# Capacitors and Timing Circuits

### Overview

Timing circuits are very important in digital electronics for making time delays, digital pulses with a well defined period and clock pulses that oscillate at a fixed reliable frequency. The key component in timing circuits is a capacitor. The lesson looks at how a capacitor behaves and how it can be used with a resistor to give a voltage that changes slowly with time. Monostable circuits use a resistor and capacitor to give a single output pulse of a fixed duration. Astable circuits use a resistor and capacitor to produce a digital output that changes between on and off repeatedly with at a fixed frequency. Calculations of time period and frequency are explained with many examples.

### Learning Objectives

• Understand the function of a capacitor
• Use capacitor values in calculations
• Recognise that an RC network can produce a time delay
• Understand how the voltage across a capacitor (charging or discharging) varies with time
• Appreciate the action of a monostable circuit
• Know the details of a 555 monostable circuit
• Calculate the period of a monostable circuit
• Appreciate the action of an astable circuit
• Know the details of a 555 astable circuit
• Understand the meaning of period, frequency, mark and space when describing the output of an astable circuit
• Calculate the period, frequency and mark:space ratio for a 555 astable

### Lesson Content

Reading: Read the page about capacitors paying particular attention to the units and how to convert between them. It is important to be able to use units of µF and nF in equations. When building circuits it is important to recognise the difference between electrolytic and non-electrolytic capacitors and get the polarity of electrolytic capacitors correct. The second part of the page about storing charge and the structure of capacitors is more for interest and background information.

Video (30 sec): Watch the short video about how not to treat a capacitor.

Reading: Having read about capacitors, now read the page about how to use capacitors to create timing circuits by combining them with a resistor. Understand the concept of a time constant (R × C) and the idea that the time for the voltage to halve can be easily calculated. Learn the equation T½ = 0.7 × R × C. Finally, recognise and understand the graphs for charging and discharging.

Video (8 min): Reinforce your understanding with this video that demonstrates an RC timing circuit.

Reading: Read the first four sections and make notes about the ideal and 555 based monostables. Pay attention to the timing diagram and the concept of a falling edge for the trigger. Study the circuit and operation of a 555 based monostable. It is important to know the good and bad points about this monostable as it is not an ideal monostable, but it is very useful.

Video (10 min): Watch the video about the 555 monostable to see how it is built on protoboard, how to draw out the circuit and how to use the relevant equation to calculate the time period.

Reading: Read about the NAND gate monostable, which is a much better monostable in many respects, and then think about the final section which describes a very poor monostable but, in doing so, helps to think about how monostables actually work. Make sure that the equations for the time periods of the 555 and NAND monostables are familiar.

Video (7 min): Watch the video about the NAND monostable to see how it is built on protoboard, how to draw out the circuit and how to use the relevant equation to calculate the time period.

Interactive White Board Video (7 min): Follow the video through several examples of how to calculate time period, resistor values and capacitor values for the 555 monostable. Each example is explained in detail.

Reading: Read the first seven sections of the astable page. Each section is about a different aspect of the 555 astable circuit. Make sure that to understand the different equations and how to go about calculating the required resistor and capacitor values to achieve a given frequency, period or mark-space ratio.

Video (8 min): Watch the video to see how to make a 555 astable circuit on protoboard, draw the circuit, make use of the reset pin and observe the output on an oscilloscope.

Interactive White Board Video (12 min): Follow the video through several examples of how to calculate time period, resistor values and capacitor values for the 555 astable. Each example is explained in detail.

Reading: Read the final two sections about alternative astable circuits. Both the relaxation oscillator and the NOR gate astable have mark-space ratios of 1:1 unlike the 555 astable.

Exercises: Complete the problems to check your understanding.

### Lesson Review

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

Notes: PDF download. All about capacitors, identifying electrolytic and non-electrolytic and how to use the units of capacitance.

Presentation: Powerpoint download. RC networks used as timing circuits including the equations for time constant and half value time as well as graphs of charging and discharging.

Notes: PDF download. Resistor - Capacitor networks used as timing circuits including the idea of a time constant and the time period.

Presentation: Powerpoint download. The 555 and NAND gate monostables explained along with the relevant equations to calculate the time period.

Notes: PDF download. Both the NAND gate and 555 based monostables explained along with the equations to calculate the period in each case.

Presentation: Powerpoint download. The 555 astable circuit explained along with the relevant equations to calculate the time period and mark-space ratio.

Notes: PDF download. The 555 astable explained in detail along with the equations to calculate the period in each case. Other astable circuits are considered briefly.

### Self Assessment

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

Quiz: Questions about capacitors, capacitor values and RC circuits.

Quiz: Questions about monostable and astable circuits.

### Self Evaluation

I can:

• Describe the action of a capacitor and use the equation Q = C × V
• Explain how an RC circuit can be used to produce a time delay
• Describe how the voltage across a charging capacitor in an RC circuit varies with time
• Describe how the voltage across a discharging capacitor in an RC circuit varies with time and interpret decay graphs
• Use the equation T½ = 0.7 × R × C for an RC circuit
• Describe the action of a monostable circuit including the need for a trigger
• Understand what is meant by a falling edge trigger
• Draw the circuit diagram for a 555 based monostable and use the equation T = 1.1 × R × C
• Draw the circuit for a NAND gate monostable and use the equation T = 0.7 × R × C
• Describe the action of an astable circuit in terms of period, mark and space
• Understand the meaning of frequency and mark:space ratio when describing the output of an astable circuit
• Draw the circuit diagram for a 555 based astable
• Use the appropriate equations for period, frequency, mark, space and mark:space ratio for a 555 astable
• Interpret voltage-time graphs for monostable and astable circuits