Medium is the Message Flashcards
Sensors that respond to the environment
doorbell responds to pressure of a finger, street lamps have sensors to measure the brightness of the sky, weather conditions
Position sensor
measures how far something has moved from a reference position - simple position sensor depends on rotary potentiometer
Where are fibre optic cables used most?
in long distance communication systems such as telephone systems and computer networks
How is most communication transferred?
by signals sent along a cable - electrical signals can be sent along cables made of copper or other good electrical conductors, alternatively visible or infrared radiation can be sent along fibre optic cables
How do fibre optic cables work?
the signal travels inside a thin transparent fibre meeting the surface at an angle of incidence greater than the critical angle therefore it undergoes total internal reflection and is confined to the interior of the fibre
Dispersion in a fibre optic
spreading means that a signal that starts as a sharp pulse is smeared out after travelling along the fibre
Multipath dispersion
the refractive index is the same throughout the fibre so the speed of propagation of the light through it is the same regardless of the route. Therefore when different bits of radiation take different routes the result is that a sharp pulse gets spread out and the longer the fibre the worse things get
How can multipath dispersion be reduced?
by limiting the range of angles of incidence that result in total internal reflection - this can be achieved by surrounding the core fibre with a cladding material whose refractive index is slightly lower tan that of the core to produce a stepped index fibre
Attenuation
loss of intensity - in metal cables occurs where there is a transfer of energy from the signal to the cable due in part to its resistance and leakage through the insulation
What are charge coupled devices (CCD) used for?
used to obtain 2D images
How do CCDs work?
They’re made up of pixels. When a photon is absorbed by a pixel, a photoelectron is released from the semi-conductor but remains trapped in the pixel. As more photons are absorbed, more electrons are released. If an image is projected onto a CCD then charge builds up in each pixel according to the number of photons that have reached it, in this way recording the image.
How is a CCD image read?
the electrons are electrically shunted along the CCD step by step producing a small pulse of current as each ‘package’ reaches the edge. The size of each pulse can then be processed and transmitted
CCD images
If the image is bright then only a short exposure time is needed as the CCD images are produced so rapidly there is no noticeable time delay
Capacitor
device that stores charge (energy - by separating electric charge)
When a capacitor becomes charged…
…electrons simply redistribute themselves between the plates making one positive and one negative - overall no net charge
Capacitance =
Charge / Voltage (C=Q/V)
Energy in a capacitor can be worked out by…
…the area under a charge voltage graph
CCD read-out
Each packet of charge has to be moved to an output system and measured. The CCD is designed so that each tiny light sensitive capacitor is separated from its neighbour by another capacitor that remains unaffected by light. First a temporary voltage is applied to the ‘empty’ capacitor so that the charge spreads into it. The first capacitor’s voltage is then reduced to zero, leaving all the charge on the adjacent one. This process is repeated until each capacitor’s initial charge gets to an output sensor where the voltage across the capacitor is finally measured.
Are LEDs conductors?
they’re semi-conductors
Semi-conductors
are materials that conduct less well than metals but they are not insulators
Doping
Adding atoms of another element - Semi-conductors conduct much better if a small number of atoms of another element are introduced to the crystal to make the average number of electrons per atom slightly more than four or less than four (this lowers the resistivity)
P type semi-conductor (p stands for positive but the atom is actually neutral)
the average number of outer electrons per atom is less than four. Some of the atoms have only 7 electrons around them with space for another one - a hole. An electron from a neighbouring atom can easily move into the ‘hole’, allowing good electrical conduction and leaving a hole next to another atom. If an electron moves from the left into a hole, then the hole appears to move to the right.
N type semi-conductor
If the average number of electrons per atom is more than 4, then the extra electrons cannot fit into the shell containing 8 electrons so they have to occupy a higher energy level shell. These higher energy electrons can move more easily, reducing the resistivity
A place where the 2 types of semi-conductor meet is called…
a junction (this is used in an LED)
