Electronic control: from switches to chips
Electric circuits carry current to make things work. Electronic circuits go further: they control the current using components made of semiconductors. Control grew from hand switches and relays (electric magnets that move a switch), to vacuum tubes (1900s), to transistors (1947) and finally integrated circuits (chips) holding billions of transistors.
A control system has an input (switch, sensor), a process (transistor, chip) and an output (lamp, LED, motor, buzzer).
Basic components and their symbols
| Component | Job | Symbol (described) |
|---|---|---|
| Cell / battery | Gives energy (voltage) | Long thin line (+) and short thick line (−); a battery repeats the pair |
| Switch | Opens or closes the path | A line hinged up from a gap |
| Resistor | Limits current | A small rectangle (zig-zag in some books) |
| Variable resistor / potentiometer | Adjustable resistance, e.g. volume knob | Rectangle with an arrow through or onto it |
| Capacitor | Stores charge | Two short parallel plates |
| Diode | One-way current | A triangle pointing at a bar |
| LED | Diode that gives light | Diode symbol with two arrows pointing out |
| LDR / thermistor | Resistance changes with light / temperature | Resistor with arrows in / with a slanted line |
| Transistor (NPN) | Switch or amplifier | Circle with base line; arrow on the emitter pointing out |
Passive components (resistor, capacitor, inductor) cannot add energy or amplify. Active components (transistor, chips) can control or amplify using a power supply.
Resistors and the colour code
A resistor sets the current using Ohm's law: I = V ÷ R. Its value is printed as coloured bands:
black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9. Band 1 and 2 are digits, band 3 is the number of zeros (multiplier), band 4 is tolerance (gold ±5%, silver ±10%).
Example: orange–green–brown–gold = 3, 5, ×10 = 350 Ω ± 5%.
Resistors also have a power rating (often ¼ W). Power P = I²R must stay below it or the resistor overheats.
Capacitors, diodes, LEDs and transistors
- Capacitor: two metal plates separated by an insulator. It charges up when connected to a supply and gives the charge back later. Uses: smoothing the output of a charger, timing (with a resistor, time ≈ R × C), camera flash. Unit: farad (F); common values are µF. Electrolytic capacitors have + and − legs.
- Diode: lets current pass from anode to cathode (the band end) only. Used to turn AC into DC (rectifying) and to protect circuits from a battery put in backwards. A silicon diode drops about 0.7 V when conducting.
- LED: a diode that glows. It needs about 2 V (red) to 3 V (blue/white) and about 10–20 mA, and must have a series resistor. The longer leg is the anode (+).
- Transistor: three legs – base, collector, emitter. A small base current (about 0.7 V on the base of a silicon NPN) lets a much bigger collector current flow. Used as a switch (on/off) or an amplifier (a small signal becomes a big copy).
Analogue and digital signals
An analogue signal can take any value in a range, like the changing voltage from a microphone or a temperature sensor. A digital signal has only two levels: 0 (low) and 1 (high). Amplifiers are analogue circuits; logic gates and microcontrollers are digital. An analogue-to-digital converter changes one into the other.
Building circuits: drawings, materials, simulation and soldering
- Plan: draw the circuit diagram with standard symbols, and a parts list (value, rating). Technical documents also include a board layout and wiring drawing.
- Simulate: test it in free circuit-simulation software and check currents before buying parts.
- Prototype: push parts into a breadboard; no solder needed, easy to change.
- Make it permanent: mount parts on a stripboard or printed circuit board (PCB) and solder them.
Materials: conductors (copper wire and tracks, tin-plated legs), insulators (PVC sleeving, the fibreglass board, heat-shrink tube), solder (a tin-based alloy, lead-free today) with flux inside. Boards are drilled and cut; wires are stripped, cut and crimped.
Soldering steps: clean and tin the hot iron tip (about 330 °C) → hold the tip on the pad and the leg together for 1–2 s → feed solder onto the joint, not the iron → remove solder, then the iron → a good joint is shiny and shaped like a small cone → trim the extra leg.
Safety: use a stand for the iron, work in fresh air (flux fumes), wear eye protection when trimming legs, never touch the tip, switch off and unplug when done, and never work on mains voltage.
Try it: an LED circuit
With a 9 V battery, one red LED and a few resistors (or the 3D free play): predict the current for 330 Ω, 470 Ω and 1 kΩ with I = (9 − 2) ÷ R. Which is brightest? Which is safest? Now turn the LED round: what happens and why?
Key formulas and definitions
- Ohm's law: V = I × R
- LED series resistor: R = (V_supply − V_LED) ÷ I_LED
- Power: P = V × I = I² × R
- Colour code: digit, digit, × 10^(band 3), tolerance
- RC timing: time ≈ R × C (seconds, with R in Ω and C in F)
Worked examples
1. Read the resistor: yellow–violet–red–gold.
4, 7, ×10² → 4700 Ω = 4.7 kΩ, ±5%.
2. A red LED (2 V, 20 mA) is run from a 9 V battery. What resistor is needed?
R = (9 − 2) ÷ 0.02 = 7 ÷ 0.02 = 350 Ω. Use the next standard value up, 390 Ω.
3. With a 5 V supply and a 150 Ω resistor, what current flows through a 2 V LED?
I = (5 − 2) ÷ 150 = 3 ÷ 150 = 0.02 A = 20 mA.
4. What power does the 350 Ω resistor in example 2 use? Is a ¼ W resistor fine?
P = I²R = 0.02² × 350 = 0.0004 × 350 = 0.14 W. Yes, 0.14 W < 0.25 W.
5. A 100 µF capacitor charges through a 10 kΩ resistor. Roughly how long is the delay?
t ≈ R × C = 10 000 × 0.0001 = 1 s.
6. A street light must come on at dusk. Name the input, process and output parts.
Input: an LDR (its resistance rises in the dark). Process: a transistor that switches on when the LDR makes the base voltage rise. Output: a relay and lamp (or an LED).
Common mistakes
- Connecting an LED straight to a battery with no resistor; it draws too much current and burns out.
- Putting a diode, LED or electrolytic capacitor in backwards; they have a + and − side.
- Reading the colour code from the wrong end; the tolerance band (gold/silver) goes on the right.
- Melting solder on the iron tip and dripping it onto the joint, giving a dull 'dry joint' that does not conduct well.