Chapter 2: Timing Circuits

Question 1

Vc is equal to 0.5V0 after how long

Answer 1

0.69RC

Question 2

For a charging capacitor Vc≈V0 after how long?

Answer 2

5RC

Question 3

For a discharging capacitor Vc≈ ? after 5RC

Answer 3

0V

Question 4

How is the time constant of a circuit calcultated?

Answer 4

RC
R=value of resistor, Ohms,Ω
C=Value of capacitor, Farads,F

Question 5

What is time constant?

Answer 5

The rate that the capacitor charges/discharges at a rate determined by the size of both the resistor and capacitor

Question 6

What are monostable circuits useful for?

Answer 6

Creating delay sub-systems of a configurable length. For example, they can keep an outside lamp on for 20 seconds when triggered by a movement sensor

Question 7

What is the ideal monostable behaviour

Answer 7

A monostable circuit has one stable state, either logic 0 or logic 1. It remains in that state until triggered into the opposite logic state by an external signal. After a predetermined time, the output returns to the original stable state. This behaviour is shown in the voltage / time graphs for initially low or high situations.

Question 8

What is the basic building block of any monostable timer?

Answer 8

The RC network

Question 9

What is the negative effect of the RC network in a monostable circuit?

Answer 9

Where an output current flows, the timing is disturbed and the delay depends on the size of the current, which is not a desirable feature

Question 10

How can the negative effect of the RC network be stopped?

Answer 10

Buffer the RC network with a component such as a NOT gate.The output
current then comes from the power supply to the NOT gate and not from the RC network itself. The RC network controls only when the NOT gate switches.

Question 11

Name the two types of 555 timer circuits

Answer 11

555 astable and 555 monostable

Question 12

On a 555 monostable, what is the minimum value of resistance on the timing resistor?

Answer 12

1kΩ

Question 13

The current is limited by the timing resistor on a 555 circuit, what does this prevent?

Answer 13

It prevents the components from overheating

Question 14

State the equation associated with the time delay of a 555 monostable circuit.

Answer 14

T=1.1RC

Question 15

What is mechanical bounce?

Answer 15

A switch is simply two metal bars, separated by air when switched ‘off’ and pressed into contact when ‘on’.However, metal is ‘springy’ and when the metal bars flick into contact, they bounce off again and can do so a number of times. The effect is the same as if the user had closed and opened the switch several times rapidly.

Question 16

State 2 ways in which mechanical bounce can be cured

Answer 16

Using the RC network and using a monostable circuit.

Question 17

Explain how the RC network is used to cure mechanical bounce

Answer 17

Connecting, to the switch, a capacitor in parallel with the resistor, creating the
discharge circuit. On first contact, the switch connects the capacitor to the positive supply rail and the capacitor charges rapidly to that voltage. Further bounces of the switch contacts do little, as the capacitor has not had time to discharge much.

Question 18

State the limitations of using an RC network to cure mechanical bounce

Answer 18

-if the time constant of the RC network is too large, the action of the circuit will slow, since the slow capacitor discharge renders the circuit insensitive to switch presses for some time;
- if the time constant of the RC network is too small, some of the switch bounces will not be
suppressed.
-the approach lies on the fact that the voltage across the capacitor changes slowly

Question 19

Explain how using a monostable can cure switch bounce

Answer 19

Monostable debounce circuits rely on the
fact that further trigger pulses, occurring
before the monostable ‘times out’, are
ignored. The output pulse begins on the first trigger pulse. Additional trigger pulses occurring before the end of the monostable pulse are ignored.

Question 20

Does the astable circuit have a stable state?

Answer 20

No-Its output switches continually between logic 1 and logic 0. It is sometimes known as a ‘pulse generator’ or a ‘clock’.

Question 21

On an astable circuit, what is the ‘mark-space ratio’ ?

Answer 21

The ‘mark’ refers to the time ‘on’ at logic 1, and the ‘space’ refers simply to the time ‘off’ at logic 0.
Note: the mark is always longer than the space

Question 22

On an astable circuit what is the minimum value of resistance for R1 and R2?

Answer 22

1kΩ to prevent overheating

Question 23

State one way to create an astable sub-system?

Answer 23

Use a Schmitt inverter.

Question 24

Draw a Schmitt inverter sub system

Answer 24

The schmitt inverter input is connected to a capacitor (which is grounded). The input is also connected to a feedback resistor, which sits between the input and output of the Schmitt inverter

Question 25

Draw a 555 monostable

Question 26

Draw a 555 astable

Question 27

Is the value of a capacitor on pinout 5 of the 555 chip important?

Answer 27

No-it is not included in calculations
but it’s approximate value is 10nF

Question 28

Assume that the capacitor of a Schmitt inverter astable circuit is uncharged initially. What would be the output of the Schmitt?

Answer 28

Logic 1

Question 29

If the Schmitt inverter is at logic 0, will the capacitor charge or discharge?

Answer 29

Capacitor C begins to charge through the resistor R and the voltage at the input of the Schmitt NOT gate starts to rise.

Question 30

When the Schmitt inverter voltage becomes logic 1, what happens to the output and does the capacitor charge or discharge?

Answer 30

Its output changes to logic 0. The capacitor now starts to discharge through R and the voltage across C falls.