5  Electricity Applications

5.1 Learning Objectives

• Calculate LED current limiting resistor values using Ohm’s Law to prevent component damage in IoT circuits
• Design voltage dividers to interface 5V sensors with 3.3V microcontrollers using resistor ratio calculations
• Analyze power budget for battery-powered IoT devices by calculating duty cycles and average current consumption
• Build series and parallel resistor networks and measure voltage/current distribution using multimeters
• Select and configure passive components (resistors, capacitors, inductors) for real-world IoT applications including sensor pull-up resistors, signal filtering, and DC-DC converters

In 60 Seconds

Passive components (resistors, capacitors, inductors) shape circuit behavior without external power. Real-world IoT applications include LED current limiting, sensor pull-up resistors, and power budget calculations that determine battery life for deployed devices.

Key Concepts

• LED Current Limiting Resistor: Calculated as R = (Vsupply - Vf) / If; the LED forward voltage Vf is typically 1.8-2.0 V (red), 2.1-2.5 V (green/yellow), 3.0-3.5 V (blue/white); use If = 5-20 mA for standard indicator LEDs
• Voltage Divider for Sensor Interface: Two resistors in series produce an intermediate voltage: Vout = Vin x R2 / (R1 + R2); used to read resistive sensors (thermistors, LDRs) as voltage changes, and to drop 5 V sensor outputs to 3.3 V for MCU ADC inputs
• Pull-Up Resistor for Digital Inputs: A resistor from VCC to a digital input ensures the pin reads HIGH when no device is driving it; typical values: 4.7-10 kohm for I2C, 10-47 kohm for button inputs; prevents floating inputs from reading random values
• Battery Life Calculation: Runtime = Battery_capacity_mAh / Average_current_mA; for duty-cycled IoT devices: Iavg = Iactive x duty_fraction + Isleep x (1 - duty_fraction); essential for sizing batteries in wireless IoT sensor nodes
• Power Supply Decoupling: 100 nF ceramic capacitor in parallel with 10 uF electrolytic placed close to each IC’s power supply pins; ceramic absorbs high-frequency transients, electrolytic provides bulk charge for slow current demands; prevents supply noise from affecting circuit performance
• Fuse Protection: A fuse in series with the power supply that opens (breaks) when current exceeds its rated value; prevents fire and board damage from short circuits and overcurrent conditions; fuse current rating should be 20-50% above maximum normal operating current
• Circuit Protection Diode: A series diode protecting against reverse-polarity power supply connections; simple but effective for field-deployed devices where connectors may be accidentally reversed; adds one diode forward voltage drop (0.3-0.7 V) to the supply path
• Current Measurement with Shunt Resistor: A small value resistor (0.01-1 ohm) in series with the power supply enables current measurement via voltage sensing: I = V_shunt / R_shunt; allows monitoring of battery current without expensive current sensor ICs in low-cost IoT applications

For Beginners: Passive Components

Resistors, capacitors, and inductors are the three basic building blocks found in almost every electronic circuit. A resistor is like a narrow section of pipe that limits water flow (limits electrical current). A capacitor is like a small tank that stores and releases water (stores and releases electrical charge). An inductor is like a heavy flywheel that resists changes in flow. Together, they shape how electricity behaves in your IoT circuits.

⏱️ ~12 min | ⭐⭐ Intermediate | 📋 P06.C04.U09

Passive components don’t require external power and don’t generate power.

5.1.1 Comparison Table

Component	Property	Unit	Symbol	Function	IoT Application
Resistor	Resistance	Ohm (Ω)		Limits current	LED current limiting, pull-up/down
Capacitor	Capacitance	Farad (F)		Stores charge	Power smoothing, filtering
Inductor	Inductance	Henry (H)		Stores magnetic energy	DC-DC converters, RF circuits

5.1.2 Capacitors

Function: Store electrical energy as electric charge (like tiny, fast-charging batteries)

Typical Values:

• µF (microfarad): 10^-6 F - Power supply decoupling
• nF (nanofarad): 10^-9 F - Signal filtering
• pF (picofarad): 10^-12 F - High-frequency circuits

IoT Applications:

• Smoothing power supply voltage for microcontrollers
• Filtering noise from sensor signals
• Energy storage for low-power devices

5.1.3 Inductors

Function: Store energy as magnetic fields when current flows

Typical Values:

• mH (millihenry): 10^-3 H - Power inductors
• µH (microhenry): 10^-6 H - RF circuits

IoT Applications:

• DC-DC converters (boost/buck regulators)
• EMI filtering
• Wireless charging coils

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5.2 How It Works: Voltage Dividers in IoT Sensor Interfaces

A voltage divider is one of the most fundamental circuits in IoT systems, allowing you to interface sensors operating at different voltages with microcontrollers. When you connect two resistors in series between a voltage source and ground, the voltage divides proportionally based on the resistor values.

Step 1: Identify the Basic Principle

Consider a 5V sensor connected to a 3.3V microcontroller. Connecting them directly would damage the microcontroller’s input pin. A voltage divider solves this:

5V ──┬── R1 (1kΩ) ──┬── V_out (to MCU pin) ──┬── R2 (2.2kΩ) ── GND

Step 2: Calculate the Output Voltage

The voltage divider equation determines V_out:

\[V_{out} = V_{in} \times \frac{R2}{R1 + R2}\]

For our example: \[V_{out} = 5V \times \frac{2.2k\Omega}{1k\Omega + 2.2k\Omega} = 5V \times \frac{2.2}{3.2} = 3.44V\]

This is still slightly high for a 3.3V MCU (exceeds absolute maximum rating). Adjust R1 to 1.5kΩ:

\[V_{out} = 5V \times \frac{2.2k\Omega}{1.5k\Omega + 2.2k\Omega} = 5V \times \frac{2.2}{3.7} = 2.97V\]

Now the signal is safely within the 0-3.3V range.

Step 3: Consider Current Draw

Voltage dividers constantly draw current (wasted power). Total resistance should be high enough to minimize current:

\[I_{divider} = \frac{V_{in}}{R1 + R2} = \frac{5V}{3.7k\Omega} = 1.35 \text{ mA}\]

For battery-powered devices, increase resistor values proportionally (15kΩ + 22kΩ) to reduce current to 135 µA while maintaining the same voltage ratio.

Step 4: Account for Loading Effects

The MCU input pin has input impedance (typically >100kΩ for GPIO, ~1MΩ for ADC). If the divider resistance is too high, the input impedance creates a parallel path that affects the output voltage. General rule: keep total divider resistance below 1/10 of the input impedance.

Real-World Application Example

The DHT22 temperature/humidity sensor outputs 5V logic levels. To interface with an ESP32 (3.3V GPIO): - Use 1.8kΩ (R1) and 3.3kΩ (R2) - V_out = 5V × (3.3 / 5.1) = 3.24V (safe for ESP32) - Current draw = 5V / 5.1kΩ = 0.98 mA - Power loss = 5V × 0.98mA = 4.9 mW (acceptable for continuous operation)

This simple two-resistor circuit protects expensive microcontrollers from overvoltage damage in mixed-voltage IoT systems.

Putting Numbers to It: Try Different Voltage Divider Values

viewof vdiv_vin = Inputs.range([1, 24], {value: 5, step: 0.1, label: "Input voltage (V)"})
viewof vdiv_r1 = Inputs.range([100, 100000], {value: 1800, step: 100, label: "R1 (Ω)"})
viewof vdiv_r2 = Inputs.range([100, 100000], {value: 3300, step: 100, label: "R2 (Ω)"})

vdiv_vout = vdiv_vin * vdiv_r2 / (vdiv_r1 + vdiv_r2)
vdiv_current = vdiv_vin / (vdiv_r1 + vdiv_r2) * 1000
vdiv_power = vdiv_vin * vdiv_current
vdiv_safe = vdiv_vout <= 3.3

html`<div style="background: var(--bs-light, #f8f9fa); padding: 1rem; border-radius: 8px; border: 1px solid var(--bs-border-color, #dee2e6); font-family: inherit;">
<p><strong>Output voltage:</strong> ${vdiv_vout.toFixed(2)} V ${vdiv_safe ? "(safe for 3.3V MCU)" : "(WARNING: exceeds 3.3V!)"}</p>
<p><strong>Divider current:</strong> ${vdiv_current.toFixed(2)} mA</p>
<p><strong>Power dissipation:</strong> ${vdiv_power.toFixed(2)} mW</p>
<p style="color: var(--bs-secondary, #6c757d); font-size: 0.9em;"><em>For battery operation, scale both resistors up by 10× (${(vdiv_r1/100).toFixed(0)}kΩ + ${(vdiv_r2/100).toFixed(0)}kΩ) to reduce current to ${(vdiv_current/10).toFixed(2)} mA.</em></p>
</div>`

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5.3 Real-World Applications in IoT

⏱️ ~15 min | ⭐⭐ Intermediate | 📋 P06.C04.U10

5.3.1 Application 1: Fan Speed Control

Concept: Adjust resistance to control motor current

Figure 5.1: Fan speed control circuit showing variable resistor controlling current flow to motor, demonstrating speed regulation through resistance adjustment

5.3.2 Application 2: LED Current Limiting

Problem: LEDs will burn out if connected directly to power supply

Solution: Add a resistor to limit current

Example Calculation:

• Power supply: 5V
• LED forward voltage: 2V (typical red LED)
• Desired current: 20mA (0.02A)

\[R = \frac{V_{supply} - V_{LED}}{I} = \frac{5V - 2V}{0.02A} = 150Ω\]

Use a 150Ω or 220Ω resistor (standard value)

5.3.3 Application 3: Sensor Pull-up Resistors

Why needed: Many IoT sensors have open-drain outputs that need pull-up resistors

Typical values: 4.7kΩ or 10kΩ

Application: I2C communication (covered in Chapter 4)

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5.4 Hands-On Labs

⏱️ ~30 min | ⭐⭐⭐ Advanced | 📋 P06.C04.U11

5.4.1 Lab 1: Build Your First LED Circuit

Objective: Build a basic LED circuit with current limiting resistor.

Materials Needed (or use TinkerCAD simulator):

• 1× LED (any color)
• 1× 220Ω resistor
• 1× Push button switch
• 1× 9V battery (or 5V USB power)
• Breadboard and jumper wires

Circuit Diagram:

Figure 5.2: Basic LED circuit breadboard diagram showing battery, resistor, LED, and button switch connections for hands-on learning

Instructions:

1. Connect resistor to positive battery terminal
2. Connect LED positive (long leg) to other end of resistor
3. Connect LED negative (short leg) to one terminal of button
4. Connect other button terminal to battery negative
5. Press button → LED lights up!

Measurements to Record:

• Measure voltage across the LED using multimeter
• Measure current through the circuit
• Calculate and verify resistance using Ohm’s Law

Expected Learning:

• Current only flows when circuit is complete
• Resistor limits current to safe level for LED
• Practice reading circuit diagrams
• Verify Ohm’s Law with real measurements

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5.4.2 Lab 2: Voltage Divider for Sensor Interfacing

Objective: Create a voltage divider to scale down 5V to 3.3V for microcontroller ADC input.

Materials Needed:

• 1× 1kΩ resistor (R1)
• 1× 2kΩ resistor (R2)
• 1× 5V power supply (or USB)
• Multimeter
• Breadboard and jumper wires

Circuit Diagram:

Figure 5.3: Voltage divider circuit diagram showing two resistors in series dividing 5V input to 3

Instructions:

1. Connect R1 (1kΩ) from 5V to middle point
2. Connect R2 (2kΩ) from middle point to ground
3. Measure output voltage at middle point
4. Calculate expected voltage: \(V_{out} = 5V \times \frac{2kΩ}{1kΩ + 2kΩ} = 3.33V\)
5. Compare calculated vs measured values

Expected Learning:

• Voltage dividers reduce voltage proportionally
• Essential for interfacing 5V sensors with 3.3V microcontrollers
• Predict output voltage from resistor ratio values

Extension:

• Try different resistor values (4.7kΩ and 10kΩ)
• Calculate current through the divider
• Determine power dissipation

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5.4.3 Lab 3: Power Budget Analysis for IoT Device

Objective: Calculate total power consumption and estimate battery life for a battery-powered IoT sensor.

Scenario: Environmental sensor with these components: - ESP32 microcontroller: 160mA active, 10µA deep sleep - DHT22 sensor: 1.5mA when reading - LoRa radio: 120mA transmit, 15mA receive, 1µA sleep - Status LED: 20mA when on

Operating cycle (every 5 minutes):

1. Wake from sleep (0.1s)
2. Read sensor (2s)
3. Transmit data (0.5s)
4. Return to sleep (297.4s)

Instructions:

1. Calculate duty cycles:
   • Active time: 2.6s per 300s = 0.87%
   • Sleep time: 297.4s per 300s = 99.13%
2. Calculate average current for each component:
   • ESP32: (160mA × 0.0087) + (0.01mA × 0.9913) = 1.40mA
   • DHT22: 1.5mA × (2/300) = 0.01mA
   • LoRa: (120mA × 0.5/300) + (0.001mA × 299.5/300) = 0.20mA
   • LED: 20mA × (0.5/300) = 0.03mA
   • Total average: 1.64mA
3. Calculate battery life:
   • Battery: 2000mAh Li-ion (3.7V)
   • Battery life: 2000mAh / 1.64mA ≈ 1220 hours ≈ 51 days
4. Use Python PowerBudget calculator to verify

Expected Learning:

• Duty cycle dramatically affects battery life
• Sleep modes are critical for IoT devices
• Power budget analysis guides component selection

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5.4.4 🧪 Interactive Lab: Series vs Parallel Resistor Networks

🎮 Try It Yourself: Explore Series and Parallel Circuits

What you’ll do: Build and test resistor networks in series and parallel configurations, measuring voltage, current, and resistance.

What you’ll learn:

• How resistors combine in series vs parallel
• How voltage and current distribute in each configuration
• When to use series vs parallel in real IoT circuits

Estimated time: 12 minutes

🎯 Interactive Challenges:

Try these experiments:

1. Series Resistance Challenge: Calculate the total resistance of the 3 series resistors (each 1kΩ). Then measure it with the multimeter.
   💡 Hint

   For series: R_total = R1 + R2 + R3 = 1kΩ + 1kΩ + 1kΩ = 3kΩ. Resistances simply add up in series.
2. Parallel Resistance Challenge: Calculate the total resistance of the 3 parallel resistors (each 1kΩ). Is it higher or lower than one resistor?
   💡 Hint

   For equal resistors in parallel: R_total = R / n = 1kΩ / 3 ≈ 333Ω. Parallel resistance is always LOWER than any individual resistor.
3. Current Distribution Challenge: In the series circuit, measure current at different points. In parallel, measure current through each branch. What pattern do you see?
   💡 Hint

   Series: Current is the SAME everywhere (Kirchhoff’s Current Law). Parallel: Current DIVIDES among branches, but sum equals total current.
4. Voltage Division Challenge: Measure voltage across each resistor in series. Then in parallel. What’s the difference?
   💡 Hint

   Series: Voltage divides proportionally (5V/3 ≈ 1.67V each). Parallel: Voltage is the SAME across all resistors (5V each).

Part A: Series Configuration

Figure 5.4: Series resistor circuit showing three 1kΩ resistors connected end-to-end with total resistance of 3kΩ and current of 1

Instructions:

1. Connect three 1kΩ resistors in series
2. Measure total resistance: Expected = 3kΩ
3. Calculate current: I = 5V / 3kΩ = 1.67mA
4. Measure voltage across each resistor (should be ~1.67V each)

Part B: Parallel Configuration

Figure 5.5: Parallel resistor circuit showing three 1kΩ resistors connected side-by-side with total resistance of 333Ω and total current of 15mA

Instructions:

1. Connect three 1kΩ resistors in parallel
2. Measure total resistance: Expected = 333Ω (1kΩ/3)
3. Calculate total current: I = 5V / 333Ω = 15mA
4. Measure current through each resistor (should be ~5mA each)

Comparison Table:

Configuration	Total Resistance	Total Current	Voltage per Resistor	Current per Resistor
Series	3kΩ	1.67mA	1.67V	1.67mA
Parallel	333Ω	15mA	5V	5mA

Expected Learning:

• Series: Resistance adds, current stays same
• Parallel: Resistance decreases, current divides
• Determine when to use each configuration in IoT circuits

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5.5 Kirchhoff’s Laws: Analyzing Real IoT Circuits

When IoT circuits get more complex than a single loop – for example, a sensor node with an MCU, LED indicator, and pull-up resistor all sharing a power supply – you need Kirchhoff’s Laws to predict voltage and current at every point.

5.5.1 Kirchhoff’s Current Law (KCL)

The Rule: Total current entering a node equals total current leaving that node. Current cannot appear or disappear – it must flow somewhere.

Why IoT Engineers Care: KCL tells you how much current your battery must supply when multiple components share a power rail.

5.5.2 Kirchhoff’s Voltage Law (KVL)

The Rule: The sum of all voltages around any closed loop in a circuit equals zero. Every volt the battery provides must be “used up” by components in the loop.

Why IoT Engineers Care: KVL tells you the voltage at every point in the circuit, which is critical for ensuring sensors and MCUs receive the correct operating voltage.

5.5.3 Worked Example: ESP32 Sensor Node Power Circuit

Scenario: You are designing a battery-powered air quality sensor node with these components on a shared 3.3V rail:

• ESP32-C3 microcontroller: draws 80 mA when active
• BME280 environmental sensor (I2C with 4.7 kOhm pull-ups): draws 1 mA
• Status LED (green, V_forward = 2.1V) with current-limiting resistor: target 10 mA
• MQ-135 gas sensor heater: draws 150 mA from a separate 5V rail

The power comes from a 3.7V LiPo battery through a 3.3V LDO regulator (AMS1117-3.3).

Step 1: Apply KCL at the 3.3V Power Node

All components connect to the 3.3V rail. By KCL, the total current the regulator must supply equals the sum of all branch currents:

\[I_{total} = I_{ESP32} + I_{BME280} + I_{LED} + I_{pullups}\]

Calculate the pull-up resistor current. Two 4.7 kOhm pull-ups (SDA and SCL) connect from 3.3V to the I2C lines. When the line is pulled low (logic 0):

\[I_{pullup} = \frac{V}{R} = \frac{3.3V}{4.7\text{k}\Omega} = 0.70 \text{ mA per line}\]

Both lines pulled low simultaneously (worst case):

\[I_{pullups} = 2 \times 0.70 = 1.4 \text{ mA}\]

Total 3.3V rail current:

\[I_{total} = 80 + 1 + 10 + 1.4 = 92.4 \text{ mA}\]

Step 2: Apply KVL to the LED Loop

Trace a loop from the 3.3V rail through the current-limiting resistor, through the LED, to ground:

\[V_{supply} - V_{resistor} - V_{LED} = 0\]

\[3.3V - V_{resistor} - 2.1V = 0\]

\[V_{resistor} = 3.3V - 2.1V = 1.2V\]

Now find the resistor value for 10 mA:

\[R = \frac{V_{resistor}}{I_{LED}} = \frac{1.2V}{0.01A} = 120\Omega\]

Use a standard 120 Ohm resistor. Verify power dissipation:

\[P_{resistor} = I^2 \times R = (0.01)^2 \times 120 = 0.012 \text{ W}\]

A standard 1/8 W (0.125 W) resistor is sufficient.

Step 3: Apply KVL to Check Regulator Headroom

The LDO regulator needs a minimum dropout voltage of 1.0V (AMS1117 datasheet). Apply KVL to the battery-regulator loop:

\[V_{battery} - V_{dropout} - V_{output} = 0\]

\[V_{battery(min)} = V_{dropout} + V_{output} = 1.0V + 3.3V = 4.3V\]

But a LiPo battery drops to 3.0V when discharged. At 3.5V battery:

\[V_{available} = 3.5V - 3.3V = 0.2V < 1.0V \text{ dropout}\]

Problem found by KVL: The regulator cannot maintain 3.3V output when the battery drops below 4.3V. A LiPo spends most of its discharge curve between 3.5-3.8V, so the LDO will drop out frequently.

Solution: Replace the LDO with a buck-boost regulator (e.g., TPS63000) that can maintain 3.3V output from inputs as low as 2.5V.

Step 4: Total Power Budget

Component	Voltage	Current	Power
ESP32-C3 (active)	3.3V	80 mA	264 mW
BME280 sensor	3.3V	1 mA	3.3 mW
Status LED	3.3V	10 mA	33 mW
I2C pull-ups	3.3V	1.4 mA	4.6 mW
MQ-135 heater	5.0V	150 mA	750 mW
Total		242.4 mA	1,055 mW

With a 2000 mAh LiPo at 3.7V (7.4 Wh), battery life during continuous active mode:

\[\text{Battery life} = \frac{7.4 \text{ Wh}}{1.055 \text{ W}} \approx 7 \text{ hours}\]

This confirms the MQ-135 heater dominates power consumption (71%). Duty-cycling it (heat 30s, read, sleep 270s) would reduce average heater current to 15 mA and extend battery life to approximately 25 hours.

Key Takeaway from Kirchhoff’s Laws

KCL answered: “Can my regulator handle the total current?” (92.4 mA – yes, AMS1117 supports up to 1A).

KVL answered: “Does my LED get the right current?” (yes, 120 Ohm resistor) AND revealed a hidden problem: “Will the LDO maintain output as the battery discharges?” (no – must switch to buck-boost).

These two laws catch problems that Ohm’s Law alone cannot: KCL catches overcurrent on shared rails, and KVL catches voltage headroom failures in multi-component loops.

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5.6 Quiz 3

Test your understanding of electricity fundamentals with these questions.

Question 1: Ohm’s Law Basic

A circuit has 12V voltage and 4Ω resistance. What is the current?

A) 3A B) 48A C) 8A D) 16A

Show Answer

Answer: A) 3A

\[I = \frac{V}{R} = \frac{12V}{4Ω} = 3A\]

Question 2: Power Calculation

An IoT device runs on 5V and draws 100mA (0.1A). How much power does it consume?

A) 0.5W B) 50W C) 5W D) 0.05W

Show Answer

Answer: A) 0.5W

\[P = V \times I = 5V \times 0.1A = 0.5W\]

Question 3: Resistor Selection for LED

You need to limit current to 20mA for a 2V LED powered by a 5V supply. What resistor value?

A) 100Ω B) 150Ω C) 220Ω D) 330Ω

Show Answer

Answer: B) 150Ω

\[R = \frac{V_{supply} - V_{LED}}{I} = \frac{5V - 2V}{0.02A} = 150Ω\]

In practice, use 150Ω or the nearest standard value (220Ω).

Question 4: Series Resistors

Three resistors (100Ω, 220Ω, 330Ω) are connected in series. What is the total resistance?

A) 217Ω B) 550Ω C) 650Ω D) 54Ω

Show Answer

Answer: C) 650Ω

For series resistors: \(R_{total} = R_1 + R_2 + R_3 = 100Ω + 220Ω + 330Ω = 650Ω\)

Question 5: Parallel Resistors

Two 1kΩ resistors are connected in parallel. What is the total resistance?

A) 2kΩ B) 1kΩ C) 500Ω D) 250Ω

Show Answer

Answer: C) 500Ω

For parallel resistors: \(\frac{1}{R_{total}} = \frac{1}{R_1} + \frac{1}{R_2} = \frac{1}{1kΩ} + \frac{1}{1kΩ} = \frac{2}{1kΩ}\)

Therefore: \(R_{total} = \frac{1kΩ}{2} = 500Ω\)

For identical resistors in parallel: \(R_{total} = \frac{R}{n}\) where n is the number of resistors.

Question 6: Voltage Divider

A voltage divider uses 1kΩ (R1) and 3kΩ (R2) resistors to divide a 12V input. What is the output voltage across R2?

A) 3V B) 4V C) 9V D) 12V

Show Answer

Answer: C) 9V

\[V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2} = 12V \times \frac{3kΩ}{1kΩ + 3kΩ} = 12V \times \frac{3}{4} = 9V\]

Question 7: Battery Life Calculation

An IoT sensor draws 2mA continuously from a 1000mAh battery. How long will the battery last?

A) 50 hours B) 200 hours C) 500 hours D) 2000 hours

Show Answer

Answer: C) 500 hours

\[Battery\ Life = \frac{Battery\ Capacity}{Current\ Draw} = \frac{1000mAh}{2mA} = 500\ hours\]

This equals approximately 21 days.

Question 8: Power Dissipation in Resistor

A 220Ω resistor carries 50mA (0.05A) current. How much power does it dissipate?

A) 0.055W B) 0.55W C) 5.5W D) 11W

Show Answer

Answer: B) 0.55W

\[P = I^2 \times R = (0.05A)^2 \times 220Ω = 0.0025 \times 220 = 0.55W\]

This requires at least a 1W rated resistor for safe operation (with safety margin).

Question 9: Current Flow Direction

In conventional current flow notation used in circuit analysis, current flows:

A) From negative to positive B) From positive to negative C) In both directions simultaneously D) Only in AC circuits

Show Answer

Answer: B) From positive to negative

By convention, current flows from positive (+) to negative (-), even though electrons physically flow from negative to positive. This historical convention is used in all circuit analysis and design.

Question 10: Component Power Rating

You need to select a resistor that will dissipate 0.3W of power. Which power rating should you choose for safe operation with a 2× safety margin?

A) 1/8 W (0.125W) B) 1/4 W (0.25W) C) 1/2 W (0.5W) D) 1 W

Show Answer

Answer: D) 1 W

With 2× safety margin: \(0.3W \times 2 = 0.6W\)

The next standard power rating above 0.6W is 1W.

Safety margins prevent overheating and ensure long-term reliability. Common practice is to use components rated for at least 2× the expected power dissipation.

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Key Takeaway

Passive components (resistors, capacitors, inductors) shape every IoT circuit: resistors limit current and create voltage dividers, capacitors filter noise and store energy, and inductors enable efficient power conversion. The power budget calculation (average current = sum of duty-cycle-weighted component currents) is the single most important analysis for battery-powered IoT deployments.

For Kids: Meet the Sensor Squad!

Sammy the Sensor and friends learn about building real circuits!

Sammy the Sensor was excited – today was Build Day! The Sensor Squad was going to build their very first LED circuit. “First, we need a resistor,” said Max the Microcontroller, holding up a tiny striped cylinder. “Without it, Lila could get hurt!”

Lila the LED looked worried. “Hurt? How?” Max explained: “If too much electricity flows through you, you’ll burn out – like eating too much candy too fast gives you a tummy ache. The resistor is like a speed bump that slows the electricity down to a safe amount.”

Bella the Battery was ready to power everything. “I have 9 volts of energy!” she announced proudly. Max did some quick math: “Lila needs about 2 volts, so the resistor needs to handle 7 volts. Using Ohm’s Law – that’s V divided by I – we need a 220 ohm resistor to keep the current at a safe 20 milliamps.”

They connected everything on the breadboard: Bella to the resistor, the resistor to Lila, and Lila back to Bella through a button switch. When Sammy pressed the button – POP – Lila lit up bright red! “I’m glowing!” Lila cheered. “And I feel perfectly safe thanks to that resistor!”

“That’s the magic of circuits,” said Max. “Every part has a job: Bella provides energy, the resistor keeps everyone safe, and the button gives us control. That’s how EVERY LED in your toys, phones, and smart home gadgets works!”

5.6.1 Key Words for Kids

Word	What It Means
Resistor	A part that slows down electricity, like a speed bump for electrons
Capacitor	A tiny bucket that stores electricity and releases it quickly
LED	A special light that runs on very little electricity
Breadboard	A board with holes where you plug in parts to build circuits
Voltage Divider	Two resistors that share voltage, like splitting a pizza between friends
Battery Life	How long a battery lasts before it runs out of energy

5.7 Concept Relationships: How Electrical Concepts Connect

Base Concept	Builds To	Requires Understanding Of	Applied In
Voltage (V)	Current flow, Power	Ohm’s Law	Voltage dividers, Battery selection
Current (I)	Power consumption, Heat	Ohm’s Law, KCL	Power budgets, Component ratings
Resistance (R)	Voltage drop, Power loss	Ohm’s Law	Pull-up resistors, Current limiting
Power (P = V × I)	Battery life, Heat dissipation	Voltage, Current	Component selection, Thermal management
Ohm’s Law (V=IR)	All circuit analysis	Voltage, Current, Resistance	Every IoT circuit design
Series Circuits	Voltage dividers	Ohm’s Law, KVL	Sensor level shifters, Multi-cell batteries
Parallel Circuits	Current distribution	Ohm’s Law, KCL	Redundant sensors, LED arrays
Capacitors	Filtering, Energy storage	Charge, Voltage	Power supply decoupling, Signal conditioning
Inductors	DC-DC converters	Magnetic fields, Energy	Buck/boost regulators, RF circuits
KVL	Loop analysis	Series circuits, Voltage	Power rail debugging, LED current limiting
KCL	Node analysis	Parallel circuits, Current	I2C pull-ups, Multi-load power budgets
Power Budgets	Battery life calculation	Power, Current, Duty cycle	IoT deployment planning

Key Insight: Every IoT electrical design decision starts with Ohm’s Law (V=IR) and branches into power analysis (P=VI), circuit topology (series/parallel), and component selection (passive elements). Kirchhoff’s Laws (KVL and KCL) tie these concepts together for complete circuit analysis.

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Chapter Summary

Electricity is the foundation of all IoT systems, driven by the flow of electrons through conductors. Understanding the relationship between voltage (electrical “pressure”), current (flow rate), and resistance (opposition to flow) through Ohm’s Law (V = I × R) is essential for designing and troubleshooting IoT circuits. Power consumption (P = V × I) determines battery life and energy requirements for IoT deployments.

Circuit configurations significantly impact system behavior: series circuits share current but divide voltage, while parallel circuits share voltage but divide current. Series configurations are useful for voltage division and cumulative resistance, while parallel configurations provide redundancy and current distribution. Understanding these principles enables proper component selection and circuit design.

Passive components shape circuit behavior without requiring external power: resistors limit current and create voltage dividers, capacitors store energy and filter signals, and inductors resist changes in current. Real-world circuits combine these components to condition sensor signals, filter noise, store energy, and protect sensitive electronics.

Practical application of electrical principles includes power budget calculations (ensuring battery capacity meets device requirements), voltage regulation (maintaining stable supply despite load variations), and component selection (choosing resistor wattage, capacitor voltage ratings). Safety margins (typically 2×) prevent component failure and ensure long-term reliability in IoT deployments.

5.8 Practice: Match and Order

5.9 See Also

Within This Module:

• Electricity Introduction - Foundational concepts of voltage, current, and resistance
• Ohm’s Law Deep Dive - Advanced worked examples for IoT applications including heaters, motors, and complete power budgets
• Common Electricity Pitfalls - Unit confusion, voltage drops, power limits, and AC vs DC mistakes

Related Topics:

• Electronics: Doping & Diodes - Active semiconductor devices built on electrical principles
• Electronics: Transistor Selection - Choosing BJT vs MOSFET for IoT switching and amplification
• Power Management - System-level power architecture and battery optimization
• Analog-to-Digital Conversion - How electrical signals become digital data
• Sensor Interfacing - Electrical considerations for connecting sensors to microcontrollers

External Resources:

• All About Circuits - Ohm’s Law - Interactive tutorials with circuit simulators
• SparkFun Learn - Voltage, Current, Resistance - Beginner-friendly explanations with visual aids
• TI Power Management Fundamentals - Industry reference for power circuit design

Common Pitfalls

1. LED Resistor Calculated for Wrong Operating Current

Many beginners size LED resistors for the maximum rated current (20 mA) because it is listed prominently in the datasheet. At 20 mA, LEDs are near their thermal limits and consume unnecessary power. Use 5-10 mA for indicator LEDs — they are still clearly visible and last orders of magnitude longer. Size the resistor for your chosen If, not the maximum rated If.

2. Voltage Divider Loading Error

When a load (ADC input, voltmeter, next stage circuit) is connected to the voltage divider output, it appears in parallel with R2 and changes the voltage. If the load resistance is comparable to R2, the output voltage drops significantly below the calculated value. Use an operational amplifier buffer if the load impedance is too low to maintain divider accuracy.

3. Battery Rated Capacity vs. Usable Capacity

Battery rated capacity (mAh on the label) assumes discharge to a cutoff voltage (typically 2.5-3.0 V for Li-ion, 1.0 V for alkaline). Devices with brownout reset voltages of 2.8-3.0 V stop working before the battery is fully discharged. Usable capacity is typically 80-90% of rated capacity. Apply this derating factor to battery life calculations for realistic estimates.

4. Forgetting that Reverse Protection Diodes Drop Supply Voltage

A series protection diode drops 0.3-0.7 V off the supply voltage. A 3.3 V system with a diode sees 2.6-3.0 V at the circuit. For 3.3 V logic that requires VCC > 2.7 V minimum, this can bring the supply dangerously close to the minimum operating voltage. Account for diode drops in power supply headroom calculations, or use a P-channel MOSFET for near-zero-drop reverse protection.

Label the Diagram

5.10 What’s Next

Next Chapter	Why It Matters
Electronics: Doping & Diodes	Progress from passive components to active semiconductor devices — learn how doping creates diodes and transistors that enable amplification, switching, and complex IoT functionality
Ohm’s Law Deep Dive	Return to the foundational equations with advanced worked examples for IoT applications including heaters, motors, LED calculations, I2C pull-ups, and complete power budget analysis
Common Electricity Pitfalls	Avoid the most common mistakes in IoT circuit design: unit confusion, voltage drops, power limits, and AC vs DC errors

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