Circuits and Electronics(in a Nutshell)
AEM 1905, Fall 2008
Electrical circuits are a lotlike (water) plumbing systems
in which water circulates (except discharging of wateroutside of the closed piping
system is not allowed).
Electrical current consists of charged particles (usually electrons) flowing
through metal wires like water current consists of water molecules flowing
through hollow pipes.
Electrical current is defined as flowing from positive to negative (also known
as from high voltage to low voltage) just like water flows from regions of high pressure to regions of low pressure.
A battery can push electrical current through a circuit (which is a continuous connection of wires and components) just as a water pump can push water through a closed plumbing system.
The amount of electrical current is measured in amperes (amps) and the “pressure” with which it is pushed is measured in volts (V) just as water
current can be measured in gallons/minute and water pressure can
come in pounds/square inch (psi).
One needs to have a “complete circuit” for current to flow (out of the + end of
the battery, through the wires and components, then back into the – end
of the battery) just as one needs a closed network of pipes for a plumbing
system to be able to circulate water from the pump, through the pipes,
then back to the pump again.
Components can be placed “in series” (i.e. one after another) so that
electrical current must pass through them one after another before
returning to the battery. Components in series have the same electrical current passing through them.
On the other hand, components can be placed “in parallel” (i.e. one beside
another) so that the electrical current must chose which one component to pass through, bypassing the others.
Components in parallel have the same voltage drop but usually carry different amounts of current, with more current flowing through the easiest path (i.e. the path with the least resistance).
In terms of (water) plumbing, a water heater would be in series with a room hot-water
radiator – the water flows through the water heater then the exact same water molecules
go on to the radiator. On the other hand, two hot-water room radiators might be plumbed in parallel – the water would choose to go either to the living room
radiator or to the bedroom radiator before returning to the water heater, but no single
water molecule would go through both radiators in 1 trip.
Components and vocabulary
Battery: pushes on the electrical charges (though none will actually flow if the circuit
isn’t complete) – the battery has two terminals labeled positive (+) and negative (-)
– the most negative voltage region in the circuit, often the negative end of the battery, is sometimes called “ground” or 0 volts – the
amount of push the battery supplies is the “battery voltage”, often 1.5 V or 9 V or 12 V (with respect to ground) – as a battery gets worn out (or if it gets too cold!) its voltage
will go down until the battery is too weak to continue to push current through the circuit
symbol
Wire: provides a path through which electrical current can flow –
ideally a wire has no resistance
symbol
Resistor: serves as a current path but limits the amount of electrical current
flow and reduces the pressure (i.e. drops the voltage) – resistance is
measured in Ohms (Ω) – for resistors it doesn’t matter which way + and – are
connected – when current flows through a resistor the resistor may get
hot (i.e. it “dissipates” energy), so resistors can also be used as heaters
symbol
Switch: place where a current path can be mechanically opened and closed, to
start or stop the flow of electrical current – switches are used to turn
things ON and OFF – place the switch in series with the component(s) it is meant
to control, like a battery symbol
Capacitor: serves as a place to temporarily store electrical charge,
like a temporary battery – “charge it up” (store electrical charge) then
“discharge it” (temporarily produce electrical current) – capacitance is
measured in Farads (F) – electrolytic capacitors are ones in which it matters which way + and – are
connected
symbol
Diode: serves as a one-way valve, only allowing current to flow one
direction under normal circumstances – an LED (light
emitting diode) is a diode (often red or green) that glows when current flows through it – diodes must be
inserted the right way around for the circuit to operate correctly
symbols
Voltage regulator: a chip that can be powered by a range of voltages but uses internal circuitry to drop the voltage to output a very stable voltage (e.g. a “5 V regulator” might be able to able to be
able to run off any voltage from 6 V up to 20 V, but it always outputs exactly 5 V) –
this is handy for providing a constant voltage to components even when dealing with batteries that can vary in voltage and circuits that can vary in overall resistance
IC (Integrated Circuit, AKA chip): a silicon chip with many tiny transistors
on-board which can be programmed to make decisions (a microprocessor chip), to store digital information (a memory chip), to convert digital input to analog form (DAC), or vise versa (ADC), etc. –
connects to other components through its multiple legs, called pins – be very
careful never to put a chip in backward!
ADC (Analog to Digital Converter): a chip that takes analog (continuous) voltage
input, perhaps from a sensor, and converts it to digital form for ease of use
Breadboard: a board into which components
can be plugged and unplugged, allowing one
to build and check circuits without having to be as permanent as
soldering them together
PCB (Printed Circuit Board): insulating board onto which
components can be soldered, with metallic traces etched into
the board to make electrical connections without having to
use external wires
Perf. Board (Perforated (Circuit) Board): insulating board onto
which components can be soldered, with no metallic traces etched between holes like on a PCB – using perf. board is more
permanent than using a breadboard but you need to connect components with
external wires
Transistor: 3-leg device used in logic circuits so that a small/weak electrical
current at one point can control a much larger/more-powerful electrical current
elsewhere in the circuit
Sensor: a device, often powered using +5 V and ground (+0 V) connections, that has a third
output the voltage of which varies predictably and reproducibly as
some physical parameter changes like temperature or air pressure – needs to be “calibrated” (i.e. the output needs to be checked using
known physical conditions) so output values can be correctly
interpreted
Socket: a dummy set of receptacles that matches the pins on a chip – the socket is
soldered onto the board and the chip snaps into it so that the chip
can be replaced (carefully!) without resoldering if it goes bad
Cable: a wire or set of parallel wires connecting
components together – for example, sensors often use a 3-wire cable with the wires
used for +5 V, ground (+0 V), and signal (output voltage)
Battery pack: a device for holding multiple (identical)
batteries, either in series (+ of one battery attached to the – of the next, in which case the total voltage is the sum of the battery
voltages) or in parallel (all + terminals connected together, all – terminals connected together, in which case the battery pack has the same voltage as each
individual battery, but it will last longer (i.e. can provide current for a longer amount of time))
Audio jack: used to make a pull-before-flight pin to start a flight
computer just before we let go without having to open up a payload box
Male and female headers: used to allow quick
electrical connections between sensors, flight
computers, for programming, etc.
Shrink wrap: plastic insulation tubing one can slide over exposed metal,
like a solder joint, to insulate it electrically from
nearby wires – shrink wrap contracts (shrinks!) when heated with a heat gun – think ahead; you might need to put the
shrink wrap on before you do the soldering
Using a multimeterto make resistance andvoltage measurements
Multimeter: a device with two probes (red (positive) and black (negative)) with which one can make resistance,
voltage, and current measurements – for resistance and voltage measurements (all we’ll be doing in this class) always plug the black probe into the socket labeled “COM” and the
red probe into the socket labeled “V Ω mA”
Measuring resistance in Ohms (Ω). Turn the dial to a value on the Ohms scale larger than the resistance you expect. Touch the two probes to either end of the resistor (preferably when
it is not part of any circuit, lest there be alternate current paths around the resistor). Adjust the dial downwards as
necessary. Read the value of the resistance in Ohms, kiloOhms, or MegaOhms. (Note – you can also read off
resistance values by using resistor color codes.)
Measuring DC voltage (i.e. battery-type voltage). Turn the dial to a value on the DC Volts scale larger than the voltage you expect. Touch the two probes to the two points on the
circuit between which you want to know the voltage change, trying to put the black probe on the point with the lower
voltage (i.e. closer to “ground”). Adjust the dial downwards as necessary and read off the value in volts. Notice that the reading is the voltage drop from the red (positive) probe to the black (negative) probe, so a positive reading means that the red probe is indeed at a more positive voltage than the black probe. On the other hand, a negative reading means the red probe is more negative than the black probe. (See
next slide for photos of a voltage reading being made.)
Using a multimeter to measure the voltage of a battery. Notice that the red probe goes to the +
terminal and the black probe goes to the – terminal to get a positive value on the screen.
Comments on someactual circuits
The circuit diagram forthe heater circuit:
3 batteries in parallel (so they last longer)3 resistors in series (which will get hot)
1 switch (to turn it ON and OFF)
Sensor for theweather station:
1 pressure gauge (0 to 15 psi)1 temperature gauge
4-wire cable: +5V, ground, press. output, temp. output
Animation of a 555 timer chip circuit,similar to the one we built.
http://courses.ncsu.edu:8020/ece480/common/htdocs/480_555.htm
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