In today’s world, we use many electrical circuits to carry out basic and complex tasks. Some tasks are such that we expect an electronic system to automatically complete without manual interference.

Take a simple example:

When there is someone prowling in the dead of the night in a bank, we would expect an electronic system to automatically detect the presence of an intruder and respond by flashing security lights or sounding an alarm.

So, we have two important words here:

  1. Detecting
  2. Responding

Hence, to automatically detect a change in the environment and respond to this stimulus, electronic systems make use of devices known as transducers.

Any electronic system can be considered to consist three basic parts:

  1. Input transducers/ sensors.
  2. Output transducers.

Now we’ll be explaining what each part does and their examples:

Part What it does Example
Input transducers/ sensors. input transducers detect changes in the environment Light dependent resistors, thermistors, microphones, sensors etc.
Processors. Processors decide on what actions to take CPU in a computer
Output transducers. Output transducers convert the electrical energy supplied by the processor into another form to give an output Light emitting diodes, buzzers, motors, heaters, relays etc.


Input transducers

Light dependent resistors (LDR)
What is a LDR? A LDR is a type of ‘variable resistor’ whose resistance depends on the amount of light falling on it.
How does an LDR work? An LDR is made up of a material which does not conduct electricity too well (such as graphite). In the dark, a LDR has an extremely huge resistance. However, when light is shone on the LDR, its resistance decreases, letting electric currents flow through it.
Uses of LDRs LDRs are used in burglar alarm systems and systems involving security lights.

LDRs are also used in some digital clocks.


What is a thermistor? Thermistor is another type of resistor, whose resistance depends on its environment. (In this case, the resistance depends on the surrounding temperature.)
How does a thermistor work? For some thermistors, the resistance decreases as the temperature increases.

For other thermistors the resistance increases as the temperature increases

Uses of thermistors Some thermistors are used in circuits designed to prevent overheating.

Thermistors are also used in temperature probes

Thermistors can also be used to switch a relay.


Output Transducers

What is a relay? A relay is a type of switch that works using an electromagnet.
How does a relay work? When relay is used there are two circuits:

1.    The electromagnet coil of the relay is in one circuit

2.    The switch is in another circuit.

When a current flows through the relay coil in the first circuit it becomes magnetised. It pulls on the switch in the second circuit, causing it to close and allowing a current to flow in the second circuit.


The second circuit often involves a large voltage which would be risky for an operator to switch or which could not be switched by a normal electronic circuit.

Uses of relays Relays are often used to switch the mains supplies in houses

Relays are also used in electric bell/alarms


Light Emitting Diode (LED)
What is an LED? A LED is a diode made from the semiconductor gallium arsenide phosphide.

(A diode is a component that allows electric current to flow in one direction only.)

How does an LED work? When a current passes through a LED, it gives out coloured light.

Note:  If the voltage passes 5V, a lot of damage can be caused (Be careful!)

Uses of LEDs LEDs are used in indicator lamps on computers, and electrical component such as radios.

Many clocks, calculators, video recorders and measuring instruments have seven-segment LED numerical displays.

LEDs are also used in traffic lights.

Semiconductor diodes

A diode is a component that allows electric current to flow in one direction only.

A diode is a non-ohmic conductor. It is useful for converting alternating current (which varies back and forth) into direct current (which flows in one direction only).

This process is known as a rectification and the diode acts as a rectifier.

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