1 of 33© Boardworks Ltd 2009. 2 of 33© Boardworks Ltd 2009 Potential dividers

1 of 33 © Boardworks Ltd 2009


Potential dividers



Using sensors in potential dividers

VIN

VOUT

0 V 0 V

R1

R2

By replacing either resistor with an input device, the potential divider can be used as an input sensor for a control system.

A potential divider uses two series resistors to reduce a high input voltage (VIN) to a lower output voltage (VOUT).

Replacing R2 with a switch will produce a simple pressure sensor, while LDRs and thermistors can be used to produce light and temperature sensors respectively.


Light sensors



Control systems


What are logic gates?

Logic gates are electronic switches that process information.

This chip contains four NOT gates.

They are called gates because they open to produce a high output signal only when they receive the correct combination of input signals.

A large number of logic gates can be incorporated in one electronic chip.

Most logic gates have multiple inputs, which are used to determine a single output.


Logic gate voltage

Microchips contain logic gates, which use this binary code to send and store information.

Digital systems have two states, ON and OFF. These simple electronic states are represented in binary code: ON is called logic 1; OFF is called logic 0.

Logic gates receive multiple signals in these two states. Different combinations of signals lead to different outputs.

Microchips can be easily damaged by high voltages. Due to this the voltages for the two logic states are standardized.

Logic 1 is 5 V, while logic 0 is 0 V.


Types of logic gate


Logic gates

AND•For the output to be high 1•Both inputs must be high 1

OR•For the output to be high 1 •One input must be high 1

NOT•If the input is high 1 •The output is low 0


NAND & NOR

NAND

•The opposite output from

an AND gate

•Output high if both A & B are not both high

NOT

•Equivalent to a OR gate

•With the output inverted by a

NOT gate

Output is high if both A & B are NOT high


Logic gates summary



Truth tables for three inputs


Three input truth tables question


Truth tables for four inputs


A burglar alarm

The alarm keeps sounding until the owner enters the code into the control panel, regardless of the input it receives.

Can you identify the input sensor, the processor and the output?

A burglar alarm is a great example of a control system.

input = movement sensor

processor = alarm unit

output = siren and lights

How does this happen?



Making use of logic output

There are many different output devices for an electronic system. These include motors, buzzers and lamps.

In a logic system both current and voltage are very small to prevent damaging the gates.

This limits the range of output devices which can be run on a logic system.

In order to make full use of logic systems, we must find a way to control a high voltage component using a low voltage logic system: a relay allows this.


Introduction to relays

A relay is an electromagnetic switch which allows one circuit to control another circuit by turning it on and off.

The starter motor in a car uses a current of several hundred amps, and is controlled by a relay, making it safe and easy to operate.

A relay is used to isolate a high voltage circuit, controlling it with a low voltage counterpart.

There are a number or possible reasons for this: To allow the use of a smaller, more practical switch. To isolate the user from a dangerous high voltage. To allow a logic system to control high voltage

components.


How does a relay work?

A relay uses an electromagnet to operate a switch.

As electricity flows in the input circuit, the coil becomes an electromagnet.

The coil attracts the steel switch in the output circuit, closing the switch.

Electricity flows in the output circuit driving the motor.input output

steel switch

coil

circuit symbol for a relay


Separating mains and logic voltage

Relays allow us to separate the output from an electronic control system, of either 0 V or 5 V, from the mains voltage of 230 V.

This allows electronic control systems to turn on mains appliances, which they would otherwise be unable to run.

It also protects the user from the dangerously high mains voltage.

Some electronic control systems do have their own output device. This serves as an indicator, clearly showing the user the logic state of the circuit.


LEDs as indicators

0-5 V

0 V

In real circuits, we must use a protective resistor in series with the LED.

LEDs are ideal indicators for a logic circuit – why is this?

They require only a low current and voltage to emit light.

Their two states, on or off, match the binary states of logic 0 and 1.

This ensures a voltage of around 0.7 V across the LED, preventing the current from becoming damaging.


Outputs and relays summary



Glossary


Anagrams


Multiple-choice quiz

Documents

1 of 33© Boardworks Ltd 2009. 2 of 33© Boardworks Ltd 2009 Potential dividers