TRANSISTORS AS SWITCHES

Durability

TRANSISTORS AS SWITCHES

Silicone Chips now contain over a million transistors for only a few pence each.

Size

Efficiency

Cost

Common Formats

Advantages:

How Transistors Work……..

The transistor is a bipolar device made up from 3 layers of semi conductor materials

N type (poor Conductor)

P type (good Conductor)

P type

How Transistors Work……..

N

P

Base

P

Emitter

Collector

The transistor layers are termed:

How Transistors Work……..

N

P

Apply Signal

+0.6 [Volts]

Resistance of N type Semi-conductor

Decreases

P

C

B

E

How Transistors Work……..

N

P

Apply Signal

+0.6 [Volts]The Transistor becomes an excellent conductor

Across the collector and emitter

P

C

B

E

The BC 108 is common general-purpose transistor. The diagram below shows the position of the legs when viewed from underneath the case.

The transistor has to be connected into circuits correctly. The arrowhead on the emitter indicates the direction of ‘conventional’ current flow that is, opposite to the electron flow.

How Transistors Work……..

How Transistors Work……..

b

c

e

0v

If you apply Voltage Directly

Or apply Voltage Directly

How Transistors Work……..

The Transistor must be protected from short circuit by placing loads at the base and the collector

b

c

e

How Transistors Work……..

b

c

e

Build the following circuit using crocodile clips

c

b

e

How Transistors Circuits are built….

Using input transducer devices that convert a change in physical conditions into a change in resistance and voltage. This can then be processed in an electrical network based on a voltage divider circuit.

Transistor

Circuit Group

How Transistors Circuits are built….

Example 1: a simple voltage divider can be used to fine tune the signal to the transistor

Voltage Divider Circuit Group

How Transistors Circuits are built….

Example 2: a thermistor in the voltage divider can be used to trigger the transistor at a given temperature

Thermistor Divider Circuit Group

How Transistors Circuits are built….

Example 3: a LDR in the voltage divider can be used to trigger the transistor at a given light Level

‘LDR’ Divider Circuit Group

Build a circuit on a bread board

Problem: Mr & Mrs Beverage go regularly to their local clubby. When coming home, usually too worse for drink. They stot and fumble in the dark trying to open the door for ages, causing him to swear loudly and wake up Mr Walker next door.

Brief: Design Mr Beverage a system that will bring on a Light when it gets dark and has an adjustment that can alter for summer time- leaving Mr Beverage more money for drink.

Investigate: LDR voltage divider configuration to provide a “darkness” sensorDesign & Build: A proto type using a breadboard