Motivating Students with a Cigar Box Amplifier project

Besides my full-time gig as an engineer, I also teach part-time at a local community college. The circuits class is a continuation from the basic AC/DC fundamentals class and introduces the students to active components like diodes, transistors and op-amps.  I tend to be a bit out of the box and decided that  it’s more enriching to drive the class towards a project based learning experience.  Instead of paying for the typical lab manual and building “static circuits” each week in lab to expand on the theory, I suggested to the class to put the money into the materials required beyond the lab stock of devices to build a stereo  amplifier that they can plug their iPhone or MP3 player into and listen  to.

With only 15 weeks for a semester,  the labs turned into project updates and planning. The class started by learning about power budgets and input and output requirements. The first task was to decide how much power they could afford given the cost of the alternative lab manual.

Understanding diodes turned into a power supply design with a power budget to drive up to a total of 15W of power to two 8 ohm speakers.  Basic transistor circuits turned into figuring out bias and gain for the given inputs and requirements for the next stage of the amplifier. Op-amps turned into learning about a tone controls through active bandpass filters.  The final stage was the power amplifier and because of schedule restraints the class decision was to use a linear Class A design though inefficient, was still able to provide enough output to have acceptable volume to the speakers.

I had the students create their own schematic using ExpressPCB tools and eventually create a PCB that still fell within budget.  They  learned about design reviews and tradeoffs  in a project between scope, costs and time.

I had the class practice soldering on breadboards to build prototypes of sections of the power supply and amp. They had to develop a test plan to verify  the specifications. This way when the final layout was complete on the PCB, they were ready to solder the complete board  and experienced with troubleshooting as well as making measurements against the design specifications.

I found a cigar company willing to sell some empty wooden cigar boxes to house the stereo parts as a nice finish.  At the cost of $2/box, this fit well within the budget.  Another student volunteered to come up with a wooden insert to  mount the speakers and the PCB.

Overall, the project was a big success. The output was delivering only  4.5W per channel but still loud enough to enjoy at a desk. There was no distortion to deal with since all the stages were basically linear class A.  All the students ended up with a  working cigar box stereo amplifier that they learned so much from. A few students wanted to make it more power efficient. They asked about turning the final stage to a class D operation.  Over the summer and with a small Maker Faire grant from the school, they actually went on to build and explore the challenges of designing a Class D amplifier to fit in the same cigar box housing.