Design Ideas tackle "common" design problems, common-mode voltage and LED aging

Engineers have for many years used passive-CMV (common-mode voltage) inductive cancellation. Sometimes called a "humbucker transformer" because the power mains’ 60-Hz "hum" is often a dominant CMV component. In the Design Idea "Actively driven ferrite core inductively cancels common-mode voltage," an active humbucker toroidal transformer is scaled up to handle any number of inputs and outputs.

The Design Idea in "Improved optocoupler circuits reduce current draw, resist LED aging" overcomes the problems of excessive power consumption, uncertain switching, and LED aging. It lends itself well to wide ac-range applications.

The CD4017 Johnson decade counter finds use in simple circuits ranging from sound effects to LED displays. In the Design Idea "Cascade two decade counters to obtain 19-step sequential counter," the counter’s outputs are normally low and go high only at their respective decoded time slot. Each decoded output remains high for one full clock cycle. Be sure to catch the two author-submitted video clips of the circuit in action.

At least two classic ways exist to address applications requiring sampling of a sum of  two analog voltages. They both involve using power-hungry, low-value precision resistors. The Design Idea "Dual-input sample-and-hold amplifier uses no external resistors" presents a summing sample-and-hold amplifier that obviates these problems.