Inputs, outputs and processes in systems used by designers

Purpose of input devices

Input devices take a signal from the physical or ‘real world’, and turn it into an electronic signal that a process device, such as a microcontroller, can understand and act upon.
  • examples of real world signals include light level, temperature and pressure
  • examples of electronic signals include voltage and current

How the system will work is the ‘process’, and the ‘input’ is the part of the system that enables the process to start happening:


Sensors can be used to detect changes in light level, temperature and pressure. They are used in a wide range of products, from night lights to security alarms and central heating systems.

Types of sensors

A light-dependent resistor (LDR) is a special type of resistor whose resistance changes with the light level. As the light gets brighter, its resistance decreases. It can therefore be used as a simple light sensor. Examples of products include solar garden lights and street lighting.

Showing the standard electrical symbol for a light-dependent resistor (LDR) sensor alongside a photograph of the component.Light-dependent resistor (LDR)

A thermistor works in a similar way except it responds to changing temperature levels. Usually its resistance decreases as the temperature increases, but some thermistors can work in the opposite way where resistance increases as temperature increases. An example of a use for a thermistor is in a heating system. It can be used to check the temperature in a room and trigger the turning on of a heater if it is too cold.

Showing the standard electrical symbol for a thermistor sensor alongside a photograph of the component.Thermistor

A moisture sensor works when water connects two metal tracks or probes. Anything that contains water can act as a conductor of electricity, eg soil. Commercial plant farms make use of moisture-sensing technology to ensure plants are not over or under watered.

The standard electrical symbol for a moisture sensor shown alongside a photograph of a moisture sensor.Moisture sensor

A piezoelectric sensor changes mechanical motion or force into electrical energy and can produce an electrical pulse from pressure, eg by hitting it. The change in pressure sends an electric pulse, providing a circuit with an input signal. They are used in microphones, where soundwaves create pressure that makes the electrical pulse.

The standard electrical symbol for a piezoelectric sensor shown alongside a photograph of a piezoelectric sensor.Piezoelectric sensor

Input devices provide an input signal to a circuit. Along with sensors, there are other components that can provide an input signal to control a circuit, eg a switch like a single-pole single-throw switch (SPST) that can turn a light on or off.


Switches allow current to flow through them when the contacts inside are joined together. They are usually named after how they work. For example, a push-to-make switch allows current to flow (or a signal to be passed on for processing) when pressed, therefore ‘making’ the circuit. A push-to-break switch does the reverse and ‘breaks’ the circuit.

Other examples of switches include:

  • rocker switch
  • tilt switch
  • micro switch
  • reed (magnetic) switch
The standard switch symbols for push-to-make and push-to-break switches alongside a photograph of the switch.

Push-to-make (PTM) and push-to-break (PTB) switch

Other control devices

Resistors limit the flow of current, protecting delicate components from being overloaded. Fixed-value resistors do not change their resistance, but with variable resistors it is possible to vary the resistance. This helps control the flow of current and protects delicate components from being overloaded.

The standard electrical symbol for a resistor alongside a photograph of a resistor.Resistor

A variable resistor allows the user to adjust the resistance by turning a knob. When used in a product like an automatic nightlight, the user can adjust the sensitivity of the light-dependent resistor (LDR) to trigger an output.

The standard electrical symbol for a variable resistor shown alongside a photograph of a variable resistor.Variable resistor

A transistor is a semi-conductor that amplifies a signal from a sensor - it is like a small electronic switch. Transistors have three legs:

When a small voltage at the base connection is detected, the transistor is switched on and a large current can flow in through the collector and out through the emitter.

The standard electrical symbol for a transistor alongside a photograph of a transistor.Transistor

Integrated circuits

Dual in line (DIL) IC packages are integrated circuits (IC) that are protected inside a plastic housing, with two rows of metal legs (which is why it’s called ‘dual’). The IC is usually placed inside a chip holder that is soldered to a circuit board - each leg is numbered to identify its function. As they are small and lightweight they are often used in computers, cameras, microwaves and TVs.

A collection of black microcontrollers of varying sizes and length of pins.
Dual in line integrated circuit

Microcontrollers are controllable ICs. The most common type of microcontroller is a programmable interface controller (PIC) that can be constantly reprogrammed to complete a variety of tasks, such as controlling timing or temperature in a microwave or running specific wash cycles in a washing machine. PICS are available in a variety of memory sizes and with varying numbers of pins - each pin can attach to an input or output, so, as pins increase, so does the number of devices that can be attached. The more memory or pins available, the greater the cost of the PIC. They are used in most modern electrical products, such as TVs and stereos.

A computer numerical controlled (CNC) microcontroller based on the microchip with USB data input on a white background.