TOPIC: ELECTRONICS 4.3 UNDERSTANDING TRANSISTORS CHAPTER HIGHLIGHT: Transistor

1. A simple junction transistor consists of a crystal of one type of doped semiconductor which is sandwiched between two crystal of opposite type

npn transistor

pnp transistor

p-type semiconductor sandwiched with n-type semiconductor

n-type semiconductor sandwiched with p-type semiconductor

2. Current flows in transistor

i. Based current, Ib controlled the collector current, Ic ii. If Ib = 0, so Ic = 0 iii. Small change on Ib caused the big changes in Ic.

3. transistor as current amplifier

Switch S is off Ib = 0, so Ic = 0 Ib = Base current Ic = Collector current

Switch S is on There is a small current at base Ib. This will cause a large current in the collector, Ic.

4. transistor as automatic switch

Fire alarm system

At room temperature When the fire is on, the temperature is increased

At room temperature Fire is on Thermistor resistance is high Temperature of thermistor increases Potential difference between XY is high Resistance of thermistor is reduced Potential difference between YZ is low Vxy is low Ib is very small Vyz is high Ic is small Ib increased Ic cannot turn on the relay Ic increased and turned on the relay Bell is not activated Bell is ringing

ACTIVITY A: 1. The flow of the _______ current depends on __________ current. 2. Figure below shows a circuit which can be used as an early morning alarm.

The alarm will activate when a light is detected.

(a) (i) Named component R1, L, M and N R1 : L : M : N : (ii) What is the function of the resistor R3?

(iii) If the current flowing through N is 0.5 mA and potential difference crossing the terminals N is 1.0V. How many resistance of R3?

(b) (i) What is the change to R3 resistance when it is exposed to a light?

(ii) What is the changes to the potential difference crossing R1 when it is exposed to a light?

SOLUTION ACTIVITY A:

1. collector, base 2. (a) (i)

R1 : Light Dependent Resistors (LDR) L : Diode M : Relay switch N : npn transistor

(ii) limited base current

(iii) R = IV =

0005.00.1 = 2 x 10-3 Ω

(b) (i) R1 resistance decreases (ii) potential difference across R1 decreases

ACTIVITY B

Figure above shows a transistor used in an electronic alarm system. (a) What is type of the transistor used?

(b) At room temperature, the thermistor resistance is 2 k Ω. Calculate the potential difference between :

(i) PR (ii) PQ (iii) QR

(c) Explain a relationship between thermistor resistance and temperature.

(d) When a fire ccured, describe what will happened to : (i) potential difference across QR (ii) base current

(iii) collector current

SOLUTION ACTIVITY B:

(a) npn transistor (b) (i) 6 V

(ii) potential difference PQ = Ω+

Ω2)k(10

2k x 6 V

(iii) potential difference QR =potential difference PR - potential difference PQ

= 6 V – 1 V = 5 V

(c) Thermistor resistance decreases when temperature increases

(d) (i) potential difference across QR increases

(ii) base current increases (iii) collector current increases

ACTIVITY C In the circuit shown in figure below, a 4700Ω resistor is connected in series with a light-dependent resistor (LDR), which has a resistance of 3300Ω, and with a 12 V power supply.

(a) Calculate the current through the LDR. (b) Calculate the voltage across the LDR.

(c) The resistance of the LDR decreases as brighter light falls on it.

Describe and explain how the voltage across the LDR changes as the brightness of the light that falls on it increases.

(d) The LDR and resistor are connected to a transistor, as shown in Figure 1.2.

(i) When the LDR is in the dark, lamp L glows brightly. Explain briefly why this happens.

(ii) When the light on the LDR is bright enough, lamp L does not

glow. Explain briefly why this happens.

SOLUTION ACTIVITY C:

(a) current = RV =

3300470012+

= 1.5 × 10-3 A

(b) voltage = IR = 1.5 × 10-3 × 3300 = 5.0 V

(c) As brightness increases, the resistance of the LDR decreases. Since

the arrangement is a potential divider, the voltage across the LDR decreases.

(d) (i) In the dark, the LDR has a high resistance. The transistor is ‘on’ and

a voltage close to 12 V is now applied across the lamp.

(ii) In the light, the LDR has a low resistance. The transistor is ‘off’ and a voltage close to 0 V is now applied across the lamp.

ASSESSMENT : Objective Question 1. The diagram shows the symbol for a transistor.

Which of the following shows the correct name of the electrode P and the type of the transistor?

Electrode P Type of transistor

A Emitter pnp* B Collector pnp C Emitter npn D Collector npn

2. The diagram shows a transistor used as an automatic switch. Bulb P will

light up if potential difference across S is equal or more than 7.2V. In the beginning, resistance of the light-dependent resistor R is 1 M Q. When exposed to light, resistance of R decreases.

What is the value of R when P starts to light? A 200V B 400V* C 600V D 800V E 900V

3. In which circuit does the bulb light up?

4. In which circuit will the light-emitting diode (LED) light up when switch is

on?

Structured Question

1. Name two main types of transistors and draw their symbols.

(i) (ii)

2. State two uses of transistor. (i)____________________ (ii)_______________________

3. If there are no flow of the base current, the current collector

is___________

4. A change that __________ in the base current can caused changes that___________ in the collector current.

5. Figure below shows a light dependent resistor (LDR), resistor R and S, a

light emmiting diod (LED), a transistor and a battery that will be connected to form a circuit. The LED emmits lights when it is in a bright surroundings.

(a) (i) State one function of a transistor. __________________________________ [1 mark] (ii) Complete the circuit (iii) Give one reason why LED emmits light in a bright surrounding. _________________________________________ [2 mark]

(b) What modification is required to the circuit so that the

LED will emmits light when the surrounding become dark? _______________________________ [1 mark]

(c) An alarm is needed which emmits sound when there is

a fire. Two modifications have to be made to the circuit in (a)(ii) by replacing electronic components.

(i) State one electronic component which needs to be replaced. Give a reason for your answer. _____________________________

_____________________ [2 mark] (ii) Name two electronic component that are needed

to replace the unsuitable components. ____________________________ [2 mark]

(iii) In the space below, draw a circuit diagram to show

the new circuit.

[2 mark]

6. Two similar bulbs, P and Q are connected to a transistor in the base circuit and collector circuit respectively, as shown in figure below. When S is switched on, bulb P does not light up but bulb Q light on. Then the base circuit is connected to two batteries as shown in figure below. When S is switched on, bulb P lights up but bulb Q lights up more brightly.

Using this information; a. make one suitable inference, [1 mark] b. state one appropriate hypothesis that couldnbe investigated, [1 mark] c. describe how you would design an experiment to test your

hypothesis using transistor and other apparatus. In your description, state clearly the following: (i) aim of the experiment (j) variables in experiment (k) list of apparatus and materials (l) arrangement of the apparatus (m) the procedure of the experiment, which includes the method

of controlling the manipulated variable and method of measuring the responding variable

(n) the way you would tabulate the data (o) the way you would analyse the data

[10 mark]

Answer (Assessment) Objective question 1. A 2. B 3. A 4. A Structured Question 1.

2. (i) transistor as current amplifier

(ii) transistor as automatic switch

3. zero 4. small, big 5. (i) current amplifier

(ii)

(iii) Resistance LDR decrease when the potential difference LDR decreases. Base current across through S increases

(a) replace the resistance of R with the LDR

(b) (i) LED are needed to be replaced by electronic component that can

produce the wave sounds (ii) Bell and thermistor

(iii)

6. (a) Collector current in a transistor is affected by the base current [1]

(b) As base current increases by a little amount, the collector current will increases by a large amount. [1]

(c) (i) to investigate the relationship between collector current and base current in a transistor. [1]

(ii) variables:

- manipulated : base current - responding : collector current - constant : Type of transistor used [1]

(iii) Transistor pnp, resistor R=10kΩ, battery, miliammeter,

microammeter, switch, connecting wire [1]

(iv)

[1] (v) A battery is connected across MN. The switch is switched on.

Reading of microammeter is recorded. This is the value of base current Ib. Reading of miliammeter that corresponds is recorded. This is the value of collector current, Ic. The experiment is repeated by connecting 2, 3, 4 and 5 bateries across MN. The corresponding values of Ib and Ic are recorded. [4]

(vi)

No. Of battery Base current, Ib/micrometre

Collector current, Ic/milimetre

1 2 3 4 5

[1]

(vii) Graph of collector current, Ic againts base current, Ib is plotted.

[1] [10]

mnza

Ic

Ib 0

TOPIC: ELECTRONICS 4.4 ANALYSING LOGIC GATES CHAPTER HIGHLIGHT:

5. Types of logic gates and their truth tables - Logic gates are electronic devices which have only two output states. - The potential of the output is either ‘HIGH’ (5 – 6 V) or ‘LOW” (0 V).

Types of logic gates

Logic

symbol

Boolean

expression

Truth table

Action of logic

gates

Characteristic of logic

gates

NOT

A = Y

input output

A Y 0 1 1 0

The output is high if the input is not high. The output is always the opposite of

the input.

The output is opposite to the input. It

is an inverter.

AND

A • B = Y

input outputA B Y 0 0 0 0 1 0 1 0 0 1 1 1

The output is

high only if input X and input Y

are high.

If both inputs is ‘1’ will give

output ‘1’

OR

A + B = Y

input outputA B Y 0 0 0 0 1 1 1 0 1 1 1 1

The output is

high when either X or Y or both

are high.

When either; input is ‘1’, output is ‘1’

NAND

BA • = Y

input outputA B Y 0 0 1 0 1 1 1 0 1 1 1 0

The output is

not high only if input X and

input Y are high.

The output is opposite to the AND

gate output

NOR

BA + = Y

input outputA B Y 0 0 1 0 1 0 1 0 0 1 1 0

The output is

not high if either input X or input

Y are high.

The output is opposite to the OR gate

output

6. Logic gates in combination i.

The truth table

Input Logical operation Output

A

B C

⎟⎟⎠

⎞⎜⎜⎝

⎛AFrom

D

⎟⎟⎠

⎞⎜⎜⎝

⎛BFrom

E

⎟⎟⎠

⎞⎜⎜⎝

⎛BFrom

F

⎟⎟⎠

⎞⎜⎜⎝

⎛CB

From,

G

⎟⎟⎠

⎞⎜⎜⎝

⎛ED

From,

Y

⎟⎟⎠

⎞⎜⎜⎝

⎛GF

From,

0 0 1 0 0 0 1 1 0 1 1 1 1 1 0 1 1 0 0 0 0 0 1 1 1 1 0 1 1 0 0 0

ii.

The truth table

Input Logical operation Output

A

B C

⎟⎟⎠

⎞⎜⎜⎝

⎛AFrom

D

⎟⎟⎠

⎞⎜⎜⎝

⎛AFrom

E

⎟⎟⎠

⎞⎜⎜⎝

⎛AFrom

G

⎟⎟⎠

⎞⎜⎜⎝

⎛DC

From,

H

⎟⎟⎠

⎞⎜⎜⎝

⎛FE

From,

X

⎟⎟⎠

⎞⎜⎜⎝

⎛HG

From,

0 0 0 0 1 0 0 1 0 1 0 0 1 0 1 1 1 0 1 1 0 1 0 1 1 1 1 1 0 1 0 1

7. Application of logic gates Air conditioner controlling system

Input Output A B Y 0 0 0 0 1 0 1 0 0 1 1 1

Situation Logic

Warm room 1 Heat sensor Cold room 0

Day 1

Input

Light sensor Night 0

Switch on 1 Output Air conditioner Switch off 0

ACTIVITY A: State types of logic gates given :

SOLUTION ACTIVITY A: a. AND gate b. OR gate c. NOT gate d. NAND gate e. NOR gate f. NOT gate

ACTIVITY B: Build gate circuit by using AND, OR and NOT for Boolean expressions given : (a) A . B + A.B = Y (b) A .C + A.B.C = Y

SOLUTION ACTIVITY B:

ASSESSMENT : Objective Question

1. The diagram shows a logic gates circuit.

A P Q R C P Q R 0 0 1 0 0 0 0 1 0 0 1 1 1 0 1 1 0 0 1 1 0 1 1 1

B P Q R D P Q R 0 0 0 0 0 0 0 1 1 0 1 1 1 0 1 1 0 1 1 1 0 1 1 0

2. The circuit represents a Boolean algebraic principle with output Z.

Which of the Boolean algebraic expression represents the output Z? A Z = X • Y C Z = X • Y B Z = X • Y B Z = X + Y

Structured Question

1. In each logic gates combination given: (i) State Boolean expression (ii) Name their logic gate

2.

Figure above shows a combination logic gates in a circuit. Based from the arrangement, complete the truth table given.

3. A factory producing pesticide uses a logic system to ensure the volume of pesticide in a plastic bottle is within acceptable range. Figure below shows the arrangement of the logic system fixed beside a conveyor belt carrying bottles of pesticide. The radioactive detector, S1 and S2 detect the levels of radiation passing through the bottles.

Detectors S1 and S2 will give input 0 to logic gate system when they receive a low level of radiation. Detectors S1 and S2 will give input 1 to logic gate system when they receive a high level of radiation. (a) State the level of the pesticide in the bottle if the input S1 is 0.

Give a reason for your answer. __________________________________________________

__________________________________________________ [2 mark]

(b) State the level of the pesticide in the bottle if the input S1 is 1. Give a reason for your answer. __________________________________________________ __________________________________________________

[2 mark] Figure below shows two combinations of logic gates, P and Q that are suggested to develop the logic gate system.

(c) Name the logic gate M __________________________________________ [1 mark]

(d) Complete the truth tables for P and Q.

Input S1 Output S2 Output 0 0 1 0 1 1

The truth table of P

Input S1 Output S2 Output 0 0 1 0 1 1

The truth table of Q [4 mark]

(e) When the output of the logic gate system is 1, it indicates the

volume of the pesticide in the bottle is within the acceptable range.

When the output of the logic gate system is 0, it indicates the volume of the pesticide in the bottle is outside the acceptable range.

(i) Based on the output of the truth tables in (d), which combination of logic gates is suitable to be used for the logic gate system?

Combination of logic gate___________________ [1 mark]

(ii) Give two reason for your choice. 1._________________________________________ 2._________________________________________ [2 mark]

4. Ali wishes to design a security system in his house as shown in figure below. When a person crosses the infrared beam at night, the infrared sensor will send asignal to a logic gate which then light up a spotlight. The spotlight has a 240 V alternating current (a.c) supply and will only light up at night.

The circuit that could help Ali to develop the security system is shown in figure below.

(a) If the potential difference across resistor W is 2 V, what is the potential difference across the infrared sensor? _________________________________ [1 mark]

(b) Component X has a high resistance when the surroundings are dark and a low resistance in bright surroundings. Name the component X. _________________________________ [1 mark]

(c) Component S is a switch that connects the 240 V a.c spotlight

circuit. (i) What type of switch is component S?

___________________________ [1 mark]

(ii) Why is component S necessary in the circuit? ___________________________ [1 mark]

(d) The following truth table shows the operation of a logic gate in the circuit in the circuit in figure above.

Key: Dark surroundings : high input voltage at P, logic’1’ Bright surroundings : low input voltage at P, logic’0’ Person crossing the path : high input voltage at Q, logic’1’ Nobody crossing the path : low input voltage at Q, logic’0’ Switch S is on : high input voltage at R, logic’1’ Switch S is off : low input voltage at R, logic’0’

Input P Output Q Surrounding Input voltage

at P Person crossing

Input voltage at Q

Output R

Bright 0 No 0 Bright Yes Dark No Dark Yes

(i) Using the given key, complete the truth table above. [4 mark]

(ii) Name the logic gate in the circuit in figure above.

______________________________________ [1 mark] (iii) Sketch the symbol of the logic gate in (d)(ii).

Answer (Assessment) Objective question 1. B 2. A Structured Question

1.

2.

3. (a) Out of the range caused less light received.

(b) Not enough to achieve the range caused high light received. (c) OR gate

(d) Input S1 Output S2 Output

0 0 0 1 0 1 1 1 0

The truth table of P

Input S1 Output S2 Output 0 0 1 1 0 1 1 1 1

The truth table of Q (e) i. Logic gate combination : P

ii. 1. Input S1 1 and input S2 0 in the range 2. Input S1 0 and input S2 0 or Input S1 1 and input S2 1 in

the range

4. (a) Potential difference = 12-2=10v (b) X Light Depndent Resistor (c) i. S relay switch

ii. S is needed to activate the spotlight in the circuit

(d) i. Input P Output Q

Surrounding Input voltage at P

Person crossing

Input voltage at Q

Output R

Bright 0 No 0 0 Bright 0 Yes 1 0 Dark 1 No 0 0 Dark 1 Yes 1 1

ii. AND gate iii.