In 1971, Signetics—later Philips—introduced the NE555 timer, and manufacturers are still producing more than 1 billion of them a year. As "NE555 timer sparks low-cost voltage-to-frequency converter" shows, by adding a few components to the NE555, you can build a simple voltage-to-frequency converter for less than 50 cents.

The circuit in "Optoisolators compute watts and volt-amperes" is an elaboration on that older circuit. It uses optoisolaters in a bridge circuit to measures not only watts, but also volt-amperes and so makes possible an estimation of power factor—watts divided by volt-amperes.

"Single-supply circuit measures -48V high-side current" uses an AD629 difference amplifier and an AD8603 operational amplifier, both from Analog Devices, to measure current at -48 to -60V and operates from a single positive-power supply, reducing circuit cost and complexity.

"Three-state switch interface uses one microcontroller pin" presents a way to sense three states of an SPDT (single-pole/doublethrow) switch with a center-off state, using only a single pin of Atmel's ATmega8 microcontroller. The Design Idea even provides a simple program for the circuit.