Cathleen Shamieh

Cheat Sheet / Updated 02-02-2023

Electronics is more than just schematics and circuits. By using various components, such as resistors and capacitors, electronics allows you to bend electric current to your will to create an infinite variety of gizmos and gadgets. In exploring electronics, use this handy reference for working with Ohm’s, Joule’s, and Kirchhoff’s Laws; making important calculations; determining the values of resistors and capacitors according to the codes that appear on their casings; and using a 555 timer and other integrated circuits (ICs).

Article / Updated 09-17-2021

You need a closed path, or closed circuit, to get electric current to flow. If there's a break anywhere in the path, you have an open circuit, and the current stops flowing — and the metal atoms in the wire quickly settle down to a peaceful, electrically neutral existence. A closed circuit allows current to flow, but an open circuit leaves electrons stranded. Picture a gallon of water flowing through an open pipe. The water will flow for a short time but then stop when all the water exits the pipe. If you pump water through a closed pipe system, the water will continue to flow as long as you keep forcing it to move. Open circuits by design Open circuits are often created by design. For instance, a simple light switch opens and closes the circuit that connects a light to a power source. When you build a circuit, it's a good idea to disconnect the battery or other power source when the circuit is not in use. Technically, that's creating an open circuit. A flashlight that is off is an open circuit. In the flashlight shown here, the flat black button in the lower left controls the switch inside. The switch is nothing more than two flexible pieces of metal in close proximity to each other. With the black button slid all the way to the right, the switch is in an open position and the flashlight is off. A switch in the open position disconnects the light bulb from the battery, creating an open circuit. Turning the flashlight on by sliding the black button to the left pushes the two pieces of metal together — or closes the switch — and completes the circuit so that current can flow. Closing the switch completes the conductive path in this flashlight, allowing electrons to flow. Open circuits by accident Sometimes open circuits are created by accident. You forget to connect a battery, for instance, or there's a break in a wire somewhere in your circuit. When you build a circuit using a solderless breadboard, you may mistakenly plug one side of a component into the wrong hole in the breadboard, leaving that component unconnected and creating an open circuit. Accidental open circuits are usually harmless but can be the source of much frustration when you're trying to figure out why your circuit isn't working the way you think it should. Short circuits take the wrong path Short circuits are another matter entirely. A short circuit is a direct connection between two points in a circuit that aren't supposed to be directly connected, such as the two terminals of a power supply. Electric current takes the path of least resistance, so in a short circuit, the current will bypass other parallel paths and travel through the direct connection. (Think of the current as being lazy and taking the path through which it doesn't have to do much work.) In a short circuit, current may be diverted from the path you intended it to flow through. If you short out a power supply, you send large amounts of electrical energy from one side of the power supply to the other. With nothing in the circuit to limit the current and absorb the electrical energy, heat builds up quickly in the wire and in the power supply. A short circuit can melt the insulation around a wire and may cause a fire, an explosion, or a release of harmful chemicals from certain power supplies, such as a rechargeable battery or a car battery.

Article / Updated 06-18-2020

The figure shows the front and back of one type of mini-speaker. Speakers usually come with leads attached. The leads are twisted together to keep things neat and tidy. You attach the leads to components in your circuit so that electrical current passes from your circuit into the speaker. The speaker then converts the current into sound. A typical speaker contains two magnets and a cone made of paper or plastic (see the following figure). The black material you see in the mini-speaker shown is the paper cone. One of the speaker's magnets is a permanent magnet (meaning that it is always magnetized) and the other is an electromagnet. An electromagnet is just a coil of wire wrapped around a hunk of iron. If no current passes through the coil of wire, the electromagnet is not magnetized. When current passes through the coil of wire, the electromagnet becomes magnetized and gets pulled and then pushed away from the permanent magnet. The cone is attached to the electromagnet, so when the electromagnet moves, the cone vibrates, creating sound (which is just moving air). If you look closely at the back of the speaker, right, you might be able to see that one side of each lead wire is sticking through the back of the black cone. Those wires are connected to the coil inside the speaker. By connecting the other side of the lead wires to your circuit, you control the flow of current through the coil. Depending on what your circuit is doing, current may or may not flow through the coil, and you may or may not hear sound coming from the speaker.

Article / Updated 08-29-2016

A pushbutton switch is a type of tactile (meaning touch) switch, which is an on/off switch that is activated when pressure is applied to it (usually by a finger). The figure shows one type of pushbutton switch. Each of the eight switches is a normally open, momentary single-pole, single-throw (SPST) pushbutton switch. That's a lot of words just to tell you that this type of switch Makes or breaks a connection between two points Is normally open, or off (that is, the connection is broken) Is temporarily closed, or on (making the connection), when you press its button Is off again after you release its button Note that the pushbutton switch shown has four pins. Inside the switch, the top-left pin is connected to the top-right pin, and the bottom-left pin is connected to the bottom-right pin. These pin pairings just give you two ways to access each side of the switch connection; you are still making or breaking a single connection. The single make-or-break connection that the pushbutton switch controls is between the top pins and the bottom pins. You can use either one of the top pins for one side of the connection in your circuit and either one of the bottom pins for the other side of the connection in your circuit.

Article / Updated 08-29-2016

Pot is the shortened name for a potentiometer. A potentiometer (pronounced "poe-ten-shee-AH-meh-ter") is a variable resistor. The pot enables you to vary the blink rate of the LED without changing any components in your circuit. Pots come in various shapes, sizes, and values, but they all have the following things in common: They have three terminals (or connection points). They have a knob, screw, or slider that can be moved to vary the resistance between the middle terminal and either one of the outer terminals. The resistance between the two outer terminals is a fixed (constant) resistance, and it is the maximum resistance of the pot. This resistance doesn't vary even when the knob, screw, or slider is moved. The resistance between the middle terminal and either one of the outer terminals varies from 0 Ω to the maximum resistance of the pot as the knob, screw, or slider is moved. The maximum resistance (between terminals 1 and 3) of the pot — 10 kΩ — is stamped on the back of its case. If the control knob is positioned at the midpoint of its full range, the resistance between terminals 1 and 2 will be 5 kΩ and the resistance between terminals 2 and 3 will be 5 kΩ. As you turn the knob, the two variable resistances — that is, the resistance between terminals 1 and 2 and the resistance between terminals 2 and 3 — change, but their sum is always the maximum resistance of the pot. For instance, say you turn the knob so that the resistance between terminals 1 and 2 is 2 kΩ. In this case, the resistance between terminals 2 and 3 is 8 kΩ. As you vary the resistance between terminals 1 and 2 from 0 Ω to 10 kΩ, the resistance between terminals 2 and 3 varies the opposite way — that is, from 10 kΩ to 0 Ω.

Article / Updated 08-29-2016

Discrete components are individual electronic devices, such as resistors, capacitors, LEDs, and transistors. You connect the components as you build circuits. An integrated circuit (IC) contains anywhere from a few dozen to many billions (yes — billions!) of circuit components packaged in a single device that can fit into the palm of your hand. The components in an IC aren't just randomly thrown together in the package. They are connected to form a miniature circuit that performs one or more functions, such as counting or adding two numbers. Among the most complex ICs are the microprocessors that do most of the work involved in running your laptop, tablet, smartphone, and other devices. Microprocessors — which contain millions (or billions) of tiny transistors and other components — perform many functions and are often called the brains of computing.

Article / Updated 08-29-2016

Look at the two 2N3906 PNP transistors shown here. Do they look like the 2N3904 NPN transistor? Yup. Does it matter if you get the 2N3904 and 2N3906 transistors mixed up? Yessirree, Bob! In this project, you use both a 2N3904 NPN transistor and a 2N3906 PNP transistor, so you really need to keep track of which is which. The good thing about these types of transistors (especially compared to those troublesome photoresistors) is that their model numbers are stamped on their cases, so you can always tell at a glance which one is which. Like the 2N3904 transistor, the 2N3906 transistor has three leads, also called terminals or pins. These pins connect to the base, collector, and emitter inside the transistor case, as shown here and indicated in the pin diagram on the product packaging or datasheet (check the Internet). The pin diagram for the 2N3906 transistor is the same as that of the 2N3904 transistor. Depending on where you get your transistors, the pin assignments may or may not be the same as the pin assignments shown here. Check the packaging or documentation that comes with your transistor to determine which pin is which. When you plug your transistors into the solderless breadboard as you build the circuit in this project, you need to make sure that you use the correct transistor (that is, 2N3904 or 2N3906) as instructed and that you orient the transistor leads correctly.

Article / Updated 08-29-2016

A photoresistor, which is also called a light-dependent resistor (LDR) or photocell, consists of a piece of semiconductor material that exhibits an interesting characteristic: It acts like a resistor except that the value of resistance depends on how much light is shining on it. You can see an assortment of photoresistors in this figure, left, and a close-up of one photoresistor on the right. Note that these little devices have no identifying marks, such as a model number, which makes working with them a bit sketchy, er, loads of fun! Pinning down the exact resistance of a photoresistor is like trying to catch a feather as it floats down through the air. Part of the problem is that you never really know exactly how much light is shining — that is, unless you happen to own a special device called a light meter, which measures light energy. (Add it to your holiday wish list or borrow one from a photographer friend!) In general, a photoresistor works like this: In bright light, its resistance is relatively low (usually less than 10 kΩ). In darkness, its resistance is relatively high (usually more than 1 MΩ).

Article / Updated 08-29-2016

Diodes do a simple but important job: They allow current to flow in just one direction. A special kind of diode — a light-emitting diode, or LED — is often used for the purpose of, well, lighting up. You have to be careful to orient the LED the correct way, or current won't flow at all. Another kind of diode performs the important task of preventing current from flowing the wrong way in your circuit. This type of diode — a 1N4148 (or 1N914) diode — doesn't light up. It just does its job as a one-way valve for current without calling attention to itself. Pictured here, the 1N4148 diode is a signal diode. A signal diode is designed to handle relatively small currents and voltages. Diodes that can handle large current and voltages are known as power diodes. The black band at one end of the 1N4148 diode indicates the cathode (negative side). Current flows from the anode (positive side) to the cathode, but not the other way. (Some types of diodes have a silver band instead of a black band to indicate the cathode.) The 1N914 diode is considered equivalent to the 1N4148 diode.

Article / Updated 08-29-2016

To get an idea of how important a transistor is in a circuit, think about a circuit that changes the dimming time of an LED. This circuit is set up with a resistor and a capacitor to dim the light from an LED over a predictable time interval. But the longer the time interval, the less brightly the LED glows — even before it dims as the capacitor discharges! Do you know why the LED glows less brightly when the dimming time is extended? The answer is in the RC time constant. The RC time constant determines how long it takes the capacitor to discharge, which, in turn, determines how long it takes to dim the LED. To extend the dimming time, you increase either the resistance (the R) or the capacitance (the C), so that the RC time constant is larger. But huge capacitors are hard to find (and very impractical), so increasing the resistance is the better way to greatly extend the dimming time. Increasing the resistance successfully increases the RC time constant — but it also weakens the current flowing through the LED. (Remember, more resistance means less current.) A very large resistance restricts the current so much that the LED doesn't glow brightly when it's first turned on. What if you wanted to, say, turn on the lights over a stage and then bring down the lights slowly as the curtain opens? Or turn on the dome light in your family's car when you open the car door, and dim the light slowly when you close the car door? The fact that the large resistance produces a weak current can be a problem: The lights won't ever glow brightly — even when they first turn on! Using a transistor to boost the weak current solves the problem. By placing a transistor between the resistor-capacitor combination and the LED-resistor combination, you essentially jack up the current so that the lights glow brightly when they first turn on! The way it works is this: You feed the weak current coming from the resistor-capacitor part of the circuit into the base of the transistor. You use that weak base current to control a stronger current flowing from the collector to the emitter, and you use that stronger current to power the LEDs so that they shine brightly (that is, before they dim). This figure lays out your plan of attack for this project. It's useful to visualize what's happening with a block diagram like this because you can easily lose track of the big picture when you start plugging components into the breadboard.