Electronics / Bench / Schematics
Bench 09

Schematics &
datasheets A schematic shows connectivity, not physical layout. Learn its grammar and you can build someone else's circuit correctly; learn which parts of a datasheet to read and you stop being intimidated by forty-page PDFs.

needs the component pagesreading skill
Tier 0 · Groundsymbols

Reading a circuit diagram

Module 0.1

Symbols and designators

The vocabulary
DesignatorPartSymbol cue
RResistorZigzag (US) or plain rectangle (IEC)
CCapacitorTwo lines; one curved or a + marks polarised
D / LEDDiodeTriangle into a bar; arrows mean it emits light
QTransistorThree legs; arrow on the emitter shows NPN or PNP
U / ICIntegrated circuitRectangle with numbered pins
LInductorCoils or a filled bar
SWSwitchA break in a line
J / PConnectorHeaders, sockets, terminal blocks
Y / XCrystalRectangle between two plates

The grammar

  • Power at the top, ground at the bottom, signal left to right. A schematic drawn this way is readable at a glance; one that is not is a warning about its author.
  • Crossing wires only connect where there is a dot. Modern practice avoids four-way junctions entirely to remove the ambiguity.
  • Repeated ground and supply symbols are all one node. They are not separate grounds — the symbol is a label, saving a rat's nest of lines.
  • Net labels do the same for signals: two wires labelled SDA in different corners are connected.
Module 0.2

Breadboards

Which holes connect to which
┌──────────────────────────────────────┐ │ + + + + + + + + + + + + + + + + + + │ power rail — connected HORIZONTALLY │ − − − − − − − − − − − − − − − − − − │ (often split in the middle!) │ │ │ a b c d e ← 5 holes connected │ │ ───────── VERTICALLY in a column │ │ ╪╪╪╪╪╪╪╪╪ centre channel — DIP chips straddle it │ ───────── so each pin gets its own column │ f g h i j │ │ − − − − − − − − − − − − − − − − − − │ │ + + + + + + + + + + + + + + + + + + │ └──────────────────────────────────────┘

Three faults account for most breadboard trouble, and all are invisible at a glance:

  1. Split power rails. Many boards break the + and − rails at the midpoint. If half your circuit is dead, bridge the two halves with a jumper.
  2. Components sharing a column. Two legs in the same five-hole group are connected whether you meant it or not — a very easy way to short a component out entirely.
  3. Worn contacts. Cheap boards lose grip, especially with thick component legs. An intermittent circuit that responds to being poked is usually this, not your soldering or code.
Breadboards are not suitable for everything: high currents (the contacts have real resistance), high frequencies (the parallel strips form capacitance), and anything you want to keep. They are for proving a circuit before committing it — which is exactly what you should always do.
Tier 1 · Mechanicsdatasheets

Reading a datasheet efficiently

Module 1.1

Four sections, in the order that matters

You do not read these front to back
1Typical application circuit — go here first. The manufacturer has drawn the working circuit with correct capacitor values. It is usually right and always a better starting point than a forum post.
2Absolute maximum ratings — destruction thresholds. Never design here; this is the cliff edge, not the operating range.
3Recommended operating conditions — where the part works as specified. Design here.
4Electrical characteristics — the numbers you calculate with. Always read the test conditions: a gain quoted at 10mA does not hold at 500mA, and an R_DS(on) quoted at V_GS=10V is meaningless for a 3.3V pin.

Also check the package and pinout. The same part number exists as a through-hole TO-92 you can solder easily and a surface-mount SOT-23 you probably cannot yet — and the pin order can differ between packages of the same part. Orient from the physical marker (notch, dot, or flat face) every single time; assuming pin order from memory is how transistors get connected backwards.

Where to find them: search the exact part number plus "datasheet", and prefer the manufacturer's own PDF over aggregator sites. For modules rather than chips (a sensor breakout, a buck converter), the seller's page and the underlying chip's datasheet are two different documents — you often need both.
Drill 1

A datasheet lists absolute maximum V_CE of 40V. Your circuit runs at 38V. Is that acceptable?

Far too close. Absolute maximum means "beyond this, damage may occur", with no guarantee of correct behaviour approaching it — and switching an inductive load produces spikes well above the nominal rail. Standard practice is to run at 50–80% of the rating; for 38V you would want a part rated 60V or more.
Referencesearchable

Glossary