Frequently Unasked Questions

Loyola Blakefield

Introduction to EngineeringMarch 8, 2012

Bob Rassa, Raytheon Space & Airborne Systems

RCRassa@Raytheon.com

Observations. . .

• Many of you used “flow of electrons” in your narratives---– This is true, technically, BUT to include that in your discussion is like

discussing the car wheels going around when you describe driving.

• Instead of saying that the electrons (or current) flows to the light bulb or the resistor etc when you turn on the power switch, you say that the bulb is illuminated when the switch is pressed (or similar) – in other words, describe the observable function vs. the internal workings.

• We also don’t describe the functions of components as “translating energy flow around the circuit board” – We say “power is provided to” or “the signal goes to”

• Buttons don’t “send power to” something; they Activate it

Symbols

Battery

Ground

Switch

Crystal

+

-

SPST

xtal

Variable Resistor R

CapacitorC

+-

Variable Capacitor C

Inductor L

More Symbols

Transistors

Light-Emitting Diode

Motor

Speaker

Zener Diode

motor

D

LED

ZENER

DDiode

SPKR

Antenna antenna

Transformer

And still more---

AC Plug

Coaxial Connector

Operational Amplifier(also Pre-amp, Amp)

Resistors etc• Resistors, capacitors and inductors have multiple functions in a

circuit.– Resistors do generally limit the current in a circuit—but that

isn’t always their function– Resistors, capacitors and inductors are also used in

combination as:• Filters• Resonant Circuits• Timers

Diodes Transistors etc• Diodes are part of a class of devices known as

semiconductors– Diodes pass current in one direction only– In DC circuits they block reverse current flow– In AC circuits they pass half of the voltage cycle

• Transistors are also semiconductors– Transistors amplify current

• Integrated circuits are semiconductors– ICs are primarily transistors, with some diodes- very few resistors

are build into an IC– ICs are typically microprocessors – very small computers – but can

be other things as well, such as simple logic controllers, random access memory (RAM), and more

Crystals & Oscillators• Resistors, capacitors and inductors are used in combinations as

Oscillators – constructs that produce a higher-frequency alternating current (AC) signal– A Crystal is often used to more precisely control the

frequency of the oscillator– Oscillators use the principle of resonance – a self-reinforcing

electrical signal or sound wave- in a resonant circuit– Sound resonance is what you experience if you rub a wet

finger around the rim of a wine glass, for example– Bass speakers work by installing them in a cabinet with a

resonant frequency in the mid range of the speaker response - this resonant frequency response increases the speaker volume

Television and RadioTV and radios work by transmitting a carrier frequency that is

modulated by the desired viewing or listening content. The original broadcast TV (Ch 2-13) operated in the frequency band

– or spectrum - of 54 MHz (Megahertz) to 216 MHz. These are the VHF channels. In the middle, from 88 MHz to 108 MHz, is the FM radio spectrum.

The UHF spectrum is from 470 MHz to 890 MHz (Ch 14 to 83)The base frequency is called the Carrier or Center frequency.The content signal (audio for FM, audio and video for TV)

modulates the carrier frequency. To modulate means to cause it to vary according to the modulating signal.

The TV or FM receiver uses a detector circuit to “demodulate” the signal, meaning decode the content from the carrier

Printed Circuit BoardsPrinted circuit boards are devices that hold electronic

components and connect them together to perform a specific function or set of functions.

The components mount on the top side (component side) and are soldered to the “printed trace” on the underside.

The printed trace is the series of electrical “wires” that connect the components together.

It is called a printed circuit board since the trace appears to be “printed” on the underside; in actuality it is acid-etched there from a solid conductive sheet and then coated with insulation.

The spots on the trace where component leads protrude through and are soldered to the trace are called “pads”.

Functional Block Diagrams• A functional block diagram is a drawing that shows the basic

and critical functions of a circuit.– It generally doesn’t use electrical symbols except as

needed for clarity– It shows the basic flow of the circuit. Typical are:

• Input power• Input signal• Antennas• Signal generating devices (e.g. microphone)• Signal amplifiers, controllers, processors• Feedback loops, if any• Relays, if any• Signal outputs• Grounds as needed for clarity (not necessary to show every one)

Example: Traffic LightThe traffic light kit simulates the traffic signal at a 4-way

intersection. The function of the device is to serve as a miniature of a real traffic signal. There are 12 LEDs in a 3-lamp configuration facing each direction, in traffic signal colors of red (top), yellow (middle) and green (bottom). The system has an adjustable timer circuit controlling the lights, which determines how long the lights stay red or green in each direction. Just as in a real traffic signal, the lights go from green to yellow and then red in one direction before the lights in the cross-traffic position change to green. The system is powered by a 9-volt battery and uses a microprocessor to control the timing, which is adjustable by a potentiometer.

Questions?