In electronics, a Schmitt trigger is a special kind of comparator circuit.

A Schmitt trigger changes its output state when its input voltage level rises above a certain reference voltage; however, the output does not switch back automatically when the input voltage level sinks again unless a second, lower reference voltage is crossed.

A standard op-amp comparator simply outputs the highest voltage it can, when the positive input is at a higher voltage than the negative, and then switches to the lowest output voltage it can, when the positive input drops below the negative.

Schmitt triggers use hysteresis to guard against noise that would otherwise cause rapid switching back and forth between the two output states, when the inputs are close to the threshold.

Schmitt triggers were first invented by US scientist O. H. Schmitt; today, they are typically built using operational amplifiers, and the reference voltage levels can be adjusted by controlling the resistances of R₁ and R₂:

For instance, if the trigger is currently in the on state, the output will be at the positive power supply rail (+V_S). V₊ is then a voltage divider between V_in and +V_S. The comparator is comparing V₊ to ground. V_inR₂ must be equal to −V_SR₁ for V₊ to equal zero, so V_in must drop below −(R₁/R₂)V_S to get the output to switch. At this point, the output becomes −V_S, and the threshold becomes +(R₁/R₂)V_S to switch back on.

So this circuit creates a switching band centered around zero, with trigger levels ±(R₁/R₂)V_S. The input voltage must rise above the top of the band, and then below the bottom of the band, for the output to switch on and then back off. If R₁ is zero or R₂ is infinity (an open circuit), the band collapses to zero width, and it behaves as a standard comparator.

The speed at which the op-amp switches between power rails is the slew rate. The output can also be configured to drive digital logic.

The symbol for Schmitt triggers in electronic diagrams is a triangle with a hysteresis symbol:

External links

• Hyperphysics description and equations for an alternative, more general circuit