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