This page covers just a few of the hundreds of electronic components that can be added to a circuit.
Resistor
Semiconductive elements, called resistors, have more resistance than a metal wire, but still much less resistance than an insulator, like air or plastic.
Resistors are added to a circuit for precise control over current. If the current is too high, the circuit will get hot. If the current is too low the circuit can't do its job.
The IEC symbol for a resistor is this rectangle.
The American representation of a resistor is this squiggle, which I use because I like the way it looks.
To run this resistor circuit simulation click on the yellow voltage source then slide the EMF setting to the right.
Measurement Devices
A multimeter is a measuring device that can check voltage, current, resistance, and continuity. This makes it useful for testing batteries, checking if a wire is broken, finding a resistor's value, or measuring the voltage drop across a component.
A multimeter works by changing its internal circuit for each setting. In voltage mode it has very high resistance, so only a tiny current enters the meter while it compares two points in the circuit. In current mode it has very low resistance, so the circuit's current can flow through the meter. In resistance or continuity mode, the meter sends out a small test current and uses the response to estimate resistance.
Voltage is measured across two points, so the meter is placed in parallel with the part of the circuit you want to test. Current is measured through one path, so the circuit must be opened and the meter is placed in series. Resistance and continuity should usually be measured with the circuit powered off.
Most meters have a black probe in the COM port and a red probe in a voltage/resistance/current port. If a DC voltage or current reading is negative, the probes are probably reversed compared with the direction the meter calls positive. Current mode needs the most care because placing the meter directly across a battery can make a short circuit.
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Place the meter across the LED while the circuit is powered on and the LED is lit. Voltage is a difference between two points, so a voltmeter is connected in parallel with the component being measured.
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The meter sends a small test current through the component to measure resistance. If the circuit is powered, the outside voltage can change the reading or damage the meter.
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The probes are reversed compared with the meter's positive direction. The battery pack still has a voltage of 4.8 V, but the red probe is on the lower-voltage side.
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Current mode should be placed in series with a circuit path. Touching a current meter directly across a battery can create a short circuit because the meter has very low resistance in current mode. I can't count the number of fuses that I've had to replace on multimeters that have done this.
Diode
A diode only allows current in one direction.
In circuit diagrams a diode is represented by a triangle pointing at a line. Because diodes only allow current in one direction you have to draw them with the triangle pointing with the current.
To run this diode circuit simulation click on the yellow V(diode) then slide the EMF setting to the right.
Light Emitting Diode
A Light Emitting Diode is an efficient circuit element that produces light. LEDs are a diode. This means they only allows current to flow in one direction.
Like resistors, LEDs have resistance and cause a voltage drop.
An LED's resistance decreases with temperature. This can lead to a positive feedback loop, called thermal runaway. Thermal runaway is when a small temperature increase lowers resistance, and the lower resistance leads to increased current which leads to more temperature increases.
An LED is drawn as a diode with arrows.
To run this LED circuit simulation click on the yellow V(diode) then slide the EMF setting to the right.
LED specs
strategy
The Voltage across the LED can be between 1.85 and 2.5 Volts. Set a safe target in the middle at 2.1 V.
Use Kirchoff's voltage law to find the voltage drop across the resistor.
Use the voltage across the resistor to predict the resistance that will yield 20 mA of current with Ohm's law.
solution
$$0 = V_{\mathrm{bat1}} +V_{\mathrm{bat2}}+ V_{R}+V_{\mathrm{LED}}$$ $$0 = 1.5\,\mathrm{V}+1.5\,\mathrm{V}+ V_{R}-2.1\,\mathrm{V}$$ $$V_{R} = -0.9\,\mathrm{V}$$$$V=IR$$ $$R = \frac{V}{I}$$ $$R = \frac{0.9\,\mathrm{V}}{0.020 \,\mathrm{A}}$$ $$R = 45 \,\Omega$$
Example: How much power is used by the circuit when the LED is lit?
solution
We can find the total power used by focusing on the batteries.
$$P=IV$$ $$P=(0.020 \,\mathrm{A})(3.0 \,\mathrm{V})$$ $$P=0.060 \,\mathrm{W}$$Question: Why couldn't we use one 1.5 V battery to turn on this LED?
answer
The LED requires a voltage drop of at least 1.85 V.
One 1.5 V Battery doesn't have enough voltage.
LED specs
solution
$$0 = V_{\mathrm{bat1}} + V_{R} + V_{\mathrm{LED}}$$ $$0 = 9\,\mathrm{V} + V_{R} - 3.2\,\mathrm{V}$$ $$V_{R} = -5.8\,\mathrm{V}$$$$V=IR$$ $$R = \frac{V}{I}$$ $$R = \frac{5.8\,\mathrm{V}}{0.350 \,\mathrm{A}}$$ $$R = 16.6 \,\Omega$$
Example: What if we need to light up the LED, but we only have 100 Ω resistors. How can we get a resistance near 16 Ω?
strategy
We need to wire the resistors in parallel. As we add more resistors in parallel the equivalent resistance will drop.
$$\frac{1}{R_{eq}} = \frac{1}{R_{1}}+\frac{1}{R_{2}}+ \frac{1}{R_{3}}+\cdots$$solution
Let's try 3 resistors in parallel.
$$\frac{1}{R_{eq}} = \frac{1}{100}+\frac{1}{100}+ \frac{1}{100}$$ $$\frac{1}{R_{eq}} = \frac{3}{100}$$ $$R_{eq} = \frac{100}{3}$$ $$R_{eq} = 33 \, \mathrm{\Omega}$$Thats not enough resistance. Let's add more resistors until we get near 16 Ω
Six 100 Ω resistors in parallel are equivalent to 16.7 Ω.
Transistor
Transistors are used to increase, decrease, or stop electrical current. Like diodes and solar panels, they are made of different layers of doped silicon.
A transistor can function as an on/off switch. Current through the transistor can be set to on or off by the voltage of a third terminal. Transistors acting as switches form the logic gates used in computer processors.
Transistors are also used to boost an electrical signal. They can increase the volume of a speaker, or boost the radio signal coming from a cell phone.
Pictured above is a bipolar junction transistor. It has 3 terminals called the base, collector, and emitter. Voltage at the base terminal controls the flow of charge between the emitter and the collector terminal.
To run this NPN transistor circuit simulation click on the right yellow V(BE) then slide the EMF setting to the right.
Logic Gate
Logic gates perform basic logical operations. They can be produced from a composite of elements like diodes or transistors acting like switches.
An AND gate takes in two voltages and outputs one. If input A and input B both have high voltage, then the output will also have high voltage.
| Input A | Input B | Output |
|---|---|---|
| 1 | 1 | 1 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 0 | 0 | 0 |
An OR gate takes in two voltages and outputs one. If input A, input B, or both have high voltage, then the output will also have high voltage.
| Input A | Input B | Output |
|---|---|---|
| 1 | 1 | 1 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 0 | 0 | 0 |
An XOR gate takes in two voltages and outputs one. If either input A or input B have high voltage, then the output will also have high voltage.
| Input A | Input B | Output |
|---|---|---|
| 1 | 1 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 0 | 0 | 0 |
| Input A | Input B | Output |
|---|---|---|
| 1 | 1 | ? |
solution
The output is high when input A AND input B is high.
| Input A | Input B | Output |
|---|---|---|
| 1 | 1 | 1 |
solution
The output is high.
Multiple logic circuits can be linked together to perform more complex operations. The diagram above is for a half adder that can output the sum of two single digit binary numbers.
A full adder adds two binary numbers to a previous number. A full adder can be chained to other full adders to add binary numbers with multiple digits.
A flip-flop can be in two stable states. They are used to store binary values during a calculation.
Integrated Circuit
Integrated circuits are made by etching patterns in silicon wafers. These patterns form transistors and other electronic components. Transistors are linked to produce logic gates directly on the silicon.
Microprocessors are complex integrated circuits that perform logic and math operations. Data stored in memory is loaded at the terminals on the edge of the chip as high or low voltage. The chip performs different operations based on the imputed voltages from the memory.
Because of improvements in manufacturing, the density of integrated circuits has doubled about every 16-24 months. This trend, called Moore's law, led to smaller processors with more transistors and lower energy consumption. Moore's law, and other miniaturization trends, have successfully predicted the exponential increase in computing power starting in the 1970s.
In 2023, production began on "3 nm" scale computer chips. The distance between transistors in these chips is about 40 nm. This is nearing the 0.5 nm lattice spacing between the silicon atoms in the chip. At this scale quantum effects are starting to cause problems. For example, electrons have a chance to quantum tunnel to other parts of the circuit. These quantum problems could mean that integrated circuits will reach a limit in size, cost, and energy use.