1. Cathode Ray Oscilloscope (CRO)
Electronics
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Cathode Ray Oscilloscope (CRO)
Structure of CRO
Photo
Vertical deflection of the spot
Diagram
Horizontal deflection of the spot
Diagram
Common patterns on the screen
Photo
Scales used and buttons of control
Example

2. Electronic Devices Electronic Device 1 Electronics Semiconductor diode
Next Slide
Electronic Devices
Semiconductor diode
Diagram
Light emitting diode (LED)
Diagram
Light dependent resistor (LDR)
Diagram
Thermistor
Diagram
Reed switch
Diagram
Reed relay
Diagram
Potential divider (Potentiometer)
Diagram

3. Logic gates Logic Gates 1 Electronics Definitions of a logic gate
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Logic gates
Definitions of a logic gate
Explanation
Various kinds of logic gates
Diagram

4. Integrated circuit Integrated Circuit 1 Electronics Fire alarm
Next Slide
Integrated circuit
Fire alarm
Diagram
Safety thermostat for water heating system
Diagram
Car door warning signal
Diagram
Simple burglar alarm
Diagram
Night-time anti-theft device for cars
Diagram

5. END of Electronics

6. CRO 1 Electronics Next Slide
A Cathode Ray Oscilloscope (CRO) is shown below.

7. CRO 1 Electronics Next Slide
The structure of CRO is shown below. It can be divided into three parts, electron gun, deflection system and fluorescent system.
cathode
grid
anodes
Y-plates
X-plates
electron
beam
spot
fluorescent
screen
deflection system
electron gun

8. Back to CRO 1 Electronics Click Back to
Electron gun : It emits a narrow beam of electrons which is called cathode ray.
Deflection system : It can deflect the path of electrons vertically by applying a p.d. across the X-plates and horizontally by applying a p.d. across the Y-plates.
Fluorescent screen : It is coated with fluorescent material. When the electron beam hits the screen, light is emitted.

9. CRO 1 Electronics Next Slide
Suppose X-plates have no connection, the patterns on the screen for different kinds of input voltage for Y-plates are shown below.
screen
0 V across Y-plates means no deflection for the electron beam. The spot is at the centre of the screen.

10. CRO 1 Electronics Next Slide
screen
A constant d.c. voltage , say 3 V, is across Y-plates means deflection for the electron beam. The spot is deflected upwards. The larger the d.c. voltage, the larger the deflection is.

11. Back to CRO 1 Electronics Click Back to
screen
A non-constant a.c. voltage (50 Hz), is across Y-plates means unstable deflection for the electron beam. The spot is deflected upwards and downwards rapidly and forms a vertical straight line.
The p.d. across the Y-plates is measured by the Y-gain control (or gain control). It is calibrated in V/cm or V/div. The p.d. can then be found by measuring the deflection of the spot from the centre of the screen.

12. Back to CRO 1 Electronics Click Back to
The X-plates are connected to a special circuit called time-base circuit. It makes the spot to sweep across the screen from left to right at a steady rate and then the spot will move back to its starting point very rapidly and repeat the motion.
screen
time-base
circuit
The sweeping rate is measured by s/cm or s/div.

13. CRO 1 Electronics Next Slide
Common patterns on the screen on the CRO are shown.
Y-plate : no connection
X-plate : no connection
Y-plate : d.c. voltage
X-plate : no connection

14. CRO 1 Electronics Next Slide
Common patterns on the screen on the CRO are shown.
Y-plate : a.c. voltage
X-plate : no connection
Y-plate : no connection
X-plate : time base circuit

15. CRO 1 Electronics Next Slide
Common patterns on the screen on the CRO are shown.
Y-plate : d.c. voltage
X-plate : time base circuit
Y-plate : a.c. voltage
X-plate : time base circuit

16. Back to CRO 1 Electronics Click Back to
The control panel of a CRO is shown below.
CRO can act as a voltmeter for both d.c. and a.c.

17. CRO 1 Electronics Next Slide
An a.c. power supply is connected to the CRO as shown.
(a) What is the peak voltage of the a.c. supply?
(b) What is the frequency of the a.c. supply?
20 ms/div
5 V/div
Time base
Gain control
X-plate
Y-plate
Earth
a.c. supply

18. Back to CRO 1 Electronics Click Back to Peak voltage = 1  5 V = 5 V
Period = 4  20 ms = 80 ms = 0.08 s
 Frequency = 1/T = 1/0.08 = 12.5 Hz

19. Electronic Device 1 Electronics Next Slide
Semiconductor diode and its symbol are shown below:
symbol

20. Back to Electronic Device 1 Electronics Click Back to
Diode allows the current to flow through it only in the direction indicated. If the current wants to flow through it in opposite direction, the circuit is treated as an open circuit.
circuit closed
(lamp lighted)
circuit opened
(lamp not lighted)

21. Electronic Device 1 Electronics Next Slide
Light emitting diode (LED) and its symbol are shown below:
symbol

22. Back to Electronic Device 1 Electronics Click Back to
A LED emits light when current flows through it in the forward direction.
circuit opened
(lamp not lighted)
LED not
lighted
circuit closed
(lamp lighted)
LED lighted

23. Electronic Device 1 Electronics Next Slide
Light dependent resistor (LDR) and its symbol are shown below:
symbol

24. Back to Electronic Device 1 Electronics Click Back to
Resistance of LDR depends on the intensity of light falling on it. In the dark, the LDR has a high resistance. In the light, it has low resistance.
LED glows when LDR is
lighted by a torch.
LED dies out when the
torch is removed.

25. Electronic Device 1 Electronics Next Slide
Thermistor and its symbol is shown below:
symbol

26. Back to Electronic Device 1 Electronics Click Back to
The resistance of thermistor, which is made from a semiconductor, varies with temperature. If we heat the semiconductor, its resistance decreases.
LED glows when the thermistor
becomes hot.
LED dies out when the thermistor
becomes cold.

27. Electronic Device 1 Electronics Next Slide
Reed switch and its symbol is shown below:
symbol

28. Electronic Device 1 Electronics Next Slide
Reed switch consists of two metal contacts inside a glass envelope. When a magnet is brought near the switch, the metal become magnetized and they would be in contact with each other. If we remove the magnet, the contact disappears.
N S
magnet
buzzer
The magnet closes the circuit and
the buzzer produces sound
After removing the magnet,
the circuit becomes open and the
sound disappears.

29. Back to Electronic Device 1 Electronics N S door fixed magnet
Magnet in
the door
Click Back to
The circuit shows how a reed switch can be used for burglar alarm. If someone opens the door without permission, the alarm sounds.

30. Electronic Device 1 Electronics Next Slide
Reed relay and its symbol are shown below:
symbol

31. Back to Electronic Device 1 Electronics Click Back to
Reed relay consists of a reed switch inside a coil. When current passes through the coil, it becomes an electromagnet and the reed switch is closed. Therefore, we can control the flow of large current in a circuit with a small current. M
Circuit with small current
Circuit with large current
Close the switch would close the reed relay and hence switch on the motor.

32. Electronic Device 1 Electronics Next Slide
2 variables resistors for potential dividers are shown below:

33. Back to Electronic Device 1 Electronics Click Back to
Different kinds of potential dividers are shown below:
Sliding
contact
Potential divider circuit which contains a variable resistor.
Potential divider circuit which contains one fixed resistor and an LDR.

34. Back to Logic Gates 1 Electronics Click Back to
High potential or low potential can be fed into a device which would gives out an output voltage (may be high or low potential). This device is called a logic gate. The output depends on the input signal as well as the logic gate used.
Logic gate
Output (only one)
Input (may be one or two)
High potential is represented by (1/+/True).
Low potential is represented by (0//False).
The table that describes the behaviour of that kind of logic gate is called a truth table.

35. Logic Gates 1 Electronics Next Slide NOT Gate AND Gate Symbol : A
Output
Truth table :
1/T
0/F
Symbol : A
Output
Truth table : B
0/F
1/T

36. Logic Gates 1 Electronics Next Slide OR Gate NAND Gate Symbol : A
Output
Truth table : B
0/F
1/T
Symbol : A
Output
Truth table : B
0/F
1/T

37. Back to Logic Gates 1 Electronics Click Back to NOR Gate Symbol : A
Output
Truth table : B
0/F
1/T

38. Integrated Circuit 1 Electronics Next Slide
Fire alarm’s integrated circuit
Purpose : At room temperature, the buzzer does not sound. At fire, the buzzer sounds.
 low
potential
+ high A O

39. Back to Integrated Circuit 1 Electronics Click Back to
Normal state : At low temperature, the resistance of the thermistor is very large compared with the resistance of the variable resistor. Point A is said to be at high potential (+) since the p.d. between A and low potential line is much larger than the p.d. between A and high potential line. Hence, the output O should be at low potential () and the buzzer does not sound.
At fire : At high temperature, the resistance of the thermistor is very small compared with the resistance of the variable resistor. Point A is said to be at low potential () since the p.d. between A and low potential line is much smaller than the p.d. between A and high potential line. Hence, the output O should be at high potential (+) and the buzzer sounds.
The variable resistor can be adjusted to change the sensitivity of the circuit.

40. Integrated Circuit 1 Electronics Next Slide
Safety thermostat for water heating system’s integrated circuit
Purpose : Heater is on only when water is present and the temperature is low
 low
potential
+ high A B
heater

41. Back to Integrated Circuit 1 Electronics Click Back to
When water is present, B is at high potential (+). When temperature is low, the resistance of the thermistor is large. Therefore, the p.d. between A and low potential line is large and A is also at hight potential (+). The output would be at high potential and the reed relay would switch on the heater.
If water is not present or the temperature of the water is high, either A or B would be at low potential. The output would be at low potential and the reed relay would switch off the heater.
The variable resistor can be adjusted to change the sensitivity of the circuit.

42. Integrated Circuit 1 Electronics Next Slide
Car door warning signal’s integrated circuit
Purpose : If either of the two car doors (with the switches installed) is not closed, the LED should be on.
 low
potential
+ high A B

43. Back to Integrated Circuit 1 Electronics Click Back to
When both switches are closed, A and B are both at low potential (). The output is low potential and no current would flow through the LED. No light would be observed.
If one of the switch is open, either A or B would not be at low potential (). It is said to be at high potential (+). The output is high potential and current would flow through the LED in the indicated direction. Light would be emitted.

44. Integrated Circuit 1 Electronics Next Slide
Simple burglar alarm’s integrated circuit
Purpose : If the floor (with switch installed) is stepped or the light from a torch is detected (by the LED), the alarm should sound.
 low
potential
+ high A B O

45. Back to Integrated Circuit 1 Electronics Click Back to
If the switch is pressed, A is at low potential (). If light falls on the LDR, the resistance of the LDR would be so small that the p.d. between B and the low potential line becomes very small. B would be at low potential. Therefore, when the switch is pressed or the light is detected by LDR, either A or B would be at low potential (), the output is at high potential (+) and the alarm sounds.
When the switch is open and no light is detected, both A and B would be at hgih potential (+). The output is at low potential. The buzzer does not sound.

46. Integrated Circuit 1 Electronics Next Slide
Night-time anti-theft device’s integrated circuit (for cars)
Purpose : The motor can only be switched on by pressing the switch at the daytime.
 low
potential
+ high A B O M

47. Back to Integrated Circuit 1 Electronics Click Back to
At daytime and the switch is pressed, A is at low potential () and B is also at low potential () due to the small resistance of the LDR. Therefore, the output is high potential (+) and the reed realy switch on the motor.
When the switch is open and no light is detected, both A and B would be at high potential (+). The output is at low potential. The buzzer does not sound.