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555 Schmitt Trigger

Parts and Materials

  • One 9V Battery
  • Battery Clip (Radio Shack catalog # 270-325)
  • Mini Hook Clips (soldered to Battery Clip, Radio Shack catalog # 270-372)
  • One Potentiometer, 10 KΩ, 15-Turn (Radio Shack catalog # 271-343)
  • One 555 timer IC (Radio Shack catalog # 276-1723)
  • One red light-emitting diode (Radio Shack catalog # 276-041 or equivalent)
  • One green light-emitting diode (Radio Shack catalog # 276-022 or equivalent)
  • Two 1 KΩ Resistors
  • One DVM (Digital Volt Meter) or VOM (Volt Ohm Meter)


Lessons In Electric Circuits, Volume 3, chapter 8: “Positive Feedback”

Lessons In Electric Circuits, Volume 4, chapter 3: “Logic Signal Voltage Levels”

Lerning Objectives

  • Learn how a Schmitt Trigger works
  • How to use the 555 timer as an Schmitt Trigger

Schematic Diagram

Schmitt Triggers have a convention to show a gate that is also a Schmitt Trigger, shown below.

The same schematic redrawn to reflect this convention looks something like this:



The 555 timer is probably one of the more versatile "black box" chips. Its 3 resistor voltage divider, 2 comparators, and built in set reset flip flop are wired to form a Schmitt Trigger in this design. It is interesting to note that the configuration isn't even close to the op amp configuration shown elsewhere, but the end result is identical.

Try adjusting the potentiometer until the lights flip states, then measure the voltage. Compare this voltage to the power supply voltage. Adjust the potentiometer the other way until the LED's flip states again, and measure the voltage. How close to the 1/3 and 2/3 marks did you get?

Try substituting the 9V battery with a 6 volt battery, or two 6 volt batteries, and see how close the thresholds are to the 1/3 and 2/3 marks.

Schmitt Triggers are a fundamental circuit with several uses. One is signal processing, they can pull digital data out of some extremely noisy environments. Other big uses will be shown in following projects, such as an extremely simple RC oscillator.

Theory of Operation

The defining characteristic of any Schmitt Trigger is its hysteresis. In this case it is 1/3 and 2/3 of the power supply voltage, defined by the built in resistor voltage divider on the 555. The built in comparators C1 and C2 compare the input voltage to the references provided by the voltage divider and use the comparison to trip the built in flip flop, which drives the output driver, another nice feature of the 555. The 555 can drive up to 200ma off either side of the power supply rail, the output driver creates a very low conduction path to either side of the power supply connections. The circuit "shorts" each side of the LED circuit, leaving the other side to light up.

The 5KΩ resistors are not very accurate. It is interesting to note that IC fabrication doesn't generally allow precision resistors, but the resistors compared to each other are extremely close in value, which is critical to the circuit's operation.

Lessons In Electric Circuits copyright (C) 2000-2020 Tony R. Kuphaldt, under the terms and conditions of the CC BY License.

See the Design Science License (Appendix 3) for details regarding copying and distribution.

Revised January 18, 2010

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