Limit current
A resistor in series protects an LED — or anything — from too much current.
need a resistor · an LED · Ohm's law
do size it so the current stays in range: R = (V − Vled) / I.
Every card in this series leans on them — "a resistor," "a flyback diode," "a cap across the rails." Here are the five or six parts the whole shelf is made of, what each one does, and the little circuits you build from them.
Limits current and sets voltages — the workhorse. Color bands tell its value.
limits currentStores charge briefly. Smooths, blocks DC, times, and couples signals.
stores chargeA coil that resists change in current — filters and stores energy (see no. 3).
resists changeA one-way valve for current — rectifies, protects, and (as an LED) lights.
one-wayA switch and an amplifier — the heart of all logic (and the slayer).
switch + ampMany parts in one package — op-amps, logic, regulators, microcontrollers.
many-in-oneA resistor in series protects an LED — or anything — from too much current.
need a resistor · an LED · Ohm's law
do size it so the current stays in range: R = (V − Vled) / I.
Two resistors split a voltage — to read sensors or scale a signal.
need two resistors
do the midpoint = Vin × R2 / (R1 + R2).
A resistor gives a floating input a defined state.
need one resistor to V+ or ground
do hold the pin high or low until something drives it.
Decode the color stripes into a value.
need a resistor · a band chart
do first bands = digits, next = multiplier, last = tolerance.
A small cap by the power pins kills noise.
need a 100 nF cap per chip
do place it close to the pins — every board wants them.
A resistor + cap smooths or blocks frequencies.
need a resistor · a capacitor
do arrange them for a low- or high-pass; the corner is set by R×C.
A cap passes the AC and blocks the DC between stages.
need a series capacitor
do let the wiggle through while the offset stays put.
How fast a cap charges sets a delay.
need a resistor + cap (an RC)
do use the charge time for a delay, a blink rate, or a debounce.
Diodes turn alternating current into one-way DC.
need diodes (a bridge of four)
do pass only one direction; a cap then smooths it.
A series diode saves a circuit from a backwards battery.
need one diode in the + line
do current only flows the right way — a cheap guardian.
A diode across a coil eats the back-EMF spike.
need a diode reverse-biased across the coil
do clamp the spike when the coil switches off (see no. 3 & 5).
A Zener diode holds a steady reference voltage.
need a Zener · a resistor
do bias it in reverse; it pins the voltage to its rating.
A transistor lets a tiny signal drive a big current.
need a transistor · a base/gate resistor · the load
do a low-side switch — logic on the base, load on the collector.
A transistor makes a weak signal bigger.
need a transistor · biasing resistors
do bias it on, feed the small signal in, take the larger one out.
Four switches reverse a motor's direction.
need four transistors (or a driver chip)
do diagonal pairs on = forward or back (see no. 2).
Combine them into gates — the atoms of every chip.
need transistors + resistors
do wire them so outputs follow AND / OR / NOT.
Prototype with no soldering.
need a breadboard · jumper wires
do rows connect across, rails run the length — power on the rails.
The symbols, and how to trace a circuit.
need a symbol key
do follow each net from + to − (the slayer's a good first read).
One chip that compares, amplifies, buffers, filters.
need an op-amp · a few resistors
do the analog swiss-army knife — feedback sets what it does.
The handful of values that cover most projects.
need resistor + cap assortments · common transistors/diodes
do keep the staples on hand; you'll reach for the same dozen.
workbench series · a few parts, endless circuits · trust the meter over your memory