Resistor
Limits current or sets a voltage. In the speaker kit, a variable resistor adjusts the audio level; in the detector, an adjustment sets the operating point.
STEM CCA · Build / question / discover
Invisible forces.
Things you can build.
A speaker turns an electrical signal into moving air. A metal detector notices how nearby metal changes a coil’s behaviour. Explore the invisible steps inside both builds.
A model, not a wiring guide. Motion is greatly slowed and enlarged. The diagrams show relationships, not electron speeds, exact geometry or measured kit performance. All experiments are silent.
01
Bluetooth speaker · Motion
Why is there a magnet in a speaker?
The permanent magnet provides a steady magnetic field. Reversing current in the voice coil reverses the magnetic force on the coil. The attached cone moves with it and makes pressure changes in the air.
- 1Fixed magnet
- 2Moving voice coil
- 3Attached cone
- 4Vibrating air
The magnet does not pull directly on the paper cone. Force acts on the current-carrying coil; the coil is attached to the cone. Air particles vibrate locally as the sound disturbance travels onward.
Audio current · one repeating pattern
Change one thing. Watch what follows.
More amplitude → larger cone movement. Set it to zero: current and motion stop, but the magnet is still there.
More cycles per second → higher pitch. Changing pitch alone does not change the chosen amplitude.
Paused. Use Play or Step to follow the current.
Current crosses zero.
Predict → observe → explain
Keep pitch fixed and double the volume setting. What changes in the waveform? What stays the same? Then set volume to zero: has the permanent magnet disappeared?
A little deeper: field, force and timing
The coil sits in the magnet’s gap. Current in a magnetic field experiences a force; reversing the current reverses that force. Our arrows show an idealised cycle. Real cones have mass and suspension, so current, force and displacement need not peak together at every frequency.
02
Bluetooth speaker · Signal and energy
The signal gives instructions. The supply does the work.
An amplifier makes a small audio signal able to drive a speaker. It controls energy from the power supply. It does not create energy, and Bluetooth does not wirelessly power the speaker.
- 1PhoneDigital audio
- 2Bluetooth moduleReceive & decode
- 3AmplifierControl supply energy
- 4SpeakerCoil → cone → air
5 V supplyAmplifier + moduleElectrical energy → motion + sound + heat
Small input signal
Speaker drive
Dashed lines show the model’s supply limit. The waveform is a simplified audio signal; it does not show the amplifier’s internal circuitry.
Can a louder signal appear without power?
Raise gain to enlarge the signal. Push too far and the supply cannot support the requested output: the peaks flatten (clipping).
Powered: the output follows a larger version of the input.
Predict → observe → explain
Disconnect the supply. The phone may still contain music, but can this powered-off module and amplifier drive a speaker? Reconnect it and increase gain. Why can the output not grow forever?
Where does the energy go?
The supply provides the extra output energy. Some becomes sound, but much becomes heat in the electronics and coil. Gain changes signal size; it does not make a faster rhythm. The kit’s volume control adjusts audio level, while its separate display-sensitivity control changes the LED meter response.
03
Metal detector · Oscillator loss
A small change in the field. A clear alarm.
The CAI-175 kit uses printed coils and a transistor oscillator. Its sheet describes nearby metal causing extra loss: the oscillation weakens or stops, and the output stages switch on an LED and active buzzer.
- 1Printed coil
- 2Changing field
- 3Target material
Oscillation strength
Alarm offLED + active buzzer · silent model
Sensing oscillation · not the buzzer’s audible tone
Changing flux can induce circulating currents (eddy currents) in conductive metal. These currents dissipate energy and alter the coil’s field. This kit detects the resulting loss of oscillation; it is not a pitch-changing detector model.
Bring a target towards the coil.
NearFar
The model turns the alarm on below this oscillation strength. A higher threshold responds to a smaller loss, but is also easier to trigger by unwanted changes.
Metal is far away. Oscillation continues; the alarm is off.
Predict → observe → explain
Try plastic and metal at the same distance. Why can aluminium be detected even though a fridge magnet does not stick to it? Move metal slowly closer: what happens before the alarm changes?
What this model leaves out
The sheet identifies L1/L2 printed coils, Q1 S9018 and Q2/Q3 S9012 stages. The real adjustment sets an operating point near the oscillation threshold. Our single strength meter and alarm threshold simplify that behaviour; they are not calibrated distances or a circuit simulation. Metal type, size, orientation and magnetic properties also matter.
↳
Meet the team inside the circuit
Small parts. Different jobs.
Choose a part to see its role. A component’s job depends on where it sits in the circuit, so use the kit sheet for the actual connections.
Capacitor
Stores energy in an electric field. Depending on its position, it can smooth a supply, couple an audio signal or help a coil set an oscillation frequency.
Transistor
Uses a small input change to control a larger current. The detector uses a transistor oscillator and further transistor stages to control the alarm.
Coil
Current produces a magnetic field. A changing magnetic field can also induce a voltage. The detector uses printed copper coils; a speaker uses a moving voice coil.
Battery / power supply
Provides energy for the circuit. The speaker sheet specifies a 5 V supply; the detector sheet specifies 3–5 V. Follow your kit instructions for polarity and connections.
LED & active buzzer
An LED makes light when current flows in its forward direction. An active buzzer has its own sound-making oscillator: switching on its supply produces a tone.
Bluetooth & amplifier modules
The Bluetooth module receives and decodes audio data. The amplifier uses power from the supply to drive the speaker. The LED level-display circuit shows signal level.
Match these models to our kits
The speaker sheet is labelled TJ-56-527A: MH-M18 Bluetooth module, 4 Ω / 3 W speaker and KA2284 LED level display. The amplifier name differs between the parts list and schematic, so this page explains its role without assuming an exact chip. The detector sheet is labelled CAI-175. These models support the build; they do not replace either kit’s assembly instructions.
Keep exploring
Use the Multimeter Trainer to practise measurements, or visit the interactive resources.
Before you build
Use the kit’s specified low-voltage supply. Disconnect it before soldering or rewiring, avoid short circuits and check polarity. Solder with teacher supervision, eye protection and ventilation. Keep mains electricity outside this activity.
Sources & model notes
Kit-specific identification comes from the supplied TJ-56-527A and CAI-175 instruction sheets. The diagrams are original teaching models; the reference photos are not published.