Electric Bugle

Carl Burch
Hendrix College, 1600 Washington Ave, Conway AR 72032
501–450–1377
`burch@hendrix.edu`

Full assignment description
Training for the Digital Laptop Trainer
IC diagrams (alternative reference)

We use breadboards with TTL integrated-circuit chips when teaching about logic circuits in Computing Systems Organization. Some students find the intricacies of wiring quite a bother, and indeed we could use logic circuit simulation software such as Logisim. But many other students find that working with physical circuits is a nice change of pace from the monotony of always doing assignments entirely on a computer. It also ensures that students realize that the logic-circuit concepts they are studying are real in the sense that one can actually build working circuits with them.

It can be difficult to come up with assignments that are small enough to be built by students in the course of a week but still do interesting things. We developed an assignment in which students develop a electric bugle, which features four buttons, each corresponding to playing a different tone commonly played by bugles. These four tones (low G, C, E, and high G) can be used to play a wide variety of well-known bugle calls, including Taps, Reveille, First Call, and Mess Call.

The basic idea is simple: The digital trainer we use is capable of generating a 2 KHz square wave, and we show the students how to wire a basic buzzer. The circuit the students build divides the square wave by 6, 7, 9, or 12 depending on which of four buttons is pressed — and the square wave must not be sent to the buzzer at all if none of the buttons are pressed. (Technically, the E should be generated by dividing the wave by 7.2 — but that's not feasible with an integer counter, and a more precise tuning would require more than four bits, which is all our counter IC (74LS161) can handle. Dividing by 7 is close enough to make the tunes easily recognizable.)

The overall circuit requires using a counter IC as well as several simple logic-gate ICs. One frustrating piece of the assignment is that the square wave goes very quickly, making it quite difficult to debug the circuit. Consequently, the students often rely heavily on the instructor for debugging — though it usually doesn't take too much work to determine the problem. A fair fraction of students do manage to complete the assignment on their own.

The assignment depends on purchasing a kit of electronics supplies for each student. The kit we use, detailed on the Web page, should cost less than \$200 per student. The most expensive piece by far is Kelvin's Laptop Digital Trainer.

In our Computing Systems Organization course, this is the second and final assignment using the breadboard. The first is a more traditional, simpler assignment just to get acquainted with the trainer, ICs, and breadboards. And after this assignment, the course moves on to higher levels of the machine, so we do not have more time to spend with logic circuits. And this is just as well: Building more complex circuits on breadboards would introduce difficulties beyond the course's learning goals.