The circuit stewpot

Here in the United States, people celebrate late November’s Thanksgiving holiday by cooking a turkey. By the first of December, the leftovers are mostly gone and the picked-over carcass awaits the soup pot. This fortnight’s Design Ideas have already landed in the EDN stewpot, and there’s not a turkey in the bunch.

If you’re stuffing a class-D amplifier in your latest product, you’ll appreciate learning how an “Active-filter circuitand oscilloscope inspect a Class D amplifier's output.” You can assemble this handy test-and-measurement accessory from a quad op amp and a small handful of passive parts. For best results, package the circuit in a shielded box and power it from an isolated source to avoid creating a ground loop with your o’scope.

Our next Design Idea, “Voltage-to-pulse-width converter spares microprocessor's resources,” squashes high component-count analog-to-digital conversion circuits by implementing a single-slope ADC on one of Atmel’s popular AT89-series microprocessors. Check out this approach when you can’t afford a processor with a “real” A/D feature, but don’t expect extreme accuracy from the circuit.

Voltage references don’t get much respect in the power-output category, but our next Design Idea, “Precision voltage reference delivers 80 mA,” earns its celery by punching out a healthy chunk of current. Does this circuit’s output capability ring a Bell? If so, tell EDN’s readers about your application for a robust reference via the Design Idea’s “Feedback” feature.

It doesn’t happen often, but this Design Idea, “Two-channel audio amplifier drives stepper motor,” nicely complemints our leadoff entry. You can assemble the amplifier first, and then use the active-filter circuit to view the motor’s phase voltages. And if you need a stepper motor, don’t overlook junked computer printers as a source.

And finally, if you relish innovative power sources, this Design Idea shows how to “Get power from a telephone line without disturbing it.” How? By sipping current from the landline’s on-hook voltage supply and storing a charge in a super capacitor. A micropowered DC-to-DC converter steps up the voltage (but doesn’t isolate the output).

Connecting anything to the telephone line requires a certain amount of caution and understanding of the line’s characteristics, limitations, and potential hazards (pun intended), so do your homework before you tap into the circuit.

I seem to have exhausted my supply of culinary puns, and so it’s off to the rices with this issue’s distractions. Following up on a mention of crystal radios in my previous blog, I stumbled across the following site, which offers an in-depth examination of crystal radio design criteria. And you thought that crystal sets were just for kids’ first radios….

Electronics-related online museum sites are always fun to visit, and don’t pass up Jim's Antique Radio Museum, where “Real radios glow in the dark and are warm to cuddle up next to.” Check the plastic-cabinet 1950s radios for an idea of what passed for “styling” in the Eisenhower era. If you look hard, you can see the Conelrad markings on some frequency dial scales.

Chances are, you spend an inordinate amount of time browsing online. If you’re actually looking for specific information, mouse over to Phil Bradley’s helpful compendium of information resources and free more time for recreational browsing.

73,

Brad