Inputs Flashcards
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Sensor
Light Dependent Resistor
(LDR)
A resistor whose value changes with the amount of light falling on it.
Dark = big resistance
Light = small resistance
Can be ‘normal’ or miniature
Non-polarised
Usually used in a potential divider in switching or sensing circuits
Sensor
Thermistor
A resistor whose value changes with temperature
Hot = small resistance
Cold = large resistance
(These are for negative temperature coefficient thermistors (NTC). There are also PTC types that work the other way round - THEY ARE NOT ON YOUR COURSE)
They are usually usde as part of a potential divider in sensor and switching circuits.
They are not polarised
Sensor
Moisture sensor
Can be bought but at school or hobby level are often home-made.
Normally two metal probes set a small distance apart. Idf being used to measure the moisture in soil:
Dry = large resistance
Wet = smaller resistance
No ‘standard’ symbol
Often used in potential divider circuits
Can also be made with interlocking but not touching tracks ona pcb - when it is dry they don’t touch so resistance is very high but if some if placed in water some current passes between the tracks (not much though) and the resistance drops. Not really a moisture sensor, more of a rain sensor but works in the same way.
Sensor
Piezo electric sensor
Not often used (so loved by examiners)
It detects knocks and vibrations and so could be used in alarms or safety systems (mount it to the back of a door to detect if someome kicks it)
When you tap/hit it a small current is generated (very, very small)
Normally they are used with an amplifier circuit (which is why they are rarely used at GCSE level)
Sensor
Biosensors
Theses are sensors which convert some form of biological response into an electrical signal.
An example is a glucose monitor that warns patients of dangerous glucose levels in their blood
They are also being developed alongside ‘nano-technology’ (very small) to make very small sensors that could be built into food packaging to sense the micro-organisms that are present when food starts to go ‘off’.
A growth area at the moment is devices such as FitBit that can track your heart rate as you exercise. With more biosensors we could extend this to other areas of health monitoring.
Here is a video to watch
Switch type
Toggle
Switch types
Slide
Switch types
Rocker
Switch types
Key
Switch types
Push to make
Circuit is completed when the switch is pressed
Often (but not always) a red ‘button’
Switch types
Push to break
Circuit is broken (disconnected) when switch is pressed.
Often (but not always) a black ‘button’
Switch types
Tilt
Circuit is connected or disconnected when switch is tilted or not.
Sometimes contain a small amount of mercury but ball bearings are safer.
Switch types
Micro
Not necessarily very small (micro)
Generally not pushed by people’s fingers but by doors (fridge or car) or parts of machines (limits of movement or safety guards)
Basic switch can have different ‘actuators’ - button, lever which can detect a greater range of movement, roller which can detect something moving past the switch etc.
Switch types
Reed
These switches are activated (or closed) when a magnet is near them.
Commonly used in alarms but also elsewhere.
Sometimes cased in plastic for protection and to make mounting them easier
Switch terms
SPST
Single Pole Single Throw
There is a single ‘switch’ (pole) that can connect to only one output (throw)
You can tell it is an SPST switch because it has 2 connections.
For more information go here
Switch terms
SPDT
Single Pole Double Throw
There is a single ‘switch’ (pole) that can switch between two outputs (throws)
You can tell it is an SPDT switch because it has 3 connections.
For more information go here
Switch terms
DPDT
Double Pole Double Throw
There are two ‘switches’ (poles) that move at the same time and can each switch between two outputs (throws) - the dotted line in the symbol shows that the switches move together
You can tell it is an DPDT switch because it has 6 connections.
These are the most common type in use because you can make all of the other switches from them.
They are used as light switches in houses (as they can turn both the live AND neutral wires off making the circuit much safer)
They can also be used to reverse a motor - see the image and build the circuit to check yourself
For more information go here
