How to Detect Zero Cross points in a AC Signal?

Zero-Crossing point in terms of Alternating Current is a point where there is no voltage present. In a sine wave or other simple wave-forms, the zero cross points occurs twice during a single cycle.
The following figure show the AC signal of frequency 50Hz and 230V RMS Value. The signal is shown for the duration of 100ms for better visual understanding and that's why there are only five complete cycles present.

AC Signal Vrms230V and 50Hz Frequency

In a single cycle we get two points, one when signal transition from positive side to negative side and another when signal transition from negative side to positive side, and these points of the waveform are known as zero cross points as the voltage value at these two points is zero volts, which is highlighted in the figure below.

Zero Cross Points in a Single AC Cycle

Why we want to detect zero cross points?
So now the question arises why we need to detect the zero cross points of a AC Signal. There are various applications of the Zero Cross detection but the following are the two main applications of detecting zero cross points of an AC signals, and they are as follow:

• Measuring Time or maintaining Time Clock.
• Turning On AC Loads

Coming to the first application, the zero cross point is used for maintaining the time of various equipment, if you have seen Microwaves and other devices they start with 12:00 (Can be other than 12:00 but in most cases it is 12:00, and even on my Samsung Microwave i always see 12:00) at power-up on display and after this the time increment as per our clock, so you might think that RTC is used in this, but in most of the cases there is no RTC and they used the zero cross points to increment the clock.
In India, frequency of supply coming at home is 50Hz, which means 50cycles and 100 zero cross points in one second, depending on the method used for detecting zero cross points, one can count the number of zero cross points and increment the clock.
Similarly in America, the supply coming at home is 120V 60Hz, which means that 60 cycles or 120 zero cross points in one second, and by detecting these zero cross points, one can increment the clock and maintain the time.
Now coming to the second application of Turning on the AC Loads, before that, i would like you guys to read my previous post on Relays also.
As we know AC voltages are of high value and varies as they are of alternating nature and if loads are turned on when the signal is at peak, then you will see spark, i guess most of you guys have seen such things when turning on the ac switches like fan, motor, light etc present at your home. If you guys have not seen this, i request you to turn off the lights in your home and then try to turn on any heavy loads, you will definitely see spark.
So why this is a problem? In home this is not basically a much problem, but for Embedded Engineer, this is a problem, as we have to turn on the AC loads using our embedded devices and for that purpose we use relays, and due to this spark, the contacts of the relays can stick or weld with each other due to arching and will remain in same position after that, and only way to solve this problem is to replace the relay with the new one. So this is most common problem, for the engineers who work with AC loads, and this problem is resolved only when the relay switches at zero cross point, then there will be no spark and no sticking/welding of relay contact occurs.

Circuit for Detecting Zero cross Signal:

Circuit Diagram

The above circuit can be used to detect the zero cross point of an AC signal, as you can see clearly, first of all high wattage resistors are used to drop down the AC signal and after that a bridge is used, then the opto-coupler is used to detect the zero cross signal, whenever the zero cross point in the AC signal is detected, the logic high is detected on pin 5 of the opto-coupler.
The following images shows the output of the above circuit.

Simulation

This is not the only method to detect the zero-cross points, there are various other methods to do so. Currently there are special opto-coupler in the market, which works in both directions of the ac cycle and thus doesn't require the bridge ic to be used in the above circuit, i will try to update all such circuits in future, till then stay tuned.