Switches, diodes, LEDs, transistors and sensors are the building blocks of every electronic device. Learn how each one behaves, and how they combine into working alarm circuits.
A switch is a component controlled directly by the user to open or close a circuit. Different switch types are named by how many separate circuits ("poles") and positions ("throws") they control:
| Switch | Meaning | Typical use |
|---|---|---|
| Push button | Only makes contact while it is held down | Doorbells, keypads |
| SPST | Single-pole-single-throw โ one circuit, on/off only | Simple lamp switch |
| SPDT | Single-pole-double-throw โ one circuit, switches between two connections | Stairway light switch (two-way) |
| DPDT | Double-pole-double-throw โ two separate circuits switched together, each with two positions | Reversing a motor's direction |
A diode is a component that allows current to flow in one direction only. If it is connected the wrong way round, no current flows at all โ this protects circuits from a wrongly connected battery.
A LED (light-emitting diode) works the same way โ current can only flow through it in one direction โ but a LED also gives off light when current flows. Because of this, LEDs are very often used as a small, low-power indicator to show that a circuit is switched "ON".
At this level we only work with the NPN transistor. A transistor is a small semiconductor device that can do two important jobs:
This is why transistors are essential in sensor-based circuits: a sensor alone often cannot supply enough current to sound a buzzer directly, but it can supply enough to switch a transistor, which then drives the buzzer.
Sensors are input devices that change their electrical behaviour in response to something in the environment, so a circuit can react automatically.
| Sensor | How it behaves |
|---|---|
| LDR (light dependent resistor) | Resistance decreases as light increases: dark = high resistance, bright light = low resistance |
| +t thermistor | Resistance increases as temperature increases |
| -t thermistor | Resistance decreases as temperature increases |
| Touch / moisture detector | A "wet" finger bridges two contacts, completing the circuit and signalling a touch |
| Capacitor | Stores electrical energy, then releases it โ used for smoothing or timing |
CAPS Grade 9 requires you to be able to draw and assemble these standard circuits:
Choose a sensor, drag the slider, and watch its resistance change live. See whether the connected buzzer would switch on at the set threshold.