Designing Ideas

In case you missed out on the festivities, on March 14 (3.14…get it?) mathematicians celebrated "Pi Day" to honor the ratio of a circle's circumference to its diameter. Practitioners of electronics are no strangers to pi, either, but where's our j Day?

Turning to this fortnight's Design Ideas, our first entry, by frequent contributor Richard Kurzrok, "Audio-test accessory isolates and matches loads," describes a test-bench accessory that you may find useful if you (like many engineers) have a penchant for stuffing frequently-used circuits into little aluminum boxes instead of rat's-nesting new versions every time they're needed.

What's the most popular IC ever developed? Some would say the 741 op amp, but I'd vote for the 555 timer, first announced by Signetics in 1972 and used in everything from ultrasonic rat repellers, to the anticipating timer that switches off before you switch it on (note the publication date). Our next new Design Idea, "One oscillator drives multiple solid-state relays," uses the 555 more conventionally by supplying gate-drive power to a batch of up to 10 custom-designed MOSFET power switches.

The next Design Idea, "Low-dropout linear regulators double as voltage supervisor circuits," actually started out life with a different title: "Improving the performance of LDO voltage supervisor circuits." Can you use a low-dropout voltage regulator (suitably improved) as a voltage supervisor? Sure, you can. Would this technique work with voltage regulators other than the component cited in the Design Idea? Give it a try and report your results by adding a comment.

While we're in the mood for power-supply circuit improvement, let's try another. In "External components provide true shutdown for boost converter," the author adds a MOSFET in series with the load circuit's negative lead and feeds the converter's shutdown command to the MOSFET's gate. This technique works if you can get away with interrupting the return lead—for example, if the load in question consists of a heating element—but may pose problems when used with complex loads that include active devices driven by external control signals. For these circuits, cutting loose the ground reference can cause interesting problems.

A final thought: Although the rumor that Alabama's legislature attempted to define the value of pi as 3.0 is false (it was Indiana, in 1897), I learned as a junior tech that sometimes it's okay to let a pi equal 3. Here's how it happened. The boss handed me a coil of coaxial cable and asked, "Is this a 30-foot length?" I reached for a measuring tape and began to uncoil the cable. "You're doing it the hard way," observed the boss. "Measure the diameter of the coil in inches and multiply it by three to get the mean length of a turn. Multiply the mean length of a turn by the number of turns in the coil. Add 15% of the product to get the length in inches. Divide by 12 to get the length in feet, and you're close enough for practical purposes."

Hey, it works for me.

73 for now,

—Brad