Short words. Big pictures. Flip the switch and watch what happens. Works fully offline once loaded — add it to your home screen.
1. The simplest loop
A circuit is just a loop that electricity runs around.
1
Battery pushes electricity (like a pump pushes water).
2
Wires carry it in a circle.
3
Load uses the electricity — e.g. an LED (makes light).
4
Electricity comes back to the battery. No loop = no flow.
💡
Think of water. Battery = pump. Wire = pipe. LED = water wheel. The wheel only spins if water can loop back to the pump.
2. Flip the switch
A switch just breaks the loop. No loop → no light.
3. Why the resistor? 🤔
An LED is greedy. Without a resistor, it grabs too much current and burns out.
🪫
Battery
Pushes with a voltage — 9 V.
💡
LED
Only wants ~2 V. The extra 7 V has to go somewhere.
🛑
Resistor
Soaks up the extra 7 V as heat. It "restricts" the flow.
🔥
No resistor = LED pops. Slide the resistor down to 0 Ω and watch what happens below.
Formula the circuit uses: current = (battery 9V − LED 2V) ÷ resistor. More Ω = less current = dimmer LED. Too little = burn.
4. Adding more parts
Real boards just chain more little parts into the loop. Click each to see what it does.
🛑
Resistor
Limits current. Protects LEDs & chips.
🪣
Capacitor
Stores a tiny sip of electricity. Smooths bumps.
↩️
Diode
One-way valve. Electricity only flows the arrow way.
🚪
Transistor
Tiny switch/amplifier. A little current controls a big one.
🧠
IC / Chip
Lots of transistors in one black box — the "brain".
🔘
Switch / Button
Breaks or closes the loop on command.
🔊
Speaker
Vibrates from current → makes sound (your radio!).
☀️
Solar panel
Makes electricity from light. It IS a battery (when sunny).
👆
Tap a part to learn what it does in a circuit.
5. What you see on a pulled-apart board
The green board is just flat wires holding all those parts.
Gold lines are the wires — printed flat instead of round.
Silver dots (pads) are where parts are soldered.
Parts sit on top; traces connect them underneath in a loop.
6. Your solar panel ↔ LED question ☀️→💡
Your instinct was right — it doesn't work the way it looks. Here's why, in pictures.
💡
LED
An LED uses electricity. It's a load — like a water wheel. Electricity goes IN, light comes OUT.
☀️
Solar panel
A solar panel makes electricity. It's a source — like a battery. Light goes IN, electricity comes OUT.
So can you plug the solar panel into the LED's socket?
The LED socket is a consumer — it expects power to flow INTO it.
The solar panel is a producer — it pushes power OUT.
Plugging source → source-socket is like pouring water into the pump instead of the pipe. It won't power the radio, and can back-feed where it shouldn't.
✅ What actually works
Use the solar panel in place of the battery, not in place of the LED:
Solar + wire → radio's battery + contact.
Solar − wire → radio's battery − contact.
Check voltage: solar must match the battery (e.g. 6 V panel for a 6 V radio). Too many volts = smoke. ☠️
Solar is weak in shade — radio may stutter; add a capacitor/battery pack to smooth it.
⚠️
Same fitting ≠ same job. Connectors can look identical but carry power in opposite directions. Always trace where the two wires go before you plug in.
7. The multimeter — your circuit x-ray 🔍
A multimeter is just a measuring stick for electricity. It can't put power in — it only watches.
Electricity has 3 things worth measuring. Use the water-in-pipes picture again:
Electricity thing
Unit
Water equivalent
What it answers
Voltage (V)
Volts
Water pressure
"How hard is it pushing?"
Current (A)
Amps
Water flow rate
"How much is going through?"
Resistance (Ω)
Ohms
A narrow pipe
"How hard is it getting through?"
Extra ones you'll see: Hz (Hertz = wobbles per second, for AC/mains), ° (temperature), diode/beep (continuity test — beeps if two points are connected).
📏
It doesn't power anything
It only listens. Safe to touch probes to almost any low-voltage circuit.
🔴⚫
Two probes, always
Red = "look here". Black = "compare to here". It measures the difference between them.
7a. The dial, one symbol at a time
Click each setting to see what it does, where the wires go, and a real reading.
⏻
Off
V⎓
DC volts
V∿
AC volts (Hz)
A⎓
DC amps
Ω
Resistance
)))
Continuity/beep
⏩
Diode test
Hz
Frequency
Plug wires into:⚫ COM — black lead, always🔴 VΩ — red lead (for volts/ohms/beep)
— OFF —
-----
👆
Tap a dial setting above.
Tap a setting
Each one will explain itself here.
7b. The 3 jobs you'll actually use
📏 Job 1 — Measure battery voltage (most common)
1
Dial to V⎓ DC volts. Pick a number bigger than you expect (20 V for a 9 V battery).
2
Black lead in COM. Red lead in VΩ.
3
Touch red to +, black to −. Polarity doesn't matter — you'll just get a minus sign if swapped.
4
Read the number. 9 V battery showing 7.2 V? It's flat. Showing 9.3 V? Good.
🌊 Job 2 — Measure current (how much flows)
⚠️
Current is the one that can blow a fuse. The meter must go IN the loop (in series) — you have to break a wire and put the meter in the gap. Start on the 10 A jack, not mA.
1
Move red lead to the 10 A hole (usually separate from VΩ).
2
Dial to A⎓ (DC amps).
3
Break the circuit at one point. Touch one probe to each broken end. The current now flows through the meter.
4
Read in amps (A) or milliamps (mA). 1000 mA = 1 A.
🔔 Job 3 — Continuity beep (find broken wires)
Most useful setting of all. It beeps if two points are connected by wire. Great for tracing traces on the radio board.
1
Dial to ))). Leads in COM + VΩ.
2
Touch probes to two points. Beep = same wire/connected. Silence = gap/broken.
3
Use it to find which copper trace leads where on your radio, or check if a fuse is blown.
7c. All the letters, decoded
Symbol
Name
Use it for
Red lead goes in
V⎓
DC voltage
Batteries, solar panel output, USB, inside radios/cars
VΩ
V∿
AC voltage
Wall sockets (dangerous!), mains transformers
VΩ
A⎓
DC current
Measuring how much a device draws
mA or 10A ⚠
A∿
AC current
Mains appliances (rare for hobby)
mA or 10A
Ω
Resistance
Reading resistor values, checking a part
VΩ
)))
Continuity
Finding breaks; beeps if connected
VΩ
⏩
Diode test
Check LEDs & diodes — shows forward voltage
VΩ
Hz
Frequency
How fast AC wobbles (mains = 50 Hz UK)
VΩ
F
Capacitance
Measure capacitors (only on nicer meters)
VΩ
°C/°F
Temperature
With a special probe
temp socket
⎓ = straight line = DC (batteries, solar). ∿ = wavy line = AC (wall sockets). That's the easiest way to remember.
7d. Numbers on the dial (200m, 2, 20, 200, 1000)
These are maximums the meter can read on that spot. Pick one bigger than what you expect.
Setting says
Means
Use for
200m (on A)
up to 0.2 A (200 mA)
Small LEDs
2000m / 2m
millivolts / milliamps — tiny
Sensor signals
20
up to 20 (V or A)
12 V car, 9 V battery ✅
200
up to 200
Mains voltage (AC)
1000 / 750
up to 1000 V
Big batteries / industrial
💡
If it reads "1" or "OL" on the left, the value is too big for that setting — turn the dial one notch higher. Auto-ranging meters skip this: they pick the range themselves.
7e. Use it on YOUR radio + solar panel
Check the solar panel: dial V⎓ (20 V). Red to +, black to −, in sunlight. Write down the number — that's what it actually produces.
Check the battery it replaces: same setting, measure the old battery. Now you know what voltage the radio expects.
Match them. Solar ≈ battery volts. If solar is way higher, you need a regulator or resistors before connecting.
Find + and −: any two wires, measure V⎓. If you see +5.2, red is on +. If −5.2, red is on −.
Trace a trace: continuity ())). Touch one probe to a pin, drag the other across pads until it beeps — that's where the pin goes.
Check a suspect resistor: Ω setting, touch both ends, read value. Compare to its coloured bands.
⚠️
Never turn the dial to A (amps) and touch a battery directly. That short-circuits through the meter and blows its fuse (or worse). Amps always go in series with a load.
8. Quick safety & tips
Unplug batteries before soldering or poking around.
Test LEDs with a resistor first — cheap to protect, costly to replace.
A multimeter (€10) tells you voltage & which way current flows — game changer.
Capacitors can hold a zap even after off — don't touch big ones with bare fingers.
When in doubt: less voltage first. You can always add more; you can't un-blow a chip.
Made to start simple. Re-open it any time you pull something apart. 🔧