Hi All, I am trying to use Arduino interrupts to pick up a falling voltage. It is working ok to pick up a rising voltage. Cannot get it to work with a falling voltage. Here is my hardware circuit which I have posted before. It is much simpler than other designs I have seen. I have a positive zero voltage(not floating). In the Ltspice simulation the voltage goes to nearly 5v at zero crossing of the mains voltage and the rest of the time the output voltage is close to zero. Can't see why Arduino not picking up the falling voltage. Something wrong with the circuit perhaps?