Safe Circuits and Power

Sessions

1. Session 1 · 75 min

   What is joined to what on a breadboard?

   The breadboard: its short rows, the gap down the middle and the long rails. The multimeter's continuity mode, which beeps when two points are joined.

   In the studio

   • Blink an LED

   Hands-on

   Nothing is powered: USB and the battery holder stay unplugged. Set the meter to continuity and touch the probe tips together: it beeps. Then test pairs of holes: two holes in the same short row; the same row on the other side of the gap; two holes along one rail; the two ends of a rail (some boards split the rail in the middle). Draw what you find. Then open Blink's “Build it for real” breadboard view and compare its strips with your map.

   Evidence to collect

   A breadboard map in the learner's own drawing, with at least six continuity results (beep or no beep) marked on it.

   Predict

   Before testing, mark on a drawing which holes you expect to be joined.

   Debug

   If the meter never beeps, touch the probe tips together first: no beep there means the meter's mode or a lead's socket, not the breadboard.

   Reflect

   Which result surprised you, and how will it change where you put parts?

   ⚠ Adult checks: Nothing is powered: the board stays unplugged. The meter is on continuity (never a current range) with the black lead in COM and the red lead in the V/Ω socket, never 10A.

   Works towards: Checking before power-on

2. Session 2 · 75 min

   What value is this resistor?

   The four-band colour code: two digits, a multiplier, then the tolerance band. Checking a value with the meter's resistance (Ω) range.

   In the studio

   No studio lab: this session is off-screen.

   Hands-on

   From a mixed bag of four-band 5 % resistors (220 Ω, 1 kΩ and 10 kΩ), hold each with the tolerance band (gold) on the right. Read the bands, write the value you expect, then measure it on the Ω range with the resistor on the table, touching nothing else. Work out 220 Ω yourself from the table: red (2), red (2), brown (×10).

   Evidence to collect

   A table for at least three resistors: the colours, the expected value, the measured value, and whether it is within the tolerance band's 5%.

   Predict

   Write the value you expect from the bands before you measure.

   Debug

   If a measured value is far from the bands, check which end you read from (the tolerance band goes on the right) and that the resistor touches nothing else.

   Reflect

   Which band is easiest to misread, and how will you check it next time?

   ⚠ Adult checks: Resistors are measured out of any circuit, as the meter needs; the meter is on Ω with the red lead in the V/Ω socket, never 10A. Nothing is powered; small parts stay in their tray.

   Works towards: Checking before power-on

3. Session 3 · 90 min

   Why does an LED need a resistor, and which way round does it go?

   Polarity: the LED's long leg (anode) and short leg (cathode). The series resistor that limits the current, and the UNO R3's 20 mA per I/O pin. Measuring a voltage across a part, in parallel with it.

   In the studio

   • Blink an LED

   Hands-on

   With USB unplugged, build Blink's circuit on the breadboard: D13 → 220 Ω → LED long leg; LED short leg → GND rail. Find the flat edge on the LED's rim. After the adult check, the adult plugs in USB and uploads Blink with long on-times (3 s on, 3 s off). On DC volts, measure across the LED, then across the resistor, at their breadboard rows while it is on. Work out the current from the resistor reading: volts ÷ 220 Ω gives amps, and amps × 1000 gives mA. For example, 3.0 V ÷ 220 Ω ≈ 0.0136 A = 13.6 mA, under the 20 mA per I/O pin.

   Starting code: Blink, from File/Examples/01.Basics/Blink, with both delay(1000) changed to delay(3000); the adult uploads it. On the UNO, its LED_BUILTIN is D13.

   Pins

   D13

   LED through its 220 Ω resistor

   Evidence to collect

   An annotated circuit drawing (anode, cathode, resistor value by its colours), two voltage readings and the current worked out from them, in A and in mA, compared with 20 mA.

   Predict

   Before power, predict which leg must face the resistor, and whether the LED or the resistor will show more of the 5 V.

   Debug

   If the LED stays dark, unplug USB and check in order: the long leg's direction, the resistor's row, the GND jumper, then the D13 jumper.

   Reflect

   Explain to a partner why the current you worked out is safe for the pin.

   ⚠ Adult checks before power-on: USB unplugged while wiring; the 220 Ω resistor is in series with the LED; the long leg faces the resistor; the adult uploads. Before anyone probes: the meter is on DC volts, red lead in the V/Ω socket, never 10A; probes go on breadboard rows or rails, not at the header pins.

   Works towards: Checking before power-on

4. Session 4 · 75 min

   Where does the power come from?

   Power from USB, or from a 6×AA battery holder through the barrel jack (centre-positive); the 5V, GND and VIN pins; the power path (supply to board to pin to part and back to GND). When to unplug.

   In the studio

   • Meet the board

   Hands-on

   In Meet the board, find the USB socket, the barrel jack, 5V, 3.3V, GND and VIN. With USB unplugged, run jumpers from the board's 5V and GND to the breadboard rails; then plug in USB and measure across the rails on DC volts. Next, the adult measures the 6×AA holder on DC volts at the holder's terminals, never inside the barrel plug; then, with USB unplugged, the adult plugs it into the barrel jack and switches it on, and the learner measures the rails again. Agree the class's unplug rule: before changing any wire, and at once if anything gets warm, smells or the power light goes out.

   Evidence to collect

   A power-path drawing of the LED circuit with power and signal labelled, the rail readings on USB and on the battery holder, and the unplug rule in the learner's own words.

   Predict

   Predict the rail reading on USB and then on the battery holder: will it change, and why?

   Debug

   If the power light stays off on the battery, the adult switches off, then checks the holder's switch, the cells' direction and the reading at its terminals.

   Reflect

   Draw the power path again from memory and mark where you would unplug in a hurry.

   ⚠ Adult checks before power-on: The holder reads within the board's 7–12 V at its own terminals before it goes in (below 7 V: recharge or replace the cells); only the adult plugs it in, with USB unplugged; VIN and the 5V pin are not used as power inputs; everything is unplugged (USB and battery) while anything is rewired. For each reading: red lead in the V/Ω socket, never 10A; probes go on breadboard rows or rails, not at the header pins.

   Works towards: Checking before power-on

5. Session 5 · 90 min

   Is it safe to power on, and why can't a motor run from a pin?

   A pre-power checklist the learner runs before every build. Why a motor needs a driver and its own supply: a pin's current limit, and power vs signal.

   In the studio

   • Control a motor (watched only)

   Hands-on

   Build the unit's prototype without code, with all power unplugged (USB and battery): board 5V → push button → 220 Ω → LED → GND, on the breadboard. Run the checklist before power: LED the right way round; resistor value read and measured; button in series (never straight across 5V and GND); and, before the board's 5V and GND jumpers go in, no beep on continuity between the 5V rail and the GND rail. Then fit the jumpers; after the adult check, plug in USB and test. Then, in Control a motor › Find the fault › “The motor will not run (1)”, look at a motor wired with no driver (watched only, never built) and say in your own words why a pin cannot run the motor.

   Evidence to collect

   The unit's evidence: checked wiring, shown by the learner's completed pre-power checklist for the LED-and-switch circuit, signed by the adult, and the learner's own explanation of why a motor needs a driver and a separate supply with a shared GND.

   Predict

   Before power, predict what the LED does with the button up and down, and what the meter shows across the rails.

   Debug

   Planted-fault hunt, with all power unplugged: the adult changes one thing (an LED turned round, or the wrong resistor) and the learner finds it with the checklist before power-on.

   Reflect

   Which check on your list would have caught the planted fault fastest, and why?

   ⚠ Adult checks before power-on: With all power unplugged (USB and battery), before the board's 5V and GND jumpers go in, continuity between the 5V and GND rails does not beep; the resistor is in series; the button is in series with the LED, never straight across 5V and GND. No motor is wired in this unit.

   Works towards: Checking before power-on