Topic 7: Timing Circuits

Question 1

What are the two main types of capacitors?

Answer 1

Polarised (has a + or - on the left) and Non-Polarised

Question 2

What does a charging capacitor graph look like?

Answer 2

Upwards curve, like an ‘r’, steep gradient

Question 3

What does a discharging curve look like?

Answer 3

Downwards curve, like an ‘l’, steep gradient

Question 4

For the T=1.1RC rule for monostable, what is the relationship between R, C and T?

Answer 4

When R is kΩ, C is µF and T is ms
When R is MΩ, C is µF and T is s

Question 5

What are the parts of a 555 Monostable?

Answer 5

Bottom Right - CTRL, connected to capacitor, connected to ground
Bottom Left - GND, connected to ground
Left Side Bottom - TRIG, line out with circle
Left Side Middle - THR, threshold, line out and up to discharge
Left Side Top - DIS, horizontal line between resistor and capacitor
Top Left - NOT Reset, connected to Vcc
Top Right - Vcc, connected to Vcc

Question 6

What is an astable?

Answer 6

No stable output state, switch from OV and supply voltage
Sometimes called pulse generator

Question 7

How do you find the time the output is on in astable?

Answer 7

On - Mark
Off - Space
f = 1/(T1 +T2)

Question 8

How to interpret an astable oscilloscope trace?

Answer 8

Amplitude - Lowest to highest vertical distance
Frequency - 1/ Sum of mark and space (e.g mark = 4ms, space = 2 ms , cycle 6ms, 1/6 = 166.6 Hz)

Question 9

If the period is in seconds, what will the frequency be?

Answer 9

Hz

Question 10

If the period is in milliseconds what will the frequency be?

Answer 10

kHz

Question 11

What are the parts of a 555 Astable?

Answer 11

Bottom Right - CTRL, connected to capacitor, connected to ground
Bottom Left - GND, connected to ground
Left Side Bottom - TRIG, horizontal line between second resistor and capacitor
Left Side Middle - THR, threshold, line out and down to TRG
Left Side Top - DIS, horizontal line between two resistors
Top Left - NOT Reset, connected to Vcc
Top Right - Vcc, connected to Vcc