The solution depends on what you want to achieve. If you only want to turn the lamp on and off, it is wise to use zero crossing control to limit interference - the circuit is switched when the voltage is zero (actually it is slightly greater) so you shouldn't have large changes in current - thus less interference. The actual control of the switching is done by the optocoupler so the microprocessor just needs to send a '1' or '0' to the optocoupler.


If you want to control the brightness of the lamp using phase control, then it is a little more complex. You need to detect the mains zero crossing, filter it and have it input to the microprocessor via an interrupt or timer capture. Various means can be used to determine the on time of the triac from this zero crosiing signal. The optocoupler in this instance needs to be a non zero crossing type. You will also need a choke (inductor) on the triac to limit the rate of change of the current to minimise interfernece.