Schmitt Inverter

Overview

A Schmitt circuit is a digital circuit having two possible outputs - high or low. The defining feature of a Schmitt circuit is that it has different output characteristics depending on whether the input signal is increasing or decreasing. Schmitt circuits are usually combined with logic gates and often have an inverting function, e.g. Schmitt NOT gates or Schmidtt inverters and Schmitt NAND gates. Schmitt trigger circuits can be made directly from Op-Amps giving greater control over the transfer characteristics of the circuit.

Learning Objectives

• Describe the action of a Schmitt inverter
• Describe the use of a schmitt inverter in de-bouncing signals produced by mechanical switches and analogue sensors
• Describe the use of a schmitt inverter to process slowly changing input signals
• compare the properties of transistors, comparators and Schmitt inverters as interfaces between analogue and digital systems

Lesson Content

Reading: This is a very short topic so the webpage can be read and digested in one go. Read the first three paragraphs about the nature of a Schmitt based circuit and how the Schmitt inverter differs from a conventional NOT gate. Use the fourth paragraph to contrast comparators and the use of a Schmitt inverter when dealing with slowly changing signals. Work through the final paragraph to understand how switch bounce can be a problem and how this can be resolved using an RC circuit and a Schmitt inverter.

Video (8 min): The video compares a NOT gate and a Schmitt inverter and then demonstrates how each of these works with a potential divider circuit. Watch the video and add to your notes.

Think about how a comparator, transistor and Schmitt inverter compare when used to interface a process to an output transducer. In summary, the transistors and the Schmitt inverter have fixed voltages (0.7 V for a bipolar transistor and around 3 V for a MOSFET) at which they turn on or off. Transistors can handle larger currents for the output device. A comparator has a variable threshold voltage that can be set using a potentiometer. A Schmitt inverter avoids problems when the input voltage hovers around the threshold voltage.

Lesson Review

Review your learning by working through the presentations or notes which summarise the website content.

Presentation: Powerpoint download. A summary of the Schmitt inverter with less detail than the website but with questions and a summary.

Notes: PDF download outlining the function and uses of the Schmitt inverter.

Self Assessment

Complete either the questions (pdf download) OR the on-line quiz. They are the same questions.

Questions: PDF download. Questions about the Schmitt inverter and how it is used to deal with slowly changing analogue signals and de-bounce switches.

Quiz: Questions about the Schmitt inverter and how it is used.

Self Evaluation

I can:

• Describe the action of a Schmitt inverter
• Explain the difference between a Schmitt inverter and a NOT gate
• Understand why a comparator circuit might not work as expected when dealing with slowly changing analogue signals
• Understand how a Schmitt inverter can be a better choice (than a comparator) when dealing with slowly changing analogue signals
• Describe switch bounce and appreciate some of the problems caused by switch bounce
• Explain how a monostable can be used to avoid problems associated with switch bounce
• Explain how an RC circuit can be used to avoid problems associated with switch bounce
• Appreciate that a Schmitt inverter, used with a RC circuit, is an effective way to de-bounce a switch
• Calculate the component values needed to give an appropriate time constant when de-bouncing switches with an RC circuit (or monostable)
• Qualitatively and quantitatively compare Schmitt inverters, comparator circuits, MOSFETs and bipolar transistors when interfacing analogue and digital circuits