A schematic is not a picture of a circuit — it is a map of the electrical connections, drawn so a human can follow the current. Physical layout is deliberately ignored, which is why a schematic of a sprawling breadboard can be a tidy rectangle. Two conventions do all the work: a line is a perfect conductor, and a dot at a crossing means the wires are joined while no dot means they simply pass over each other.
Resistors
A resistor limits current and creates a controlled voltage drop. It is the most common component in any circuit, and its value is printed on it in colour bands rather than numbers, because the bands are readable from any angle on a tiny cylinder.
- Band 1 — first significant digit.
- Band 2 — second significant digit.
- Band 3 — multiplier, which is how many zeros to append.
- Band 4 — tolerance: gold ±5%, silver ±10%, nothing ±20%.
- Read from the end where the bands are crowded; the tolerance band sits alone at the far end.
Resistors are manufactured in the E12 series — 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 and their decade multiples — with steps chosen so that consecutive values overlap within ±10% tolerance. This is why your calculation gives 150 Ω and the parts drawer has exactly that, but a calculation giving 143 Ω means you fit the next value up.
Capacitors
A capacitor stores charge on two plates separated by an insulator. It resists sudden changes in voltage, which gives it three common jobs: smoothing a power supply's ripple, filtering noise, and setting timing intervals together with a resistor. Capacitance is measured in farads, though real parts are in microfarads (µF), nanofarads (nF) or picofarads (pF).
Diodes and LEDs
A diode conducts in one direction only. Forward biased (anode positive) it conducts with a small fixed drop — about 0.7 V for silicon. Reverse biased it blocks. That one-way property makes diodes the basis of rectification (turning AC into DC) and of protecting circuits against reversed polarity.
An LED is a diode that emits light when forward biased. Its forward voltage depends on colour: roughly 1.8–2.2 V for red, 3.0–3.4 V for blue and white. Critically, an LED has almost no resistance of its own once conducting, so it cannot limit its own current — connect one directly across a supply and it draws until it destroys itself. It always needs a series resistor.
Transistors
A transistor uses a small current or voltage at one terminal to control a much larger current between the other two. In digital work it is used as a switch: fully off or fully on, nothing in between. A bipolar NPN transistor has a base, collector and emitter; a small base current lets a much larger collector current flow. This is exactly how a 20 mA microcontroller pin controls a 1 A motor.
| Component | Symbol cue | Job | Watch out for |
|---|---|---|---|
| Resistor | Zigzag or rectangle | Limits current, creates a voltage drop | Power rating, not just value |
| Capacitor | Two parallel plates | Stores charge, smooths and filters | Electrolytics are polarized |
| Diode | Triangle into a bar | Conducts one way only | ~0.7 V forward drop (silicon) |
| LED | Diode with arrows | Emits light when forward biased | Always needs a series resistor |
| Transistor | Circle with three leads | Switch or amplifier | Identify base, collector, emitter |
| Switch | A break with a hinged line | Opens or closes the path | SPST, SPDT, momentary vs. latching |
| Ground | Stacked shrinking lines | The 0 V reference | Everything measures relative to it |