Students start a motor whose driver has its own battery but no shared ground (nothing happens), join the grounds, separate signal, power and ground in a worked example, run the motor from the Uno's 5V and watch it reset, and fix a robot with both faults.
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Students will be able to
I can explain why separate supplies need a shared ground.
I can explain why a motor shouldn't run from the board's own 5V.
Timing
Predict and Try 5 min · Worked example 8 min · Vary 5 min · Tinkercad 8 min · Practise and Make 9 min
Before class
Model: without a common ground the driver doesn't respond (in reality the behaviour is unpredictable); a motor on the Uno's 5V resets the Uno when it starts (a brown-out). Some driver boards join the grounds for you on the board; still teach the wire.
Running the lesson
Predict: ask '5 V compared with what?'
Vary: the reset is the classic symptom students will meet on their own robots.
The hard part (on purpose)
'Separate supplies' sounds like 'separate circuits'. Draw the two halves and the one ground wire between them.
When students get stuck
What you will see
A question to ask
The fix
The motor never responds.
Is the Uno's GND joined to the battery's −?
Join the grounds.
Answers to the checks
separate supplies: The Uno is on USB; the driver and motor are on a battery. Pin 9 goes to the driver's ENA, but the Uno's GND isn't joined to the driver. When pin 9 goes HIGH, the motor… It doesn't respond reliably. The driver measures pin 9 against its own ground, which has no fixed relation to the Uno's, so the signal means nothing to it.
Start it: Start the motor. What happens?
Nothing: the driver can't read pin 9 without a shared ground.
Join them: Join the grounds.
With one shared 0 V, pin 9's 5 V means HIGH to the driver too.
Signal, power, ground: Which wires join the two halves of the circuit? The signal wires (pin to driver) and one ground wire Signals plus a common ground; the + sides stay separate.
Find the mistake: Sam's motor never responds. Click the line where Sam's reasoning first goes wrong. line 3 Line 3: separate supplies are fine, but the grounds must be joined so both sides agree on 0 V. Only the + sides stay apart.
From 5V: This motor runs from the Uno's 5V. Start it.
The motor's starting current dragged the Uno's 5V down, and the Uno reset: the code starts again from setup().
Its own battery: Give the motor its own battery (grounds joined) and start it again.
Separate power, shared ground: the motor runs and the Uno stays up.
Which wires?: A robot's Uno runs on a 9 V battery and its motors on a 6 V pack. Which wires should be joined? The two batteries' − terminals (the grounds) Join the grounds only.
Resets: A robot resets every time it starts to move. What is the most likely cause? The motors draw from the board's supply and pull it down Resets that happen when motors start are almost always a supply sagging.
fix the robot: This robot resets and its driver can't read the Uno. Fix both, then start the motor.
Its own supply for the motor, one shared ground.
Solutions
One possible solution for each task. Other correct programs are fine.
Vary: power from the Uno › From 5V
Open task: any program that does what the step asks.
Vary: power from the Uno › Its own battery
Open task: any program that does what the step asks.
fix the robot
Open task: any program that does what the step asks.
Stretch and support
Stretch: Some drivers take a separate logic supply (VCC1) and motor supply (VCC2). Which goes to which, on an L293D?
Support: A picture with signal wires in one colour, power in another, ground in black.
Exit ticket
Two supplies power a project. Which terminals do you join, and why?
The negative terminals (the grounds), so both sides measure signals from the same 0 V. Never the positives.
What each step is for
separate supplies: Committing to what happens with unjoined grounds tests the new move before the board shows it.
Try: join the grounds: Starting the motor before and after joining the grounds settles the prediction.
Worked example: what each supply does: Separating the two jobs (signal and power) and the one shared thing (ground) models the move. Sam's 'separate means unconnected' is planted after the method.
Vary: power from the Uno: Running the motor from the Uno's own 5V shows the other half of the rule: big currents pull the board's supply down and reset it.
Build it in Tinkercad: In Tinkercad, removing the ground wire between the Uno and the driver's battery shows the same failure in a fuller simulator.
grounds: Applying the rule to new systems.
fix the robot: Fixing both supply faults from one broken starting point brings the lesson together.