What is a Schmitt trigger?
A Schmitt trigger is a logic input circuit that uses hysteresis to apply positive feedback to the noninverting input of a comparator or differential amplifier. This allows the output to retain its value until the input changes sufficiently to trigger a change.
Schmitt triggers are commonly used in signal conditioning applications to remove noise in digital circuits. They are particularly effective at removing noise caused by contact bounce in switches. The Schmitt trigger's primary function is to remove noise in waveforms to prevent fluctuations from causing unpredictable output changes. A simple demonstration of a circuit that would benefit from a Schmitt trigger might be an LED that only turns ON sometimes when it is activated. Adding a Schmitt trigger to that circuit would make it easier for the LED to better define the ON and OFF signal levels.
Schmitt triggers show up in a variety of devices because of the need to for noise reduction in circuitry, especially devices that need to interface between analog and digital environments. Although most Schmitt triggers come in discrete packages or are included in logic ICs, you can still make one yourself to learn how a Schmitt trigger works.
The Schmitt trigger makes an upper and lower threshold to a wave to make triggering a device's ON and OFF states easier.
The Schmitt trigger, like many circuits, was named after its inventor, Otto Schmitt. Schmitt invented the Schmitt trigger in 1937, which he originally named the "thermionic trigger"—but Schmitt's inventions had such a profound impact on electronics that everyone called it the Schmitt trigger in his honor. Otto Schmitt is also credited with inventing the cathode follower, the differential amplifier, the chopper-stabilized amplifier, and with establishing the field of biomedical engineering.
Otto Schmitt lived from 1913 to 1998. His bachelor's degrees and PhDs were in Physics and Zoology. He always had an affinity for electronics and mathematics and invented several types of circuits that are still used today while creating artificial constructs to mimic the formation of impulses of nerve fibers. Schmitt was so gifted in the field of electrical engineering that he actually produced eight publications as an undergraduate.
A photograph of Schmitt in his later years. Courtesy of the National Academic Press.
During World War 2, Schmitt was recruited for the NDRC—later named the OSRD (Office of Scientific Research and Development)—which provided funding for top secret research for military applications. He was tasked with aiding the OSRD in finding ways to counter German submarines, which were incredibly effective at the time.
Schmitt rose to the challenge by developing the magnetic anomaly detector, often referred to as the MAD system. The MAD system detected small abnormalities in the Earth's magnetic field that were caused by submarines. The MAD system mounted on American bombers helped to end the Nazis' control of the Atlantic.
Unfortunately, many of Schmitt's achievements have been obscured due to military classification and the fact that Schmitt didn't patent many of his inventions. He actually gave the patent rights for most of his devices to the United States. Schmitt should be remembered for his brilliant inventions and humanitarian spirit.
“I’ve never wanted to be a businessman. I didn’t want to make money; I always wanted to advance ideas.”
If you'd like to learn more about the life of Otto Schmitt, his most extensive biography is "A Lifetime of Connections: Otto Herbert Schmitt, 1913-1998" by Jon M. Harkness. If you know of any other works about Schmitt's life, please share them in the comments.
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