1. Light Dependent Resistor (LDR)

An LDR is a resistor whose resistance changes with light.

• In darkness, its resistance is high (like a narrow pipe → less current).

• In bright light, its resistance becomes low (wide pipe → more current).

🔬 How it works:Made of special semiconductors. Light energy gives electrons in the material enough energy to jump and move, so more current can flow.

📍 Used in:

• Automatic lights

• Solar garden lamps

• Light-sensitive alarms

  
  

🎚 2. Variable Resistor (Rheostat/Potentiometer)

A variable resistor lets you manually control how much resistance is in a circuit.

• It has a long resistive path (like graphite or coil).

• A wiper moves over it, changing how much of the path is used.

🔬 How it works:Move the knob or slider → changes the length of the path → changes resistance → this changes the current or voltage in part of the circuit.

📍 Used in:

• Volume knobs

• Fan speed controllers

• Dimming lights

  
  

🌡 3. Thermistors

A thermistor is a resistor whose resistance changes with temperature.

There are two main types:

🔻 a. NTC Thermistor (Negative Temperature Coefficient)

• As temperature increases, resistance decreases.

• Used when we want current to flow more as something gets hotter.

🔬 Example:In a temperature sensor — when it heats up, more current flows, and the circuit detects the heat.

🔺 b. PTC Thermistor (Positive Temperature Coefficient)

• As temperature increases, resistance increases.

• This is opposite of NTC.

• Used for self-regulation and overheating protection.

  
  

⚙️ 4. Relays

A relay is an electrically controlled switch.

• Inside is a coil (electromagnet) and a mechanical switch.

• When current flows through the coil, it becomes magnetic and pulls a metal arm to close or open another circuit.

🔬 Why useful:It lets a small signal (like from a sensor or microcontroller) control a large current (like for motors or AC power).

📍 Used in:

• Cars

• Home automation

• Industrial machines

  
  

🧠 5. Sensing Circuits

These are circuits that use sensors (like LDR, thermistor) to respond to changes in the environment.

• They detect light, temperature, sound, etc.

• Then they change current or voltage.

• This can trigger an alarm, turn on a light, or send data.

📍 Examples:

• Streetlight turning on at night

• Smoke detector sensing heat

• Smart fans increasing speed in heat

  
  

🔺 6. Diodes

A diode is a device that lets current flow in one direction only.

• Made of p-n junction (positive and negative semiconductor layers).

• Blocks reverse current.

• Only starts conducting after a small voltage (called forward voltage,).

🔬 Why useful:It acts like a one-way door for current. Good for protecting circuits, or converting AC to DC.

💡 7. LED (Light Emitting Diode)

An LED is a diode that gives off light when current flows through it.

• Made of special semiconductors.

• When electrons fall from high energy to low energy in the diode, they release energy as light.

🔬 Why it glows:Electron energy jumps → releases photons (light particles).Different materials = different colors of light.

📍 Used in:

• Bulbs

• Indicators

• TV screens

• Displays