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Design & Technology

Logic and microcontrollers

Many electronic circuits have to make decisions. They look at two or more inputs and use these to determine the outputs from the circuit. The process of doing this uses electronic logic, which is based on digital switches called gates.

Electronic logic: digital and analogue signals

Logic gates allow an electronic system to make a decision based on a number on its inputs. They are digital electronic devices. Digital means that each input and output of the gates must be one of two states:

  • high, also known as 1 or 'on'
  • low, also known as 2 or 'off'
The oscilloscope readout shows a digital sine wave

Devices such as thermistorsthermistor: An electrical device whose resistance decreases as its temperature increases., microphones and light dependent resistors [LDR: A Light Dependent Resistor, or LDR, is a type of resistor which is affected by changes in light levels. A cadmium sulphide layer causes a decrease in resistance in the light and increase in the dark. ] (LDRs) produce signals that are analogue. This means that they vary over a range. Analogue signals must be processed by an analogue-to-digital converter (ADC) before they can be processed by a logic gate.

The oscilloscope readout shows an analogue sine wave

A single digital signal can be either high or low: eg a light with one switch can be on or off. However if there is more than one signal, there are more than two possible states. For example, if two signals are present there are four possible combinations: high/high, high/low, low/high and low/low.

In a logic gate each combination can be made to produce a different outcome.

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