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Preface

The information here is provided as-is and without warranty - check with more established references before making important decisions.

Electricity can be dangerous - even at lower voltages, it silently presents hazards ranging from electric shock to fires and death. If you are unsure about what you are doing, stop and seek professional advice.

Electronics

Concepts

• Direct Current (DC) flows in one direction, either steadily or in pulses.
• Alternating Current (AC) flows in alternating directions across a circuit
• Current refers to the quantity of electrons passing a given point.
  • Unit - Amperes
  • Symbol - I
• Voltage - refers to the electrical pressure or force
  • Unit - Volts
  • Symbol - V
• Resistance - refers to a cause of restriction in the flow of a current
  • Unit - Ohms
  • Symbol - R (or Ω)
• Power - refers to the work performed by a current
  • Unit - Watt
  • Symbol - p
• Potential Difference - the difference in voltage between the two ends of a conductor through which a current flows. Also known as voltage drop.

Ohm's Law

a potential difference of 1 volt will force a current of 1 ampere through a resistance of 1 ohm, or: V = I x R or I = V / R or R = V / I

Ohm's law also is used to calculate power where P = I x V (or I^2 x R)

Abbreviations and Units

Metric Prefixes for SI Units

P (PETA-) = x 1,000,000,000,000,000 (or 10^-15)

T (TERA-) = x 1,000,000,000,000 (or 10^-12)

G (GIGA-) = x 1,000,000,000 (or 10^-9)

M (MEG-) = x 1,000,000 (or 10⁶

K (KILO-) = × 1,000 (or 10³

h (HECTO-) = x 100 (or 10²)

d (DECI-) = x 1.0 (or 10^-1)

c (CENTI-) = x 0.01 (or 10^-2)

m (MILLI-) = x 0.001 (or /1000 or 10^-3)

µ (MICRO-) = x 0.000 001 (or /1000000 or 10^-6) (also mc)

n (NANO-) = x 0.000 000 001 (or /1000000000 or 10^-9)

p (PICO-) = x 0.000 000 000 001 (or /10000000000 or 10^-12)

f (FEMTO-) = x 0.000 000 000 000 001 (or /10000000000000 or 10^-15)

Resistors

A resistor is one of the most common and fundamental passive electronic components, whose primary function is to impede the flow of electric current. It introduces electrical resistance into a circuit. This resistance is a measure of the opposition to current flow. Unlike capacitors or inductors, which store energy, a resistor dissipates electrical energy, typically as heat. The unit of measurement for resistance is the Ohm (Ω).

How Resistors Work 🔥

Resistors work based on the principle of electrical resistance. Inside a resistor is a material that is not a great conductor (like carbon, metal film, or a specific alloy wire). As electrons try to flow through this material, they collide with the atoms within it, losing energy in the process. This opposition to the flow of electrons is the resistance. The amount of resistance is determined by the material used, its length, and its cross-sectional area. This relationship is defined by Ohm's Law, which states that the voltage ($V$) across a resistor is directly proportional to the current ($I$) flowing through it, with the constant of proportionality being the resistance ($R$). The formula is expressed as $V = IR$. The energy lost by the electrons is converted into heat.

File:Resistor color code chart.svg

A chart showing the color-coding system used to indicate the resistance value of a resistor.

What Resistors Are Used For 💡

The primary and most frequent use of a resistor is to limit current. For example, an LED (Light Emitting Diode) can be damaged by too much current, so a resistor is placed in series with it to ensure the current stays at a safe level. Another crucial application is in voltage division. By connecting two resistors in series, you can create a specific voltage level that is a fraction of the source voltage, which is essential for providing the correct voltage to different parts of a circuit.

Further Applications

Resistors are also used as pull-up and pull-down resistors in digital logic circuits. These ensure that a digital input pin settles at a known logical state (either high or low) when it's not actively being driven, preventing an unstable, "floating" state. They are also used in timing circuits with capacitors (RC circuits), where the resistance value helps determine the charging and discharging rate of the capacitor. From setting the gain of an amplifier to acting as heating elements, the simplicity and utility of resistors make them an indispensable component in virtually every electronic circuit ever designed.

Calculating resistance

R_t = Total Resistance

Resistors in series

• R_t = R1 + R2 + R3 (etc)

Resistors in parallel (2 resistors)

• R_t = (R1xR2) / (R1+R2)

Resistors in parallel (3 or more resistors)

• R_t = 1 / ((1/R1) + (1/R2) + (1/RN))

Resistors used as a voltage divider

• V_out = V_in x (R2/(R1+R2))

Resistor component identification

Resistor values are most famously identified using a color code, especially on through-hole components. In the common four-band system, the first two bands represent the significant digits, the third band is the multiplier (the power of 10), and the fourth band indicates the tolerance. For example, a resistor with bands of brown (1), black (0), red (2), and gold (±5%) would be 10 x 10², which is 1,000 Ohms (Ω) or 1 kΩ, with a ±5% tolerance. Precision resistors use a five-band system with three significant digit bands. However, modern Surface Mount Device (SMD) resistors are too small for color bands and instead use a numerical code. A three-digit code like "472" translates to 47 x 10², or 4,700 Ω (4.7 kΩ). Just as with inductors, the letter 'R' is used to denote a decimal point, so "5R6" means 5.6 Ω.

Resistor Colour Codes

Colour	Significant Digits (bands 1&2)	Multipliers (band 3)	Tolerence (band 4)
Black	0	1	-
Brown	1	10	±1%
Red	2	100	-
Orange	3	1000 (k)	-
Yellow	4	10000 (x10k)	No
Green	5	100000 (x100k)	colour
Blue	6	1000000 (M)	band
Violet	7	10000000 (x10M)	±20%
Gray	8	100000000 (x100M)	-
White	9	-	-
Gold	-	-	±5%
Silver	-	-	±10%

Example: a component with Yellow, Violet, Orange and Silver bands could be a 47k resistor with 10% tolerance.

Surface Mount Resistor Value Codes

3 Digit Code	4 Digit Code	Ohms (1k)		3 Digit Code	4 Digit Code	Ohms (10k)		3 Digit Code	4 Digit Code	Ohms (100k)		3 Digit Code	4 Digit Code	Ohms (1M)
102	1001	1k		103	1002	10k		104	1003	100k		105	1004	1M
112	1101	1.1k		113	1102	11k		114	1103	110k		115	1104	1.1M
122	1201	1.2k		123	1202	12k		124	1203	120k		125	1204	1.2M
132	1301	1.3k		133	1302	13k		134	1303	130k		135	1304	1.3M
152	1501	1.5k		153	1502	15k		154	1503	150k		155	1504	1.5M
162	1601	1.6k		163	1602	16k		164	1603	160k		165	1604	1.6M
182	1801	1.8k		183	1802	18k		184	1803	180k		185	1804	1.8M
202	2001	2k		203	2002	20k		204	2003	200k		205	2004	2M
222	2201	2.2k		223	2202	22k		224	2203	220k		225	2204	2.2M
242	2401	2.4k		243	2402	24k		244	2403	240k		245	2404	2.4M
272	2701	2.7K		273	2702	27k		274	2703	270k		275	2704	2.7M
302	3001	3k		303	3002	30k		304	3003	300k		305	3004	3M
332	3301	3.3k		333	3302	33k		334	3303	330k		335	3304	3.3M
362	3601	3.6k		363	3602	36k		364	3603	360k		365	3604	3.6M
392	3901	3.9k		393	3902	39k		394	3903	390k		395	3904	3.9M
432	4301	4.3k		433	4302	43k		434	4303	430k		435	4304	4.3M
472	4701	4.7k		473	4702	47k		474	4703	470k		475	4704	4.7M
512	5101	5.1k		513	5102	51k		514	3103	510k		515	5104	5.1M
562	5601	5.6k		563	5602	56k		564	5603	560k		565	5604	5.6M
622	6201	6.2k		623	6202	62k		624	6203	620k		625	6204	6.2M
682	6801	6.8k		683	6802	68k		684	6803	680k		685	6804	6.8M
752	7501	7.5k		753	7502	75k		754	7503	750k		755	7504	7.5M
822	8201	8.2k		823	8202	82K		824	8203	820k		825	8194	8.2M
912	9101	9.1k		913	9102	91k		914	9103	910k		915	9104	9.1M

---

Capacitors

A capacitor is a fundamental electronic component that stores electrical energy in an electric field. Think of it like a small, rechargeable battery that can charge and discharge very quickly. At its most basic, a capacitor consists of two conductive plates, typically made of metal, separated by an insulating material called a dielectric. This dielectric can be made of various materials, including ceramic, plastic, or even air. The ability of a capacitor to store charge is known as its capacitance, which is measured in units called Farads (F).

How Capacitors Work 🔋

Capacitors work by accumulating charge on their conductive plates. When a voltage source, like a battery, is connected across the capacitor, an electric field is created across the dielectric. This causes positive charge to build up on one plate and negative charge to build up on the other. The dielectric prevents the charge from flowing directly between the plates, so the energy remains stored in the electric field. This process is called charging. When the capacitor is connected to a circuit, it releases this stored energy in a rapid burst, a process known as discharging. The amount of charge it can store depends on its capacitance and the voltage applied.

File:Capacitor schematic with details.svg

A basic diagram showing a parallel-plate capacitor connected in a circuit.

What Capacitors Are Used For 💡

Capacitors are essential in countless electronic devices due to their ability to store and release energy quickly. One of their most common uses is in filtering. In power supplies, they smooth out fluctuations in voltage to provide a steady, clean power source for sensitive electronics. They are also crucial in timing circuits. By controlling how quickly a capacitor charges or discharges, engineers can create precise time delays, which are used in everything from blinking lights to computer clocks.

Further Applications

Another key application is in signal coupling and decoupling. In audio amplifiers, for instance, they block direct current (DC) while allowing alternating current (AC) signals (like the sound waveform) to pass through, ensuring different stages of the circuit don't interfere with each other's DC voltage levels. In camera flashes, a large capacitor is charged over a few seconds and then rapidly discharged to produce a bright burst of light. From radios and computers to mobile phones and electric vehicles, capacitors are a cornerstone of modern electronics, performing a wide range of critical functions.

Calculating Capacitance

Capacitance

• Ct = Total Capacitance

Capacitors in parallel

• Ct = C1 + C2 + C3 (etc)

Capacitors in series (2 capacitors)

• Ct = (C1 x C2) / (C1 + C2)

Capacitors in series (3 or more capacitors)

• Ct = 1 / ((1/C1) + (1/C2) + (1/CN))

Capacitors used as a voltage divider

• Vout = Vin x (C1 / (C1 + C2))

Capacitor component identification

Capacitor identification numbers are typically printed on the component's body to indicate its capacitance, tolerance, and voltage rating. The most common system, especially for smaller ceramic, film, and tantalum capacitors, is a three-digit code. In this code, the first two digits represent the significant figures of the value, and the third digit is a multiplier, indicating the power of 10. This value is almost always in picofarads (pF). For example, a capacitor marked "104" means 10 followed by 4 zeros, which is 100,000 pF. This value is more commonly expressed as 100 nanofarads (nF) or 0.1 microfarads (µF). This number is often followed by a letter that signifies the tolerance (e.g., J = ±5%, K = ±10%, M = ±20%) and a number indicating the maximum working voltage (e.g., 50V). Larger capacitors, such as electrolytic types, usually have enough space to have their full value, unit, and voltage printed directly on them, for example, "470µF 25V".

Surface Mount Capacitors

Surface-mount device (SMD) capacitors are identified by size, type, and, if marked, a 3-digit code (first two significant, third multiplier in pF) or an alphanumeric code (e.g., "A5" = 1.0µF).

Ceramic capacitors often lack markings, requiring a multimeter, while tantalum/electrolytic capacitors are polarized with a stripe indicating the negative side. 

Common Identification Methods

• 3-Digit Code (Ceramic): The first two digits are significant figures, and the third is the multiplier (number of zeros) in picofarads (pF).
  • For example, "104"= 10 x 10 ^4 pF = 100,000pF or 100nF or 0.1µF.
• Alphanumeric Code (Tantalum/Electrolytic): These often use a letter followed by a number (e.g., A5) or a voltage letter code followed by capacitance.
• Voltage Codes: Letters indicate voltage (e.g., e=2.5V, G=4V, J=6.3V, A=10V, C=16V, D=20V, E=25V, V=35V, H=50V).
• No Marking: Many small MLCC capacitors have no markings, requiring a specialised LCR meter or multimeter to check capacitance. 
• Polarity Identification (Tantalum & Electrolytic) Tantalum: A stripe or bar indicates the positive terminal.
• Electrolytic: A black stripe on the top of the canister indicates the negative terminal. 

Physical Size (Footprint) SMD capacitors are categorised by their physical dimensions (e.g., 0201, 0402, 0603, 0805, 1206, 1210), where the numbers represent length and width in inches or millimetres (e.g., 0805 is 0.08 x 0.05 inches). 

Capacitor Code System

Most common capacitor codes use a three-digit format:

• First two digits = significant figures
• Third digit = multiplier (number of zeros, in pF)

Example:

• 223J = 22 × 10³ pF = 22 nF = 0.022 µF (J = ±5% tolerance)
• 151K = 150 pF = 0.15 nF = 0.00015 µF (K = ±10% tolerance)

Capacitor Value Codes

µF	nF	pF	Code
0.00001	0.01	10	100
0.000012	0.012	12	120
0.000015	0.015	15	150
0.000018	0.018	18	180
0.000022	0.022	22	220
0.000025	0.025	25	250
0.000027	0.027	27	270
0.000033	0.033	33	330
0.000039	0.039	39	390
0.000047	0.047	47	470
0.000056	0.056	56	560
0.000068	0.068	68	680
0.000082	0.082	82	820
0.0001	0.1	100	101
0.00012	0.12	120	121
0.00015	0.15	150	151
0.00018	0.18	180	181
0.00022	0.22	220	221
0.00025	0.25	250	251
0.00027	0.27	270	271
0.00033	0.33	330	331
0.00039	0.39	390	391
0.00047	0.47	470	471
0.00056	0.56	560	561
0.00068	0.68	680	681
0.00082	0.82	820	821
0.001	1	1000	102
0.0012	1.2	1200	122
0.0015	1.5	1500	152
0.0018	1.8	1800	182
0.0022	2.2	2200	222
0.0025	2.5	2500	252
0.0027	2.7	2700	272
0.0033	3.3	3300	332
0.0039	3.9	3900	392
0.0047	4.7	4700	472
0.0056	5.6	5600	562
0.0068	6.8	6800	682
0.0082	8.2	8200	822
0.01	10	10000	103
0.012	12	12000	123
0.015	15	15000	153
0.018	18	18000	183
0.022	22	22000	223
0.025	25	25000	253
0.027	27	27000	273
0.033	33	33000	333
0.039	39	39000	393
0.047	47	47000	473
0.056	56	56000	563
0.068	68	68000	683
0.082	82	82000	823
0.1	100	100000	104
0.12	120	120000	124
0.15	150	150000	154
0.18	180	180000	184
0.22	220	220000	224
0.25	250	250000	254
0.27	270	270000	274
0.33	330	330000	334
0.39	390	390000	394
0.47	470	470000	474
0.56	560	560000	564
0.68	680	680000	684
0.82	820	820000	824
1	1000	1000000	105
1.2	1200	1200000	125
1.5	1500	1500000	155
1.8	1800	1800000	185
2	2000	2000000	205

Code	Tolerance
F	±1%
G	±2%
J	±5%
K	±10%
M	±20%
Z	+80% / −20%

---

Inductors

An inductor is a fundamental passive electronic component that stores energy in a magnetic field when electric current flows through it. Often called a coil, choke, or reactor, an inductor is typically a coil of insulated wire wound around a core. The core material can be air or a magnetic material like iron or ferrite. The ability of an inductor to store energy in a magnetic field is called inductance, and its unit of measurement is the Henry (H).

How Inductors Work 🔋

Inductors operate based on the principles of electromagnetism. When current flows through the coiled wire, it generates a magnetic field around the inductor. If the current changes, the magnetic field also changes, and this change induces a voltage (an electromotive force or EMF) across the coil, according to Faraday's law of induction. This induced voltage opposes the change in the original current, a property known as Lenz's law. In essence, an inductor resists changes in current. When current increases, the inductor tries to hold it back, and when current decreases, it tries to keep it flowing. It stores energy in the magnetic field when current is increasing and releases it when current is decreasing.

File:Inductor with magnetic field lines.svg

An inductor showing the magnetic field generated by the flow of current.

What Inductors Are Used For 💡

Due to their property of resisting changes in current, inductors are vital in many electronic applications. One of their primary uses is in filters. Paired with capacitors, they form LC circuits (or "tank" circuits) that can be used to select or block specific frequencies. This is fundamental to radio tuners and signal processing, allowing you to tune into a specific station while rejecting others. They are also essential in power supplies to smooth out the flow of direct current (DC) and filter out unwanted alternating current (AC) noise.

Calculating Inductance

Inductance

• Lt = Total Inductance

Inductors in series

• Lt = L1 + L2 + L3 (etc)

Inductors in parallel (2 inductors)

• Lt = (L1 x L2) / (L1 + L2)

Inductors in parallel (3 or more inductors)

• Lt = 1 / ((1/L1) + (1/L2) + (1/LN))

Inductors used as a current divider

• Iout = Iin x (L1 / (L1 + L2))

Further Applications

Inductors are also key components in power conversion. In devices like buck or boost converters (found in phone chargers and laptops), inductors are used to efficiently step voltages up or down. Their ability to store and release energy allows for the regulation of voltage and current. Furthermore, inductors are used as sensors to detect magnetic fields or the proximity of metallic objects. From the transformers that power our homes to the wireless charging pads for our phones and even the ignition systems in cars, inductors play a crucial role in managing and manipulating electrical energy across a vast range of technologies.

Inductor component identification

Inductor component identification numbers often follow a system similar to capacitors, but with one crucial difference: the base unit is the microhenry (µH). For many smaller inductors, a three-digit code is used where the first two digits are the significant figures and the third is the power-of-10 multiplier. For instance, an inductor marked "101" is 10 x 10¹, which equals 100 µH. A key feature of this system is the use of the letter 'R' to indicate a decimal point. So, an inductor marked "2R2" has a value of 2.2 µH, and one marked "R15" would be 0.15 µH. This code is typically followed by a letter indicating the tolerance, such as K for ±10% or M for ±20%. Just like with other components, larger inductors and chokes often have sufficient space to have their full value, including the unit (e.g., 10mH for 10 millihenries), printed directly on their casing. Some older through-hole inductors may also use a color-coding system similar to resistors.

Active Components

Semiconductors

Diodes

A diode is a fundamental semiconductor device that essentially acts as a one-way street for electrical current. It allows current to flow easily in one direction, known as the forward bias direction, while severely restricting its flow in the opposite direction, known as the reverse bias direction. This property is achieved by joining P-type and N-type semiconductor materials to form a P-N junction.

Diodes are crucial for a wide range of applications, most notably for converting alternating current (AC) to direct current (DC) in a process called rectification.

Specialised types include Light Emitting Diodes (LEDs), which produce light, and Zener diodes, used for voltage regulation.

Signal Diodes: Basics and Operation

A signal diode is a small-current semiconductor device designed to process electrical signals rather than handle high power. Unlike the Zener diodes in your reference charts, which are built for reverse breakdown, signal diodes are optimized for high-speed switching and "rectifying" small signals. They allow current to flow easily in one direction (forward bias) while blocking it in the other (reverse bias).

The most critical characteristic of a signal diode is its switching speed, or reverse recovery time. Because they have very low capacitance, they can turn on and off millions of times per second. This makes them ideal for processing high-frequency data or radio signals without distorting the waveform.

Common Applications

Signal diodes are found in a variety of low-power electronic circuits:

• Signal Clipping and Clamping: Used to prevent input signals from exceeding specific voltage limits, protecting sensitive components like the transistors seen in your pin-out tables.
• Logic Gates: They can be used to create simple "AND" or "OR" logic circuits in digital electronics.
• Demodulation: In AM radios, signal diodes extract the audio information from the high-frequency carrier wave.
• Reverse Polarity Protection: Small signal diodes protect battery-operated devices from damage if the batteries are inserted backward.

Comparison: Signal vs. Zener Diodes

Feature	Signal Diode (e.g., 1N4148)	Zener Diode (e.g., 1N4747A)
Primary Use	High-speed switching / Rectification	Voltage Regulation
Reverse Operation	Blocks current (until damaged)	Conducts safely at specific $V_Z$
Forward Voltage ($V_F$)	Typically 0.7V (Silicon)	Typically 0.7V (Silicon)
Typical Voltage Rating	Up to 100V Reverse	20V to 100V+ Breakdown

Forward Voltage Calculation

When a signal diode is conducting, it causes a small voltage drop, usually around 0.7V for silicon-based models. To calculate the remaining voltage available for a load:

1. Identify Input Voltage: Determine your source voltage ($V_{IN}$).
2. Subtract Forward Drop: Subtract the 0.7V drop from the source.
3. Apply Ohm's Law: Calculate the current ($I$) through the load ($R_L$) using the remaining voltage:

   <math>I = \frac{V_{IN} - 0.7V}{R_L}</math>

Zener Diodes: Basics and Operation

A Zener diode is a specialized semiconductor designed to allow current to flow in reverse once a specific voltage threshold, known as the Zener voltage ($V_Z$), is reached. While standard diodes are often damaged by reverse current, Zener diodes are engineered to operate safely in this "breakdown" region. As shown in technical reference charts, models like the 1N4747A are rated for specific stable voltages, such as 20V, while others like the 1N4764A reach up to 100V.

The device works by "clamping" the voltage across its terminals to its rated value, regardless of fluctuations in the input power. This is achieved through the Zener effect or Avalanche breakdown, where the diode's internal structure allows controlled conduction at a precise energy level. Because they maintain this steady state, they are primarily used as voltage regulators and reference points in power supplies to protect sensitive components from spikes.

Beyond regulation, Zener diodes are frequently found in clipping circuits to limit signal amplitudes and in surge protection devices. In more complex setups, they often provide a stable reference voltage for other components, such as the NPN or PNP transistors used in complementary pairs. Their predictable behavior makes them essential for ensuring circuit stability across various electronic applications.

Zener Diode Voltages

The following table provides the voltage ratings for the 1N47xxA series of Zener diodes:

Component	Voltage
1N4747A	20V
1N4748A	22V
1N4749A	24V
1N4750A	27V
1N4751A	30V
1N4752A	33V
1N4753A	36V
1N4754A	39V
1N4755A	43V
1N4756A	47V
1N4757A	51V
1N4758A	56V
1N4759A	62V
1N4760A	68V
1N4761A	75V
1N4762A	82V
1N4763A	91V
1N4764A	100V

Resistance Calculation

To calculate the necessary series resistor ($R_S$) for a Zener diode, you use Ohm’s Law to ensure the diode receives enough current to stay in breakdown without exceeding its power rating.

1. Identify Voltages: Subtract the Zener voltage ($V_Z$) from your input source voltage ($V_{IN}$) to find the voltage that must be dropped across the resistor ($V_R$). For a 1N4747A, $V_Z$ is 20V.
2. Determine Current: Decide on the desired operating current ($I_Z$). This must be higher than the minimum "knee current" but lower than the maximum current the diode can handle.
3. Apply Ohm's Law: Use the formula:

   <math>R_S = \frac{V_{IN} - V_Z}{I_Z}</math>

Further reading: Diode on Wikipedia

Transistors

A transistor is a semiconductor device that serves as the fundamental building block of modern electronics. Its primary functions are to act as either an amplifier, where it takes a small input signal and produces a larger output signal, or as an electronic switch, turning a current on or off.

The two main families are Bipolar Junction Transistors (BJTs) and Field-Effect Transistors (FETs).

By controlling the flow of a large current with a tiny one, transistors enable everything from simple circuits to complex computations, forming the basis of all microchips, computer memory, and processors.

Common NPN and PNP Transistors for Hobby Projects

This section lists commonly used bipolar junction transistors (BJTs) suitable for hobby electronics projects such as Arduino, Raspberry Pi, and general prototyping.

Common NPN Transistors

NPN transistors are most often used for low-side switching (load connected to +V, transistor switches to ground).

Transistor	Type	Typical Use / Notes
2N2222 / PN2222A	NPN	General purpose switching, LEDs, relays, buzzers
BC547	NPN	Low current signal transistor, very common
BC337	NPN	Higher current version of BC547
2N3904	NPN	General purpose small signal switching
TIP120 / TIP122	NPN (Darlington)	High current loads, easy to drive but inefficient (voltage drop)
BD139	NPN	Medium power, motors, lamps, audio drivers
2SC1815	NPN	Audio and signal amplification

Common PNP Transistors

PNP transistors are typically used for high-side switching or complementary designs.

Transistor	Type	Typical Use / Notes
2N3906	PNP	Complement to 2N3904, general purpose
BC557	PNP	Complement to BC547
BC327	PNP	Complement to BC337
2N2907 / PN2907A	PNP	Complement to 2N2222
TIP125 / TIP127	PNP (Darlington)	High current loads, easy to drive but inefficient
BD140	PNP	Complement to BD139, audio and motor drivers

Common Complementary Pairs

These transistor pairs are designed to work well together in push-pull or symmetrical circuits.

NPN	PNP	Typical Use
BC547	BC557	Small signal switching
BC337	BC327	Medium current loads
2N3904	2N3906	General purpose circuits
BD139	BD140	Audio amplifiers, motor drivers
TIP120	TIP125	High current loads (Darlington)
TIP122	TIP127	Higher power Darlington pairs

NPN	PNP	VCE	Ic	hFE
C945	A733	40V	0.1A	40
C1060	A670	45V	3A	20
C1061	A671	60V	3A	20
C1162	A715	35V	1A	40
C1383	A683	25V	1.5A	60
C1384	A684	50V	1.5A	60
C1663	A835	140V	0.5A	60
C1670	A840	140V	0.5A	50
C1815	A1015	50V	0.15A	70
C1826	A768	60V	4A	40
C2073	A940	150V	1.5A	40
C2229	A949	150V	0.05A	70
C2236	A966	30V	1.5A	100
C2580	A1105	120V	8A	80
C2581	A1106	140V	10A	80
C2922	A1216	180V	17A	55
C3181	A1264	120V	8A	55
C3182	A1265	140V	10A	55
C3281	A1302	200V	15A	55
C3858	A1494	200V	17A	—
D188	B545	60V	5A	30
D313	B507	60V	3A	90
D401	B546	150V	2A	40
D438	B560	80V	0.7A	75
D667	B647	80V	1A	60
D669	B649	120V	1.5A	60
D718	B688	120V	8A	55
D738	B702	160V	12A	35
D845	B755	150V	12A	55
D1063	B827	60V	7A	100

Typical Hobby Applications

• LED and LED strip drivers
• Relay drivers (with flyback diode)
• Small DC motor drivers
• Buzzers
• Logic level switching
• Audio pre-amplifiers
• Microcontroller GPIO switching (Arduino, ESP32, Raspberry Pi)

Practical Tips

• Always use a base resistor (typically 1 kΩ – 10 kΩ from microcontroller output).
• Use a flyback diode across inductive loads such as relays and motors.
• For loads above approximately 500 mA, MOSFETs are often more efficient than BJTs.
• Darlington transistors have a higher voltage drop and may require heatsinking.

Minimal Starter Set

A minimal transistor set suitable for most beginner projects:

• 2N2222 (NPN)
• BC547 (NPN)
• 2N3906 (PNP)

Transistor Lookup Table by Current Rating

This table groups common hobbyist transistors by approximate maximum continuous collector current. Always check the datasheet for exact limits, thermal requirements, and safe operating area.

Transistor	Type	Approx. Max Collector Current (Ic)	Typical Hobby Use
BC547	NPN	100 mA	Sensors, small LEDs, logic-level switching
BC557	PNP	100 mA	Complement to BC547, small signal switching
2N3904	NPN	200 mA	LEDs, small relays, general purpose switching
2N3906	PNP	200 mA	Complement to 2N3904
2N2222 / PN2222A	NPN	600 mA	Relays, small motors, buzzers
2N2907 / PN2907A	PNP	600 mA	Complement to 2N2222
BC337	NPN	800 mA	Motors, lamps, higher current loads
BC327	PNP	800 mA	Complement to BC337
2SC1815	NPN	150 mA	Audio stages, signal amplification
BD139	NPN	1.5 A	Medium motors, high-power LEDs, audio drivers
BD140	PNP	1.5 A	Complement to BD139
TIP120	NPN (Darlington)	5 A	High current loads, motors, solenoids (inefficient)
TIP122	NPN (Darlington)	5 A	Higher power loads (inefficient)
TIP125	PNP (Darlington)	5 A	Complement to TIP120
TIP127	PNP (Darlington)	5 A	Complement to TIP122

Notes

• Current ratings are typical maximums; practical safe current is often lower due to heat dissipation.
• Heatsinks may be required for BD139/BD140 and TIP12x devices at higher currents.
• For loads above ~1 A or for battery-powered projects, logic-level MOSFETs are usually more efficient than BJTs.
• Always include a flyback diode when driving inductive loads (relays, motors, solenoids).

Further reading: Transistor on Wikipedia

Integrated Circuits (ICs)

An Integrated Circuit (IC), often called a microchip or chip, is a miniaturised electronic circuit consisting of thousands or even billions of electronic components, primarily transistors, diodes, resistors, and capacitors, all fabricated on a single, tiny piece of semiconductor material, usually silicon.

By integrating all these components onto one chip, ICs can perform complex functions at incredible speeds with very low power consumption.

They are the brains behind nearly all modern electronic devices, from simple digital watches and calculators to powerful computers, smartphones, and servers.

Further reading: Integrated Circuits on Wikipedia

Microcontrollers

A microcontroller (often abbreviated as MCU) is essentially a small, self-contained computer on a single integrated circuit.

Unlike a microprocessor (like the CPU in a laptop) which requires external memory and peripherals to function, a microcontroller integrates a processor, memory (both RAM for temporary data and Flash or ROM for storing programs), and various input/output (I/O) peripherals onto one chip. These peripherals allow it to interact directly with the physical world, enabling it to read sensors, control motors, light up LEDs, and communicate with other devices. Designed for specific tasks, microcontrollers are the "hidden" brains in countless everyday items, from microwave ovens and TV remotes to cars and IoT devices, where they execute a dedicated program to manage the device's functions.

Further reading: Microcontrollers on Wikipedia

Optoelectronics

Optoelectronics (often shortened to "opto") is a branch of electronics that deals with devices that source, detect, and control light.

These components convert electrical signals into light or light into electrical signals. Key examples include Light Emitting Diodes (LEDs), which emit light when current passes through them, and photodetectors (like photodiodes or phototransistors), which generate a current when exposed to light.

Another important device is an optocoupler (or opto-isolator), which combines an LED and a photodetector in one package to transmit a signal between two electrically isolated circuits using a beam of light, providing safety and noise reduction.

Further reading: Optoelectronics on Wikipedia

Power

Batteries

A battery is a self-contained electrochemical device that converts stored chemical energy into electrical energy. It consists of one or more cells, each with three main components: a negative electrode (the anode), a positive electrode (the cathode), and a chemical medium called an electrolyte that separates them.

When a circuit is connected, a chemical reaction begins, causing electrons to flow from the anode, through the external circuit to power a device, and then to the cathode, creating a direct current (DC).

Batteries are broadly classified into two types: primary (single-use), like standard AA or AAA alkaline cells, which are discarded once depleted, and secondary (rechargeable), such as the lithium-ion batteries in smartphones and electric cars, which can have their chemical reactions reversed by applying an external power source.

Further reading: Electric Battery on Wikipedia

Electrical

Pages pertaining to house and automotive electrical systems