A zero crossing detector is used for detecting the zero crossings of AC signals. A typical AC signal is the sine wave

Sine wave

The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...

 which goes up and down the zero level. Many electronic systems need to know 'when' the signal crossed the zero level. The answer to that problem is the zero crossing detector.

Implementation

A zero crossing threshold detector is an electronic

Electronics

Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...

 circuit

Electrical network

An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current...

 that consists of an operational amplifier

Operational amplifier

An operational amplifier is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output...

 with an input voltage at its positive input (see circuit diagram). Often used in conjunction with other circuit elements, it usually functions as a simple voltage

Voltage

Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

 switch.

Operation

When the input voltage is positive, the output voltage is a positive value, when the input voltage is negative, the output voltage is a negative value. The magnitude of the output voltage is a property of the operational amplifier and its power supply. When used with a ±15 V power supply and a 741C operational amplifier, Vsat+ is approximately 13.6 V and Vsat- is approximately -14.3 V.

Applications

This detector exploits the property that the instantaneous frequency of an FM

Frequency modulation

In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...

wave is approximately given by

where is the time difference between adjacent zero crossings of FM wave