Zero Crossing Detector
A Zero Crossing Detector is a circuit that generates an output when the input crosses a reference point, in this case zero volts or ground. A Zero Crossing Detector is also called a threshold detector, but with the threshold fixed at zero volts. Normally Zero Crossing Detectors are designed using either Comparators or operational amplifiers. A comparator or Voltage Comparator is pre-designed to function as a zero crossing detector, or a comparator using a threshold of ground. Using an operational amplifier as a threshold detector requires a bit more work. There another reference design of a zero crossing detector on the voltage comparator page, using a comparator and not an Op Amp.
This circuit diagram represents a common configuration for a zero crossing detector. But this is just one of a large number of possible configurations of a zero crossing detector made using an operational amplifier. This circuit uses an LM741 only because it's so widely used and a common Op Amp.
Zero Crossing Detector
741 Package Options
the 741 Op Amp is available in the same package options as many other operational amplifiers. The through-hole packages may be either Plastic DIP Package, or Ceramic DIP Package [for military and space applications], while the surface mount versions will be in a SOIC Package. In addition the LM741 could be in a round metal TO-99 Package.
741 Off-Set Null
The 741 operational amplifier has two pins used to null the output voltage, or zero the output by adjusting input currents. Read more on the Off-Set Null Adjustment. Because the LM741 [uA741] is so common, the amplifier is used in a few other circuits which are provided to the right.
OP Amp Unused Inputs
The 741 operational amplifier is used in the circuit to represent almost any Op Amp. However if other devices are used, including dual or quad packages. Unused devices within a package should be tied off so they don't oscillate. The output of an unused operational amplifier should be tied to the minus input, and the plus input should be left open or unconnected.
A 1N914 diode is used in the schematic, although the 1N4148 diode appears in some of the application notes. Many other circuits listed on the site use the 1N914 diode, so it's reused here for that reason. Basically both diodes are equivalent and may be used interchangeably, although some data sheets classify the diodes as switching diodes while others may classify them as high speed diode. The standard outline for a 1N914 is a glass diode in a DO-35 Package Details [SOD27 package]. So any switch diode would work in this circuit, the 1N914 just happens to be one in common usage.