Circuits and Electronics (in a Nutshell) AEM 1905, Fall 2008

Electrical circuits are a lot like (water) plumbing systems in which water circulates (except discharging of water outside 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 multimeter to make resistance and voltage 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 some actual circuits

The circuit diagram for the 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 the weather 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.