Level Shifting

A surprisingly high number of questions from readers that arrive on our desks deal with the subject of the interconnection of two separate (sub) circuits that operate from different power supply voltages, or have dissimilar switching levels. How do you deal with that?There was a time once, when all digital circuits always operated from 5 V. Give or take the odd exception, there were never any interface related problems and everything could simply be tied together. Those were the days!Times change. It is now a frequent occurrence that an interconnection is required between systems operating from entirely different power supply voltages. A DCF time standard receiver module, for example, runs from a single 1.5-V penlight battery and has to be connected to a microprocessor system that runs from 5 V.