Chapter 2: Timing Circuits Flashcards
Vc is equal to 0.5V0 after how long
0.69RC
For a charging capacitor Vc≈V0 after how long?
5RC
For a discharging capacitor Vc≈ ? after 5RC
0V
How is the time constant of a circuit calcultated?
RC
R=value of resistor, Ohms,Ω
C=Value of capacitor, Farads,F
What is time constant?
The rate that the capacitor charges/discharges at a rate determined by the size of both the resistor and capacitor
What are monostable circuits useful for?
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
What is the ideal monostable behaviour
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.
What is the basic building block of any monostable timer?
The RC network
What is the negative effect of the RC network in a monostable circuit?
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
How can the negative effect of the RC network be stopped?
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.
Name the two types of 555 timer circuits
555 astable and 555 monostable
On a 555 monostable, what is the minimum value of resistance on the timing resistor?
1kΩ
The current is limited by the timing resistor on a 555 circuit, what does this prevent?
It prevents the components from overheating
State the equation associated with the time delay of a 555 monostable circuit.
T=1.1RC
What is mechanical bounce?
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.
State 2 ways in which mechanical bounce can be cured
Using the RC network and using a monostable circuit.
Explain how the RC network is used to cure mechanical bounce
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.
State the limitations of using an RC network to cure mechanical bounce
-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
Explain how using a monostable can cure switch bounce
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.
Does the astable circuit have a stable state?
No-Its output switches continually between logic 1 and logic 0. It is sometimes known as a ‘pulse generator’ or a ‘clock’.
On an astable circuit, what is the ‘mark-space ratio’ ?
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
On an astable circuit what is the minimum value of resistance for R1 and R2?
1kΩ to prevent overheating
State one way to create an astable sub-system?
Use a Schmitt inverter.
Draw a Schmitt inverter sub system
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
Draw a 555 monostable
Draw a 555 astable
Is the value of a capacitor on pinout 5 of the 555 chip important?
No-it is not included in calculations
but it’s approximate value is 10nF
Assume that the capacitor of a Schmitt inverter astable circuit is uncharged initially. What would be the output of the Schmitt?
Logic 1
If the Schmitt inverter is at logic 0, will the capacitor charge or discharge?
Capacitor C begins to charge through the resistor R and the voltage at the input of the Schmitt NOT gate starts to rise.
When the Schmitt inverter voltage becomes logic 1, what happens to the output and does the capacitor charge or discharge?
Its output changes to logic 0. The capacitor now starts to discharge through R and the voltage across C falls.
