N printer in a drawing
scale of 1/1, the drawing was accurate to
within the width of a printed line. This
allowed me to cut out the printed
drawing and use it as a template to make
the parts. This may not be the case for
every computer/printer combination, but
it was sure nice to have a template for
making my parts. TurboCad did a nice job
in my situation and saved me from having
to measure the dimensions on the drilled
32 March 2015
■ FIGURE 3. Mounting hole dimensions.
Alan Grambo was an electrical
engineer for 28 years who worked on
smart bombs for the Navy. He retired to
teach high school electronics students.
He has also worked as a forensic
engineer for lawyers and insurance
companies to investigate the causes of
accidents and fires.
He is a nationally recognized
tournament table tennis player and has
played table tennis in over 20 states,
Canada, and Finland. He now teaches
electronics to his grandkids by building
gadgets with them.
■ FIGURE 2. Locations for switches and cam.