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Basics
1. 1. Open and closed circuits — Electricity only flows round a complete loop: out of the battery’s +, through the bulb and back into its −.
2. 2. Conductors & insulators — Some materials let current flow easily — conductors, like metals.
3. 3. Volts, amps & ohms — Think of a wire as a pipe.
4. 4. Types of circuit — Open or closed, shorted, in series, in parallel or a mix: the same battery and bulbs behave very differently depending on how they are wired.
5. 5. Batteries: series & parallel — One AA cell gives 1.5 V.
6. 6. AC and DC — A battery gives direct current (DC): it always flows the same way.
7. 7. Using a breadboard — A breadboard lets you build circuits without soldering.
8. 8. Using a multimeter — A multimeter measures voltage, current and resistance, and checks continuity — but each test is set up differently: the dial picks what to measure, the red lead goes in the right socket, and the probes touch the right places.
9. 9. Kirchhoff’s two rules — Two simple rules hold in every circuit.
Resistors
1. 1. How a resistor works — A resistor makes it harder for current to flow.
2. 2. Read the colour code — Resistors are too small for printed numbers, so their value is painted on in coloured bands.
3. 3. Resistors in series — One after another, resistors make the current push through each of them in turn.
4. 4. Resistors in parallel — Side by side, resistors give the current more than one path.
5. 5. Voltage divider — Two resistors in series share the supply’s voltage between them, in proportion to their resistance.
6. 6. Variable resistor — A potentiometer is a resistor with a slider inside.
7. 7. Power and heat — Every resistor turns the energy flowing through it into heat.
8. 8. Wheatstone bridge — Two voltage dividers side by side, and a meter between their middles.
LEDs & diodes
1. 1. One way only — An LED is a diode: it lets current through in one direction only.
2. 2. Colours and volts — Each colour of LED needs a different voltage before it lights: red about 2 V, green a little more, blue and white about 3 V.
3. 3. Too much current — An LED doesn’t know when to stop: past its forward voltage, the current shoots up.
4. 4. The plain diode — A plain diode is an LED without the light: a one-way valve for current.
5. 5. LEDs in series — Two LEDs in a row share one current, but each needs its own voltage: about 2 V for a red one.
Capacitors
1. 1. Like a water tank — Before the electronics, a picture with water.
2. 2. Charge it up — A capacitor is two metal plates with a thin insulator between them.
3. 3. Fading light — With the power on, the capacitor fills up right beside the LED.
4. 4. Only while charging — Put a capacitor in line with an LED and current flows only while the capacitor is filling: the LED flashes and goes out.
5. 5. In series & parallel — Side by side — in parallel — two capacitors act like one bigger one: their capacitances add, so together they hold more and take longer to fill.
6. 6. Smoothing — A diode lets an AC supply through one way only: what comes out is DC, but in bumps — on for half of each cycle, off for the other half.
7. 7. Timed delay — Like a stair light on a timer: press the button and the capacitor fills in an instant.
8. 8. Reading the code — Small ceramic capacitors carry a three-digit code: two digits, then how many zeros follow, in picofarads.
9. 9. Two flashing LEDs — With two transistors and two capacitors, two LEDs take turns on their own for as long as the power is on.
Switches & buttons
1. 1. Push to light — A push button joins two wires only while you hold it down.
2. 2. Slide switch — A slide switch stays where you put it.
3. 3. AND & OR — Two switches in series must both be on for current to get through: that is AND.
4. 4. Two-way switch — A light on the stairs can be switched from the top and from the bottom.
Transistors
1. 1. Current control — A transistor has three legs: collector, base and emitter.
2. 2. A current switch — Give the base more current than the LED could ever use and the transistor is simply on: a switch, worked by current.
3. 3. Current gain — Between off and fully on, a transistor amplifies: the collector current is about 200 times the base current, so halving one halves the other.
4. 4. NPN and PNP — Transistors come in two kinds.
5. 5. Touch switch — Your skin conducts, just badly: across a touch pad, a few microamps get through your finger.
6. 6. Turn it round: NOT — Put the transistor beside the LED instead of in line with it, and it works the other way round.
Sensors
1. 1. Light sensor — A light-dependent resistor (LDR) is made of a material that conducts better the more light falls on it.
2. 2. Night light — Put the LDR at the bottom of a voltage divider and feed its middle to a transistor.
3. 3. Temperature sensor — A thermistor is a resistor that changes with temperature.
4. 4. Reed switch — A reed switch is two thin iron reeds sealed in a glass tube, almost touching.
5. 5. Heat alarm — The night light’s circuit, with the sensor moved to the top of the divider: a thermistor above a 2.4 kΩ resistor.
6. 6. Adjustable night light — Real sensor circuits have a small adjustable resistor, a trimmer, to set exactly when they switch.
7. 7. Hall-effect sensor — A Hall-effect sensor is a chip with a thin slice of semiconductor inside.

Electricidad y electrónica

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APR 24, 2025 14:32:18