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Subelement E6
Section E6F
Optical components: photoconductive principles and effects, photovoltaic systems, optical couplers, optical sensors, and optoisolators; LCDs
What is photoconductivity?
  • The conversion of photon energy to electromotive energy
  • The increased conductivity of an illuminated semiconductor
  • The conversion of electromotive energy to photon energy
  • The decreased conductivity of an illuminated semiconductor

When light is absorbed by a material such as a semiconductor, the number of free electrons and electron holes increases and raises its electrical conductivity. To cause excitation, the light that strikes the semiconductor must have enough energy to raise electrons across the band gap, or to excite the impurities within the band gap. When a bias voltage and a load resistor are used in series with the semiconductor, a voltage drop across the load resistors can be measured when the change in electrical conductivity of the material varies the current through the circuit.

Wikipedia Explanation

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What happens to the conductivity of a photoconductive material when light shines on it?
  • It increases
  • It decreases
  • It stays the same
  • It becomes unstable

Photoconductivity is the property of a substance to conduct electricity based upon the intensity of the light shining on it. Brighter light decreases resistance in the substance, resulting in better conductivity and vice versa.

Photoconductive materials are used in semiconductors.

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What is the most common configuration of an optoisolator or optocoupler?
  • A lens and a photomultiplier
  • A frequency modulated helium-neon laser
  • An amplitude modulated helium-neon laser
  • An LED and a phototransistor

This feature is found in in some high-frequency transceivers, and is used to transfer electrical signals between two isolated circuits by using light. It's essentially an LED focusing light on a phototransistor.

An opto-isolator is primarily used to prevent high voltages (line spikes, RF, lightning, electrostatic discharge, etc) from damaging equipment.


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What is the photovoltaic effect?
  • The conversion of voltage to current when exposed to light
  • The conversion of light to electrical energy
  • The conversion of electrical energy to mechanical energy
  • The tendency of a battery to discharge when used outside

The photovoltaic effect is the creation of voltage or electric current in a material upon exposure to light.

The standard photovoltaic effect is directly related to the photoelectric effect, though they are different processes. When the sunlight or any other light is incident upon a material surface, the electrons present in the valence band absorb energy and, being excited, jump to the conduction band and become free. These highly excited, non-thermal electrons diffuse, and some reach a junction where they are accelerated into a different material by a built-in potential (Galvani potential). This generates an electromotive force, and thus some of the light energy is converted into electric energy.

Source: Wikipedia - Photovoltaic Effect

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Which describes an optical shaft encoder?
  • A device which detects rotation of a control by interrupting a light source with a patterned wheel
  • A device which measures the strength of a beam of light using analog to digital conversion
  • A digital encryption device often used to encrypt spacecraft control signals
  • A device for generating RTTY signals by means of a rotating light source

Wikipedia definition of the device. http://en.wikipedia.org/wiki/Rotary_encoder

So, what can we do with an optical shaft encoder (optical rotary encoders)? Sound like a fancy name for a position detector. The number of applications can be enormous, but narrowing down for anything Ham related they can be used for detecting the:

  1. Position of a VFO knob
  2. Position of an rotary antenna
  3. Position of a volume control
  4. etc...

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Which of these materials is affected the most by photoconductivity?
  • A crystalline semiconductor
  • An ordinary metal
  • A heavy metal
  • A liquid semiconductor

Semiconductors are defined by their unique electric conductive behavior, somewhere between that of a metal and an insulator. High conductivity in a material comes from it having many partially filled quantum states and much state delocalization. Most common semiconducting materials are crystalline solids, but amorphous and liquid semiconductors are also known. Such disordered materials lack the rigid crystalline structure of conventional semiconductors such as silicon. A high degree of crystalline perfection is also required, since faults in crystal structure interfere with the semiconducting properties of the material.

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What is a solid state relay?
  • A relay using transistors to drive the relay coil
  • A device that uses semiconductors to implement the functions of an electromechanical relay
  • A mechanical relay that latches in the on or off state each time it is pulsed
  • A passive delay line

The key is the term "solid state" i.e. no moving parts. Otherwise it performs very similar to a mechanical relay, except for much faster and with greater reliability, so long as it is operated within its design specifications.

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Why are optoisolators often used in conjunction with solid state circuits when switching 120VAC?
  • Optoisolators provide a low impedance link between a control circuit and a power circuit
  • Optoisolators provide impedance matching between the control circuit and power circuit
  • Optoisolators provide a very high degree of electrical isolation between a control circuit and the circuit being switched
  • Optoisolators eliminate the effects of reflected light in the control circuit

Follow this link for a good explanation of the optoisolator:


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What is the efficiency of a photovoltaic cell?
  • The output RF power divided by the input DC power
  • The effective payback period
  • The open-circuit voltage divided by the short-circuit current under full illumination
  • The relative fraction of light that is converted to current

The best (and correct) answer is that the efficiency of a photovoltaic cell is the relative fraction of light that is converted to current.

Efficiency is expressed as a ratio or percentage. It can range from 0 to 1, or 0% to 100%. It does not have a unit. You can eliminate the open-circuit voltage divided by short-circuit current option because this gives you a number expressed in ohms, which is not unitless.

You can eliminate RF output power divided by input DC power because the question does not ask about the efficiency of a transmitter or any other radio, it asks about the efficiency of a photovoltaic cell (which doesn't involve RF power at all).

The effective payback period is not a term used to describe any kind of electrical phenomena.


Hint: Only the answer has the word light in it.


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What is the most common type of photovoltaic cell used for electrical power generation?
  • Selenium
  • Silicon
  • Cadmium Sulfide
  • Copper oxide

Memory Trick: Silicon Solar Cell

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What is the approximate open-circuit voltage produced by a fully-illuminated silicon photovoltaic cell?
  • 0.1 V
  • 0.5 V
  • 1.5 V
  • 12 V

This is one of those question/answer pairs that just has to be memorized. Fully-illuminated silicon photovoltaic cells produce about half a volt.

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What absorbs the energy from light falling on a photovoltaic cell?
  • Protons
  • Photons
  • Electrons
  • Holes

Photovoltaic cells (PV cells), or more commonly called solar cells or solar panels convert light into electrical current. The process is called the photovoltaic effect which is mostly the same as the photoelectric effect. In the photovoltaic effect, a "charge carrier", which is typically an electron, absorbs a photon and becomes "excited" to a higher energy level or a higher voltage. This increased voltage causes electrical current to flow.

Protons are part of the atomic nucleus and cannot normally have their energy changed and be made to move around by photon absorption.

Photons are the particles that make up all electrical magnetic radiation (light, radio waves, microwaves, etc).

"Holes" refers to a semiconductor model, and while PV cells are made of semiconductor material (silicon), holes and electrons are normally discussed when talking about transistors. It is worth mentioning that in the electron/hole model, when an electron is ejected from the PV cell, it does leave a hole, but that hole is promptly filled by a lower energy electron due to the current flow in the circuit.

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What is a liquid crystal display (LCD)?
  • A modern replacement for a quartz crystal oscillator which displays its fundamental frequency
  • A display utilizing a crystalline liquid and polarizing filters which becomes opaque when voltage is applied
  • A frequency-determining unit for a transmitter or receiver
  • A display that uses a glowing liquid to remain brightly lit in dim light

The liquid crytal material responds to an applied voltage, which changes its light refraction. This allows the material to go from nearly transparent to almost opaque, making it appear black.

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Which of the following is true of LCD displays?
  • They are hard to view in high ambient light conditions
  • They may be hard view through polarized lenses
  • They only display alphanumeric symbols
  • All of these choices are correct

An LCD display utilizes two polarizing filters along with the liquid crystal as part of the mechanism; with sufficient voltage the liquid crystal becomes polarized such that between the two filters it becomes opaque, thus becoming black (or transparent, depending on the situation).

Because it utilizes polarizing filters to do this, if you look at it through polarized lenses (filters) such as you might have on some sunglasses some orientations of the lenses may cause the display to appear entirely opaque (completely unreadable).

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