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Zero Crossing Detector circuit is basically an application of a comparator. In the article series Zero Crossing Detector with Op Amp is built using a comparator of an Op Amp IC741/351. The process of detection of this comparator is  0Volt input signal crossing point by making reference value at comparator 0Volt.  The output of the Zero Crossing Detector circuit with Op Amp is wave-shaped box that detection result of the crossroads of 0 volt input signal.

This is the Yes-No Indicator Has Zero Standby Current Circuit Diagram. This circuit produces a random "Yes" or "No" with a single button press - indicated by the illumination of a red or green LED. The circuit has two advantages over similar circuits. First, it uses just a single momentary contact pushbutton, so no on-off switch is required. When the pushbutton is pressed, an oscillator comprising the 10nF capacitor and 22kΩ resistor at pins 1 & 2 is almost immediately stopped by FET Q1, which pulls the oscillator's timing capacitor to the positive rail. However, the 220nF capacitor and 470kΩ resistor in the gate circuit of Q1 introduce a tenth of a second's delay, so that about 250 oscillations take place before the clock is stopped. Due to variations in charge on the circuit's capacitors, as well as voltage and temperature variations, and the unpredictability of when the pushbutton will be pressed, randomness is assured. The circuit has a high degr...