Keepin' It Casual
Other blogs about electronics — and there are so many — get very serious very fast. They want to first teach you some physics and electronic theory. Boo! My goal is to first get you interested in electronics. I don't want to scare you away with physics or math! A neat way to explain something is with an analogy — using a familiar thing to explain an unfamiliar thing. It works because the two things are similar. So that's what I'll do here. |
E = I ∙ R I = E / R RSERIESTOT = R1 + R2 + R3 + ... + Rn RPARTOT = 1 / (1/R1 + 1/R2 + 1/R3 + ... + 1/Rn) P = E ∙ I E = P / (I ∙ cos θ) E = P / I P = I2 ∙ Z ∙ cos θ P = I2 ∙ R P = I ∙ E ∙ cos θ E = sqrt(P ∙ R) P = (E2 ∙ cos θ) / Z P = E2 / R f = 1 / (2 ∙ π ∙ sqrt(L ∙ C)) E = I ∙ Z L = 1 / (4 ∙ π2 ∙ f2 ∙ C) C = 1 / (4 ∙ π2 ∙ f2 ∙ L) pf = P / (E ∙ I) pf = cos θ pf = R / Z G = 1 / R ZSERIES = R + j ∙ ω ∙ XL + 1/(j ∙ ω ∙ XC) G = R / (R2 + X2) |ZSERIES| = sqrt(R2 + (XL – XC)2) XC = 1 / (2 ∙ π ∙ f ∙ C) ZPAR = 1 / (1/R + 1/(j ∙ ω ∙ XL) + j ∙ ω ∙ XC) C = 1 / (2 ∙ π ∙ f ∙ XC) |ZPAR| = 1 / sqrt(1/R2 + 1/(XL – XC)2) XL = 2 ∙ π ∙ f ∙ L L = XL / (2 ∙ π ∙ f) L = μ0 ∙ μr ∙ N2 ∙ A / l H = N ∙ I / l B = μ0 ∙ (H + M) V = -N ∙ dΦ / dt ∇• E = ρ / ε0 ∇• E = 4 ∙ π ∙ k ∙ ρ ∇• B = 0 ∇⨯ E = - ∂B / ∂t ∇⨯ B = J / (ε0 ∙ c2) + (1/c2) ∙ ∂E / ∂t IF = IS ∙ (exp(n ∙ VD / VT) - 1) Argh!! |
The Analogy Begins
The Power Source
First, think of electricity as water. Water can provide power, just like electricity. In fact, waterwheels are used to get power from falling or flowing water. In cities, the water is pumped in by the water company. We also need a place to store water. It needs to be ready to use when we need it. You may have a big water tower in your town, a storage tank on the roof, a tank in your basement, or a well with a pump. In our projects, the power source is a battery — two "AA" or "AAA" dry cells. (The "AAA" is a smaller version of the "AA"). The battery is like a storage tank of water! |
 Water Wheel |
About AA Cells... The old-fashioned carbon-zinc AA dry cell is good choice when doing your first projects. It is a relatively safe and cheap source of energy. Do not use other types of batteries, like alkaline or lithium, until you become experienced and careful — they can easily damage your parts, and even hurt you, if you make any wrong connections. |
 AA Dry Cell |
Conductors are Pipes
Power gets conducted, or carried, through something. Water goes through pipes, and electricity is carried over wires. Your household plumbing is a circuit of pipes, valves, and other parts.
Pressure, like Voltage
We need pressure to get things moving. In cities, water is pumped into your house. It has pressure, provided by the water company. Pressure is needed in order to use the water. The pressure is high enough to wash dishes, clean clothes, water the lawn, take a shower, and so on — but not too high to burst any pipes!
Electrical voltage also can be high or low. Voltage is measured in a unit called the volt (from volta, an Italian name). The symbol is a capital letter "V". The common AA-cell can produce 1.5 V (1½ volts).
Current Flow
When you open a water faucet, the water flows, because the pressure inside the pipe is higher than the pressure outside the faucet. Like water, electricity flows in a circuit from a "high pressure" to a "low pressure". This flow is called current. For electricity, current flows from more-positive to more-negative voltage. We need a difference in voltage ("pressure") to get a flow going. Note to Physicists: I describe conventional current flow here, not physical. No complaints, please! |
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When a switch is flipped and the circuit is "on", then current can flow. This is like opening a water valve or turning on a water faucet.
When a circuit is "off" there is no current flow. This is like closing a water valve or shutting off a water faucet.
So... we need two things to get current to flow: a voltage difference ("pressure difference"), and a closed circuit.
Electrical current can vary. It can be high or low. We measure it in a unit called the ampere ("amp-ear", a French name), or just "amp" for short. The symbol for ampere is a capital letter "A".
The AA-cells used in our projects can supply a steady 0.1 A (1/10th of an ampere) of current for about 1 hour until they are considered fully drained. But our projects won't be drawing this much current! They will run steadily for much more than an hour.
Series and Parallel
We don't have a circuit unless we connect some parts together.
Series
When one end of a part is connected to the end of another part, it's a series connection. You could say "one part is stacked on top of another". The current flowing through one part then flows directly through the other part. It is exactly the same current. Old-fashioned Christmas tree lights are usually wired in series. If one bulb burns out, the whole string goes dark — the current can't flow — it's an "open circuit". |
 Series Circuit: Battery Powering 2 Bulbs |
About Batteries... In our projects, we will use 2 AA cells, one stacked on the other, to provide a 3-volt power supply. Each AA cell by itself is only 1½ volts (1.5 volts), but connecting two in series, their voltages add together. Three (3) volts is good voltage for a lot of nice projects. It's low enough to mostly avoid damaging our parts, in case we make mistakes. And it's too low to hurt ourselves.
Parallel
When both ends of a part are connected to both ends of another part, it's a parallel connection. The voltage across both parts is exactly the same. However, the overall current will be split between the two parts. If the two parts are fully identical, then the split will be equal (each gets 1/2 of the total current). If the two parts are not equal, then the current split will also not be equal. Some household lamps have two light bulbs. When you flip on the switch, both light bulbs light up. The two light bulbs are wired in parallel. If one bulb burns out, the other can still be lit. Its current doesn't depend on the other bulb's current. |
 Parallel Circuit: Battery Powering 2 Bulbs |
The Analogy Ends
The water-electricity analogy is not perfect. A few things with electricity and electrical circuits are not "water-like".
Polarity
Some parts are polarized by their very nature. These kinds of parts have two electrical connections, contacts, terminals, or "leads" (rhymes with "beads"). One connection is marked with a positive or plus (+) symbol. The other connection is marked with a negative or minus (-) symbol. A polarized part must be used a certain way in order for your circuit to work correctly.
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All batteries and cells like our AA-cells are polarized. If you don’t connect them correctly, the current may not flow, or it may even flow backwards — either way, your project will not work. Certain other electronic parts are polarized too.
A positive voltage means it has higher voltage level (= a higher "pressure") when compared to a negative voltage (= a lower "pressure").
Polarity doesn't easily relate to common household plumbing. We can't see or feel the water inside a copper pipe. (We can see it and feel it only once it spills out of a faucet or garden hose.)
To Loop or Not To Loop
Household water does not flow through a loop. It comes in one way, and goes out another way.
Water flows into your house under high pressure. It goes through one main pipe, goes through a meter, then through more pipes, valves, faucets,... many things. It flows into drains. This "used water" is now at a low pressure. It goes into a waste water pipe, and flows out to a main sewer pipe — a very big pipe under the street. The path is not a closed loop. (Not in my house, anyway. But if you live in a water treatment facility, then it is ☺)
A circuit is a loop. When the switch is closed and a circuit is "on", the current flows — we have a closed loop. The current flows out of the positive (+) terminal of the power source, through the circuit, and back into the negative (-) terminal of the power source. A complete circuit follows this path – positive to negative.
The current keeps flowing until either: we open the switch and turn the circuit "off"; or the power source is drained (our battery is "dead").
A Basic Circuit
Let's use a simple example to put these concepts together. A basic circuit consists of three (3) parts...
- Power Source – a Battery, for example.
- Controller – an On/Off switch is the simplest example.
- Load – something that uses power — a Light Bulb, for example.
...like the circuit that's inside a common flashlight (a.k.a. a torch in the UK).
 Switch Open: No Current Flows |  Switch Closed: Current Is Flowing |
When the switch is open, no current flows — the battery does have energy, but it isn't being used.
When the switch is closed, a complete circuit is formed — current flows, the light bulb lights up, and energy is used. Current flows out from the positive terminal on top of battery, through the red wires and through the switch. Current flows through the light bulb. If the bulb receives enough current, it lights up. The current completes its path or "loop" — it flows back to the battery through the black wire to the negative terminal.
Basic Circuit Mods
First Mod - the Load: We could build the above circuit, but you probably don't have a flashlight bulb handy. You should have an LED (light-emitting diode) in your kit of parts. Let's use an LED as the Load.
About LEDs.... actually we have to use a resistor in series with an LED. Why? LEDs are super-critical about voltage. An LED has a very specific operating voltage where it will light up. If the voltage is too low, the LED doesn't work. If the voltage is too high, the LED will burn out. It's too difficult to know exactly what this safe voltage is. And if we did know, it probably won't match the voltage from the battery or power source that we're using.
However, LEDs are not so critical about their current — they can pass anywhere from zero to the maximum rated current (which is typically 0.02 amperes = 20 milliamperes) — and be fine with it. So we can use a resistor as a simple "voltage-to-current adjuster".
Also, LEDs are polarized. The positive (+) lead must be connected to a more-positive point in the circuit then its negative (-) lead. If it's not, it won't work and may even get damaged. The negative (-) lead is the shorter lead and next to the flat on the LED's skirt.
In this circuit, the battery is two "AA" cells in series, providing a total of 3 volts. A 68-ohm resistor will set the current to a safe level (0.015 amperes = 15 milliamperes). If you don't have a 68-ohm resistor, then a 47-ohm resistor can be used (the current will be little too high, 26 milliamperes, but OK for a few minutes). Or you could use a 51, 56 or 62-ohm resistor.
Next mod - the Controller: You may not have the switch in your kit of parts. That's OK — you can just touch the wires together. But a much more interesting mod is to use a Transistor as the Controller. You should have a PNP Transistor in your kit.
About Transistors.... You must use the PNP Transistor in this circuit, not the NPN type. The PNP and NPN look nearly identical, so keep you parts in order! I will have more to say about Transistors in a separate blog. For now, be aware that a Transistor has three (3) leads and they are not interchangeable. You must connect it exactly as shown. The leads are numbered 1, 2, 3, going left to right, when the leads are pointing down and the flat side of the body is facing you.
In the above circuit, the PNP Transistor is the Controller, and acts like a switch. It's normally in an "off" state — the switch is "open" — until you touch the two Touch Pads. Your fingers should pass enough current to trigger the PNP Transistor into "on" state.
Lead #2 on the Transistor is very sensitive, and if overdriven with current, can be damaged. That's why there's a resistor connected between lead #2 and the Touch Pad. I recommended a value of 4.7k (= 4700 ohms = 4.7 kilohms), but it isn't critical. A value between 1k and 100k should be OK.
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