Chapter 7 - Timing circuits

What is an RC network?

A resistor-capacitor circuit used to produce a time delay

How does an RC network create a time delay?

Capacitor charges/discharges through resistor; time taken depends on R and C

What is a capacitor?

Electronic component that stores electrical energy as a charge, used for timing or smoothing

What is the unit of capacitance?

Farad (F), usual sub-units are microfarads (µF)

What does increasing the value of C or R in an RC circuit do to the time delay?

Increases the time delay (slower charge/discharge)

What voltage does a fully-charged capacitor have across it?

Equal to the supply voltage

What happens when a capacitor discharges through a resistor?

Voltage across it falls gradually following a predictable curve

What shape is the charge/discharge curve of a capacitor?

Exponential (quick at first, then slows)

How does the voltage across a charging capacitor vary with time?

Rises quickly at first, approaches supply voltage slowly

How does the voltage across a discharging capacitor vary with time?

Falls quickly at first, approaches zero slowly

What is a timing circuit?

Circuit that uses an RC network to provide a time delay to an output

What common component is used for accurate, repeatable timing?

The 555 timer IC

What does a monostable timer do?

Produces a single time pulse when triggered

Which formula gives the pulse period for a 555 monostable timer?

T (s) = 1.1 × R × C

If R is in MΩ and C in µF, what are the units for T?

Seconds

If R is in kΩ and C in µF, what are the units for T?

Milliseconds

What does an astable timer do?

Produces continuous square wave oscillation (pulses)

How can you visualise a timing circuit's output?

Use an oscilloscope, simulation, or a computer program set up as an oscilloscope

For a 555 astable timer, what is the mark time (time output HIGH)?

T1 = 0.7 × (R1 + R2) × C

For a 555 astable timer, what is the space time (time output LOW)?

T2 = 0.7 × R2 × C

For a 555 astable, total period = ?

T = T1 + T2

For a 555 astable, frequency f = ?

f = 1 / (T1 + T2) or f = 1.44 / ((R1 + 2R2)C)

What is the amplitude of an astable output wave?

Height from lowest (0V) to highest (supply voltage)

How do you calculate mark–space ratio for an astable?

Mark–space = Mark time ÷ Space time (T1/T2)

How do you change an RC circuit to vary timing?

Increase either R or C to get a longer time delay

What is a buffer in timing circuits?

A processing unit placed between timing circuit and output to strengthen signal and supply more current

What are the two main types of capacitors?

Polarised (electrolytic—must be correctly connected), non-polarised (may be connected either way)

Why are electrolytic capacitors used for timing?

Higher capacity for the same physical size

Why must you observe polarity for electrolytic capacitors?

Wrong polarity can cause them to overheat and possibly explode

What is the initial voltage across a capacitor when charging begins?

0V

What is the voltage across a capacitor after a long time charging?

Equals supply voltage

What is the effect on lamp or LED brightness as a capacitor discharges?

Gets dimmer as voltage falls

What is a practical example of a monostable circuit?

Pelican crossing timer, delay after a button press, automatic light-off circuit

What is a practical example of an astable circuit?

Flashing beacon, pulse generator, tone generator

How do you make timing adjustable in monostable circuits?

Use a variable resistor (potentiometer) in series with a fixed resistor/capacitor

Which values of R and C are best for monostable accuracy?

R large (but >1kΩ to limit circuit current), C value as required

When calculating from T = 1.1RC, how do you convert units if R is in kΩ and C is in µF?

T is in milliseconds (ms)

What type of waveform does an astable timer produce?

Square wave (alternating on/off)

On an oscilloscope, what does the time base control set?

Horizontal scale (ms/cm)

On an oscilloscope, what does the voltage gain control set?

Vertical scale (V/cm)

If an oscilloscope trace shows a horizontal cycle covering 6 cm at 2 ms/cm, what is one period?

12 ms (6 × 2)

If a mark is 2 cm and space is 1 cm at 2 ms/cm, what is the mark time and the space time?

Mark: 4 ms, Space: 2 ms

How to convert period to frequency?

frequency = 1/period

In a 555 monostable timer, what triggers the timing?

A falling edge (switch or trigger pulse)

What is one risk when using load devices (like lamps) directly on timing RC circuits?

Current draw affects timing accuracy

How do you safely operate a lamp with a timing circuit?

Use a buffer (transistor or two NOT gates) between timing unit and output

If a 555 monostable is triggered, what happens to output?

Output goes HIGH for a time set by 1.1RC, then returns LOW

Why use a fixed resistor in series with a variable resistor for RC timing?

Prevents setting resistance so low it damages the circuit

What determines the charge/discharge time of a capacitor?

Product of resistance and capacitance (RC)

What is 'period' of a wave?

Time for one complete cycle (mark + space)

What is the 'frequency' of a wave?

Number of cycles per second (Hz)

If f = 2 Hz, what is the period?

0.5 seconds (T = 1/f)

What is the preferred way to design accurate timers?

Use lower value capacitor, higher value resistor, avoid loads on RC circuit itself

What happens to the period if you double the capacitor value?

Period doubles

What happens to the period if you halve the resistor value?

Period halves

If you want T = 5s and use C = 100µF, what is R needed?

R = T / (1.1 × C) = 5 / (1.1 × 0.0001) ≈ 45.5 kΩ

For a 555 astable, how can you make the mark:space ratio = 1:1?

Make R2 much bigger than R1, approximately double (R2 ≈ 2 × R1)