Motors and Buzzers
Turning electricity into motion and sound.
The question
How does electricity make a fan spin or a smoke alarm beep?
What you’ll discover: how motors and buzzers turn electrical energy into motion and sound, and how to drive them safely.
In simple terms
BeginnerMagnets that push and pull
When current flows through a coil of wire, it becomes an electromagnet. Magnets push and pull on each other, and a motor uses that push and pull to make something spin. In a small DC motor, coils on the spinning part are repeatedly pushed away from and pulled toward fixed magnets around them, and a clever switch called a commutator keeps flipping the current so the push never stops.
Reverse the battery, and a simple DC motor spins the other way.
Buzzers: two kinds
- An active buzzer has its own tiny circuit inside. Connect it to the right voltage (+ to +) and it beeps by itself. It’s the easy choice for alarms.
- A passive buzzer (or piezo speaker) has no circuit inside. It only makes sound when you feed it a changing signal, but in return it can play different notes. Microcontrollers use these to play tunes (Stage 4).
Many small buzzers use a piezoelectric disc: a special crystal material that bends slightly when voltage is applied. Bending it back and forth thousands of times a second makes sound.
Safety with motors
Go deeper
IntermediateDriving motors properly
Motors need far more current than LEDs: even a small hobby motor can draw hundreds of milliamps, and much more when it first starts or if it’s stalled. That’s more than a 2N3904 transistor can handle (it’s rated for 200 mA maximum), so motors are switched with bigger transistors, such as a logic-level power MOSFET, or with a dedicated motor driver chip.
Like a relay, a motor is a coil, so it produces a voltage spike when switched off. A flyback diode (1N4001) across the motor, stripe toward +, protects the switching transistor (Lesson 9).
AdvancedBack EMF, H-bridges and noise
A spinning motor also acts as a generator, producing a voltage that opposes the supply, called back EMF. At full speed, back EMF limits the current; at standstill there’s none, which is why stall current and starting current are so much higher than running current.
To run a motor both ways under electronic control, designers use an H-bridge: four switches arranged so current can flow through the motor in either direction. Motor-driver chips contain H-bridges with built-in protection.
The brushes in small DC motors spark slightly, producing electrical noise that can upset nearby circuits and radios. A small ceramic capacitor (about 0.1 µF) soldered across the motor’s terminals reduces it, which matters to ham radio operators.
Try it
Connect a small hobby motor (rated for about 3 V) straight to a 2 × AA battery holder with a switch, with a piece of tape on the shaft as a flag so you can see it turn. Then switch off, swap the two motor wires, and switch on again.
Quick check
What makes a motor spin?
Electromagnets in the motor push and pull against permanent magnets.
What is the difference between an active and a passive buzzer?
An active buzzer has a built-in circuit and beeps on its own with steady voltage; a passive buzzer needs a changing signal, but can play different notes.
Why can’t a 2N3904 run most motors?
Motors draw hundreds of milliamps, more when starting or stalled, beyond the 2N3904’s 200 mA rating.
Words to know
- Electromagnet
- A coil that becomes a magnet when current flows.
- DC motor
- A motor that runs on direct current, reversing when the polarity is reversed.
- Piezoelectric
- A material that bends when voltage is applied, used in buzzers.
- Stall current
- The large current a motor draws when it’s powered but not turning.
Learn more
- Science World (Vancouver): Current Electricity: eight hands-on circuit activities
- VE6DOK.net: amateur radio, where electrical noise matters
- Back to Electronics School
Electronics School is part of the Discovery Zone. Every activity here runs on small batteries or a USB cable: never wall outlets. The one plug-in tool is the optional soldering iron, set up by an adult.