Input Components

Topic A1: Input Components

Identification, function and application of input components used in electronic circuits:

An input component is a component that informs an electronic system that an action has taken place. For example, the temperature in a room has fallen below 25 degrees. Generally speaking input components fall within two categories. Sensors and switches.


Sensors usually take the form of a transducer whereby some physical medium such as temperature or light is represented as an electrical value, usually resistance or voltage. Thus a change in in the physical medium will cause a change in the associated electrical value which can be read and acted upon by an electrical system.

One of the most common ways that an electrical system can make use of input sensors is through the use of a voltage divider. Lets explain further, in the next diagram we have two different electronic components. One is a fixed resistor and the other is a light dependant resistor (LDR). The resistance value of the fixed resistor will remain constant (relatively speaking) however the resistance value of the of the LDR will vary according to how much light falls upon it.


We will come back to the topic of sensors and their application a little later on once we have reached an understanding of how voltage dividers work. In order to understand voltage dividers we need a basic understanding of the 3 key elements of an electrical system and how they function together to create a circuit.

One of the easiest ways to visualize an electronic circuit is to liken it to your central heating system at home. The water in the central heating system is equivalent to the current in an electronic system. The water needs to circulate through a piping system in order to reach the radiators, the piping system and radiators represent resistance in an electronic system. Finally, in order for water to flow through the system there needs to be a pressure difference between two points in the system usually created by a pump. Voltage is the electronic equivalent of pressure difference. To summarize:

  • Current (measured in amps) =  water
  • Resistance (measured in ohms) = piping and radiators
  • Potential/pressure difference (measured in volts) = pump

This image represents the analogy nicely except for two things. I wonder if you can spot them?

Water System

Taking this a bit further next we have a real electronic circuit diagram showing a voltage divider. Remember electricity is always trying to flow from a high potential (the battery) to the circuit ground. The only thing stopping the current flowing from the battery to ground  ground are the components that are connected between.

In this case we have two 1000 Ohm resistors connected in series (one after the other). Each resistor causes a voltage drop (pressure drop) across it. You should remember that in an electronic circuit all of the voltage is used up on the components.


In the schematic (electronic circuit diagram) shown above I have connected two voltmeters. Each voltmeter is measuring the pressure drop across its associated resistor. Each voltmeter is measuring 6 volts thus:

6 + 6 = 12 volts

As stated, all of the available voltage is used up on the components (Kirchoff’s Voltage Law). The voltage at the midpoint of the two resistors is equal to 6 volts. This circuit has divided the voltage in half, hence its name: the voltage divider.

Now it just so happens to be that the voltage drop across any resistor is directly proportional to the resistance. The next slide should explain this far better than a wall of text.


Note how the voltage drop across the resistors changes as we change the value of the resistors. Note that the voltage drops are directly proportional to the value of the resistance. I have kept the values in this example very simple but the principal always holds true no matter which values are chosen.

This means that by using two fixed resistors we can set the voltage value at the dividing point to any value we desire. Now this can be easily proved using Ohm’s law but I am going to save that for another day and another page.

Back to Sensors

So what would happen if we placed a sensing component in series with a fixed resistor e.g. an LDR? The resistance of the LDR will change with the amount of light falling upon it and therefore the voltage measured across the LDR will also vary. The following video shows how this happens and how a sensing component as a part of a voltage divider can be used in a real circuit.

The only difference is that a variable resistor is used in place of a fixed resistor in the voltage divider. Using a variable resistor is a good idea as it allows us to adjust the sensitivity of the sensing component. We will talk more about applications for sensing components on other pages.

Sensor components you need to know about

On this course you will need to know about the following sensor components. In most cases, rather than providing a lengthy explanation of the function of each I have simply linked you to a helpful web resource which will explain the basics.

Light Dependent Resistor

Thermistor with a Negative Temperature Coefficient (NTC)

Moisture Sensor

Piezo Electric Sensor


The other main way of triggering an electronic system is through the use of switches. Whereas the resistance of sensors is infinitely variable (analog), switches have only two postitions, on or off (digital) The switches you will need to know about on this course are:

  • Toggle
  • Slide
  • Push to Make
  • Push to Break
  • Tilt
  • Micro
  • Rocker
  • Key

The next picture has a physical image of all the types of switch that you could use to carry out your assignments.


Using Switches

In digital systems an input value can be either on or off. Lets say we have a digital system that uses Transistor Transistor Logic (TTL). Under these circumstances:

On = 5 volts = Binary 1 = True = High

Off = 0 volts = Binary 0 = False = Low

The two different states are often referred to in various ways as shown above. The purpose of using a switch is so that it will give an output of of either 5v or 0v when it is activated. The circuit diagrams shown next will explain both states.

Providing digital 0 when a switch is closed


Providing digital 1 when a switch is closed


We can now see how easily switches can be used to interface with other electronics to provide information about what is happening in the world. As usual with electronics though I have not quite given you the whole story.

We have seen what will happen to the voltage at the dividing point in both cases with the switches closed.Now ask yourself these questions:

  • What happens to the voltage at the dividing point when the switches are open?
  • Are the resultant voltages ok to use with a TTL system? (Further research required here)
  • If they are not ok, how then can we design the circuit to fix this? (Think about the relative values of both resistors)



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