Nuts and Volts - March 2015

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 seconds. 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 work with.

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.

Author Bio 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.