# Use high-speed optocoupler with slow input signal

As known, a slow input is usually not good for CMOS digital circuit design. I wonder, if the high-speed optocouplers such as 6N137, HCPL-060L, etc, can be used with slow input signals, such as to detect a zero-crossing event in a 50Hz AC input? (I know it's a waste of money to use high speed with slow input, but that's another question.) What I should take care of? Do I need a signal conditioning before the optocoupler?

• Optocouplers are not actually digital components, even if their implementation contains one. – Ignacio Vazquez-Abrams Jun 11 '15 at 1:01

To expand on your example a bit, at 50Hz 240VAC, the mains voltage crosses the zero at about 100,000 volts/second. (100$\pi\sqrt{2}\times240$), so it's not moving all that slowly. If you want to detect the zero crossing at, say, 10V +/-20% you have to detect the voltage with an error of 20usec or less. That's assuming no voltage error (say you are using a comparator on the mains side to provide a precise transition voltage). If you allow a volt for voltage error then you must not add more than 10usec of timing error.