525  Sensor Circuit Fundamentals

Prerequisites: Electricity Fundamentals | Electronics Fundamentals

This enables: Signal Conditioning | Sensor Interfacing

525.1 Learning Objectives

By the end of this chapter, you will be able to:

• Design Voltage Dividers: Calculate resistor values for interfacing resistive sensors with ADCs
• Build RC Filters: Create low-pass filters to remove noise from sensor signals
• Implement Transistor Switches: Use transistors to control high-power loads from GPIO pins
• Calculate LED Resistors: Properly size current-limiting resistors for LED indicators
• Debug Basic Circuits: Use multimeters to diagnose common sensor circuit issues

525.2 Introduction

Sensor circuits form the foundation of IoT systems, converting physical measurements into electrical signals that microcontrollers can process. This chapter covers the essential circuit building blocks you need to interface sensors with microcontrollers like the ESP32.

TipWhat Are Sensor Circuits? (Simple Explanation)

Analogy: Think of sensor circuits like translating between languages.

Imagine you speak English, but your friend only speaks French. You need: 1. A translator (to convert the language) 2. Volume adjustment (if they whisper too quietly) 3. Noise filtering (to ignore background chatter)

Sensor circuits work the same way—they convert sensor “language” (analog voltages) into “microcontroller language” (digital numbers)!

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flowchart LR
    A[Sensor<br/>Analog Signal<br/>0-3.3V] -->|Weak Signal| B[Amplifier<br/>Boost 10x]
    B -->|Noisy Signal| C[Filter<br/>Remove Noise]
    C -->|Clean Analog| D[ADC<br/>Analog to Digital]
    D -->|Digital Value<br/>0-4095| E[Microcontroller<br/>Process Data]

    style A fill:#E67E22,stroke:#2C3E50,color:#fff
    style B fill:#16A085,stroke:#2C3E50,color:#fff
    style C fill:#16A085,stroke:#2C3E50,color:#fff
    style D fill:#2C3E50,stroke:#16A085,color:#fff
    style E fill:#2C3E50,stroke:#16A085,color:#fff

Figure 525.1: Sensor Signal Conditioning Pipeline: Amplification, Filtering, and ADC Conversion

{fig-alt=“Sensor signal processing pipeline flowchart showing five stages: sensor generating analog signal (0-3.3V), amplifier boosting weak signal by 10x, filter removing noise to produce clean analog signal, ADC converting analog to digital values (0-4095), and microcontroller processing the digital data for IoT applications.”}

NoteThe Three Main Challenges

When connecting a sensor to a microcontroller, you face three problems:

Challenge	Real-World Example	Solution
Signal too weak	Thermistor outputs only 0.01V change	Amplification - Boost the signal
Signal too noisy	Electrical interference from motors	Filtering - Remove the noise
Wrong format	Sensor speaks analog, MCU needs digital	ADC Conversion - Translate it

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flowchart TB
    subgraph Problem1[Problem 1: Weak Signal]
        P1A[Thermistor: 0.01V change] --> P1B[Too small for ADC]
    end

    subgraph Solution1[Solution: Amplification]
        S1A[Op-Amp Gain = 100x] --> S1B[0.01V → 1.0V]
    end

    subgraph Problem2[Problem 2: Noisy Signal]
        P2A[Motor interference] --> P2B[Random voltage spikes]
    end

    subgraph Solution2[Solution: Filtering]
        S2A[RC Low-Pass Filter] --> S2B[Smooth clean signal]
    end

    subgraph Problem3[Problem 3: Wrong Format]
        P3A[Analog continuous] --> P3B[MCU needs digital]
    end

    subgraph Solution3[Solution: ADC Conversion]
        S3A[12-bit ADC] --> S3B[Digital 0-4095]
    end

    Problem1 --> Solution1
    Problem2 --> Solution2
    Problem3 --> Solution3

    style Problem1 fill:#E67E22,stroke:#2C3E50,color:#fff
    style Problem2 fill:#E67E22,stroke:#2C3E50,color:#fff
    style Problem3 fill:#E67E22,stroke:#2C3E50,color:#fff
    style Solution1 fill:#16A085,stroke:#2C3E50,color:#fff
    style Solution2 fill:#16A085,stroke:#2C3E50,color:#fff
    style Solution3 fill:#16A085,stroke:#2C3E50,color:#fff

Figure 525.2: Signal Conditioning Challenges and Solutions: Amplification, Filtering, ADC

{fig-alt=“Problem-solution diagram showing three sensor circuit challenges and their fixes: (1) weak thermistor signal (0.01V change) solved by op-amp amplification with 100x gain to produce 1V output, (2) noisy signal with motor interference causing random voltage spikes solved by RC low-pass filter for smooth clean signal, (3) wrong format with analog continuous signal solved by 12-bit ADC conversion to digital values 0-4095.”}

525.3 Voltage Divider Circuit

Theory: A voltage divider is the most fundamental circuit for interfacing resistive sensors (LDR, thermistors, potentiometers) with microcontrollers. It converts resistance changes into voltage changes that ADCs can measure.

Voltage Divider Circuit

Figure 525.3: Voltage Divider: Two resistors in series create a proportional output voltage based on their ratio.

Formula: V_out = V_in × (R2 / (R1 + R2))

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      question: "A thermistor (10kohm at 25C) is placed as R1 in a voltage divider with a 10kohm fixed resistor R2 and 3.3V supply. As temperature increases and thermistor resistance drops to 5kohm, what happens to the output voltage at the junction?",
      options: [
        {text: "Decreases from 1.65V to 1.1V", correct: false, feedback: "Check your formula. With R1 on top, lower R1 resistance means less voltage drop across R1, so more voltage appears at the output."},
        {text: "Increases from 1.65V to 2.2V", correct: true, feedback: "Correct! Vout = 3.3V x (10k/(5k+10k)) = 2.2V. As the thermistor resistance decreases, less voltage drops across it, so output voltage increases."},
        {text: "Stays constant at 1.65V", correct: false, feedback: "The voltage divider output changes with resistance. That's the whole point of using this circuit for temperature sensing."},
        {text: "Increases from 1.65V to 3.3V", correct: false, feedback: "The output can never equal Vin unless R1 becomes 0 ohms. At 5kohm, significant voltage still drops across R1."}
      ],
      difficulty: "medium",
      topic: "voltage-dividers"
    }));
  }
}

TipInteractive Circuit: Voltage Divider

Try it yourself! See how voltage dividers work with variable resistances.

What This Simulates: A voltage divider circuit with adjustable resistor values, showing how output voltage changes.

How to Use: 1. Click RUN/Stop button (top right) to start the simulation 2. Right-click on resistors to change their values 3. Watch the voltmeter show output voltage 4. Observe the current flow (animated dots) 5. Hover over components to see voltage/current values

NoteLearning Points

Observe: - Voltage Division: Output voltage = Input × (R2 / (R1 + R2)) - Variable Resistance: Changing R1 or R2 changes V_out proportionally - Current Flow: Same current flows through both resistors (series circuit) - Sensor Application: LDR or thermistor replaces one resistor, ADC reads V_out

Example Calculations:

V_in = 3.3V, R1 = 10kΩ, R2 = 10kΩ
V_out = 3.3 × (10k / (10k + 10k)) = 1.65V

If R1 = LDR = 5kΩ (bright light):
V_out = 3.3 × (10k / (5k + 10k)) = 2.2V

If R1 = LDR = 20kΩ (dark):
V_out = 3.3 × (10k / (20k + 10k)) = 1.1V

Real-World Application: Every analog sensor (LDR, thermistor, flex sensor, potentiometer) uses this circuit to interface with microcontroller ADCs.

525.3.1 LDR Voltage Divider Example

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flowchart TB
    VCC["+3.3V VCC"] --> LDR["LDR<br/>(200Ω bright<br/>10kΩ dark)"]
    LDR --> VOUT["V_out to ADC<br/>(1.65V-3.24V)"]
    VOUT --> R["10kΩ Fixed<br/>Resistor"]
    R --> GND["Ground (0V)"]

    style VCC fill:#E67E22,stroke:#2C3E50,color:#fff
    style LDR fill:#16A085,stroke:#2C3E50,color:#fff
    style VOUT fill:#2C3E50,stroke:#16A085,color:#fff
    style R fill:#16A085,stroke:#2C3E50,color:#fff
    style GND fill:#7F8C8D,stroke:#2C3E50,color:#fff

Figure 525.4: LDR Voltage Divider: Light-to-Voltage Conversion Circuit

{fig-alt=“Voltage divider circuit diagram showing +3.3V VCC powering a light-dependent resistor (LDR) with variable resistance from 200Ω in bright light to 10kΩ in darkness, connected in series with a 10kΩ fixed resistor to ground. The midpoint voltage (V_out) ranges from 1.65V to 3.24V depending on light level and feeds into an ADC input for converting light intensity into digital values.”}

Real-World Applications: - Automatic Street Lights: Turn on when ambient light falls below threshold - Display Brightness: Adjust screen based on room lighting (phones, tablets) - Solar Panel Tracking: Detect sun position for maximum efficiency - Camera Exposure: Measure ambient light for automatic settings - Smart Blinds: Open/close based on sunlight intensity

525.4 RC (Resistor-Capacitor) Filter Circuit

Theory: RC circuits create time delays and filter signals. Essential for debouncing switches, filtering sensor noise, and creating timing circuits.

Time Constant: τ = R × C (tau, in seconds)

Cutoff Frequency: f_c = 1 / (2πRC)

CautionPitfall: Missing Pull-Up Resistors on I2C Lines

The Mistake: Connecting I2C sensors directly to microcontroller GPIO pins without external pull-up resistors, relying solely on internal weak pull-ups (40-100kohm typical).

Why It Happens: Many tutorials skip pull-up resistors because simple single-sensor setups “work” with internal pull-ups. Developers assume if it works on the bench, it will work in production.

The Fix: Always install external pull-up resistors on SDA and SCL lines:

• Standard I2C (100kHz): Use 4.7kohm pull-ups to 3.3V
• Fast I2C (400kHz): Use 2.2kohm pull-ups to 3.3V
• Multiple sensors or long wires (>30cm): Use 1kohm pull-ups

Calculation: Pull-up value = Rise Time / (0.8473 x Bus Capacitance). For 400kHz with 100pF total capacitance: R = 300ns / (0.8473 x 100pF) = 3.5kohm (use 2.2kohm standard value).

Symptoms of weak pull-ups: Intermittent communication failures, sensors work individually but not together, communication fails in humid/hot environments, NACK errors at higher clock speeds.

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      question: "Your I2C bus has 4 sensors with 50cm cable runs, and communication becomes unreliable at 400kHz but works at 100kHz. The internal MCU pull-ups are 45kohm. What is the most effective fix?",
      options: [
        {text: "Switch to SPI protocol instead of I2C", correct: false, feedback: "While SPI avoids pull-up issues, it requires more wires per sensor (4 vs 2) and separate chip-select lines. Fixing the pull-ups is simpler."},
        {text: "Add external 2.2kohm pull-up resistors on SDA and SCL", correct: true, feedback: "Correct! Long cables add capacitance. Strong external pull-ups (2.2kohm for 400kHz, 4.7kohm for 100kHz) provide faster rise times to overcome cable capacitance."},
        {text: "Reduce cable length to under 10cm", correct: false, feedback: "While shorter cables help, this may not be practical for your installation. Strong pull-ups can handle 50cm+ at reduced speeds."},
        {text: "Lower the I2C clock speed to 10kHz", correct: false, feedback: "This would work but sacrifices throughput unnecessarily. Proper pull-ups let you maintain 400kHz performance."}
      ],
      difficulty: "hard",
      topic: "pull-up-resistors"
    }));
  }
}

TipInteractive Circuit: RC Low-Pass Filter

Try it yourself! See how RC circuits filter high-frequency noise from sensor signals.

What This Simulates: An RC low-pass filter that smooths noisy sensor readings.

How to Use: 1. Click RUN/Stop button to start 2. Observe the input square wave (top oscilloscope trace) 3. See the filtered output (bottom trace) - sharp edges smoothed 4. Right-click resistor/capacitor to change values 5. Watch how time constant affects filtering

NoteLearning Points

Observe: - Low-Pass Filtering: High frequencies (noise) blocked, low frequencies (signal) pass through - Time Constant τ: Larger R or C = slower response, more filtering - Exponential Response: Capacitor charges/discharges exponentially - Rise Time: Output takes ~5τ to reach steady state

Sensor Applications: - Noise Filtering: Remove electrical noise from analog sensor readings - Debouncing: Smooth mechanical switch bounces - Anti-Aliasing: Filter high frequencies before ADC sampling - Smoothing: Reduce jitter in sensor measurements

Example:

R = 10kΩ, C = 100nF
τ = 10k × 100n = 1ms
f_c = 1 / (2π × 1ms) ≈ 159Hz

Filters out noise above 159Hz, preserving slower sensor changes.

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      question: "You need to filter 60Hz mains noise from a temperature sensor that updates every 2 seconds. What RC filter cutoff frequency should you choose?",
      options: [
        {text: "100Hz - just above 60Hz to block the noise", correct: false, feedback: "100Hz is above 60Hz, so it would let the 60Hz noise through. The cutoff must be well below the noise frequency."},
        {text: "1Hz - well below both 60Hz and sensor update rate", correct: true, feedback: "Correct! A 1Hz cutoff blocks 60Hz noise (40dB+ attenuation) while easily passing 0.5Hz sensor signals. Temperature changes slowly, so this works perfectly."},
        {text: "0.001Hz - as low as possible for maximum filtering", correct: false, feedback: "Too aggressive! This would take 1000+ seconds to respond to temperature changes. The filter must be faster than your measurement rate."},
        {text: "30Hz - half of 60Hz following Nyquist", correct: false, feedback: "Nyquist applies to sampling, not filtering. A 30Hz cutoff would still pass significant 60Hz noise (only ~3dB attenuation)."}
      ],
      difficulty: "medium",
      topic: "noise-filtering"
    }));
  }
}

WarningTradeoff: Hardware Filtering vs Software Filtering

Option A (Hardware RC Filter): Cutoff frequency fixed at design time (f_c = 1/2piRC), no CPU overhead, power consumption adds 0.01-0.1mW, cost $0.05-0.50 for resistor/capacitor. Provides true anti-aliasing before ADC, removes high-frequency noise physically. Cannot be adjusted after deployment, requires PCB space, component tolerance affects cutoff (±10-20%). Option B (Software Digital Filter): Cutoff frequency adjustable in code, CPU overhead 0.1-2% per filter, no additional power for passive filtering but adds MCU active time. No BOM cost increase. Allows adaptive filtering based on conditions, can implement complex filters (Kalman, median). Cannot remove aliased frequencies (must sample fast enough), adds latency of N/2 samples for N-tap filter. Decision Factors: Choose hardware filtering when noise frequencies exceed half your sampling rate (prevents aliasing that software cannot fix), when CPU resources are constrained, or when EMI/RFI is severe (motor noise, switching power supplies). Choose software filtering when filter characteristics need runtime adjustment, when implementing complex filters like Kalman or adaptive algorithms, when PCB space is limited, or when different filtering is needed for different operating modes.

525.5 Transistor Switch Circuit

Theory: Transistors act as electronic switches, allowing low-power microcontroller pins to control high-power loads (LEDs, motors, relays). Essential for driving actuators.

NPN Transistor Operation: - Base voltage > 0.7V → Transistor ON (collector-emitter conducts) - Base voltage < 0.7V → Transistor OFF (no conduction)

TipInteractive Circuit: NPN Transistor LED Switch

Try it yourself! See how transistors amplify signals to control loads.

What This Simulates: An NPN transistor switching an LED on/off, controlled by a small base current.

How to Use: 1. Click RUN/Stop button 2. Click the switch (bottom left) to toggle base current 3. Watch the LED turn on/off 4. Observe current flow through the transistor 5. Right-click transistor to see β (current gain)

NoteLearning Points

Observe: - Current Amplification: Small base current (~1mA) controls large collector current (~20mA) - Switching Action: Transistor acts as electronic switch (ON/OFF) - Saturation Mode: When ON, V_CE ≈ 0.2V (fully conducting) - Base Resistor: Limits base current to safe level

Current Gain (β):

I_collector = β × I_base
If β = 100, I_base = 0.1mA → I_collector = 10mA

Why Use Transistors: - Current Amplification: Microcontroller pin (20mA max) controls 500mA+ loads - Voltage Isolation: Separate power supply for load - Protection: Prevents damaging microcontroller with inductive kickback - Efficient: Minimal power loss in saturation

Sensor-Actuator Applications: - LED Drivers: Control high-power LEDs - Motor Control: Drive DC motors (with flyback diode) - Relay Drivers: Switch relays for AC loads - Solenoid Control: Activate locks, valves - Fan Control: PWM speed control

Design Formula:

R_base = (V_GPIO - V_BE) / I_base
R_base = (3.3V - 0.7V) / (I_collector / β)

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      options: [
        {text: "260 ohms - just enough for 10mA base current", correct: false, feedback: "10mA base current would give 1A collector current with beta=100, but leaves no saturation margin. Use more base current for reliable switching."},
        {text: "1.3k ohms - provides 2mA base current for 200mA collector", correct: true, feedback: "Correct! I_base = 200mA/100 = 2mA minimum. R = (3.3V-0.7V)/2mA = 1.3kohm. This ensures saturation with some margin while staying under GPIO limits."},
        {text: "10k ohms - limits current to protect the GPIO", correct: false, feedback: "This gives only 0.26mA base current, providing only 26mA collector current. The motor needs 200mA and won't run properly."},
        {text: "100 ohms - maximum base current for fastest switching", correct: false, feedback: "This would draw 26mA from the GPIO, exceeding its 12mA limit and potentially damaging the ESP32."}
      ],
      difficulty: "hard",
      topic: "transistor-switches"
    }));
  }
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525.6 Basic LED Circuit

Theory: LEDs (Light Emitting Diodes) require current limiting resistors to prevent burnout. Understanding LED circuits is fundamental to building indicator lights and displays.

LED Characteristics: - Forward Voltage (V_f): ~2V (red), ~3V (blue/white) - Forward Current (I_f): 20mA typical - Polarity: Anode (+) to cathode (-)

TipInteractive Circuit: LED with Current Limiting Resistor

Try it yourself! Learn how to properly drive LEDs with current limiting.

What This Simulates: An LED circuit with adjustable current-limiting resistor.

How to Use: 1. Click RUN/Stop button 2. Right-click the resistor to change resistance 3. Watch LED brightness change 4. Observe current through LED (should be ~20mA) 5. Try removing resistor (right-click → Edit → 0Ω) - LED burns out!

NoteLearning Points

Observe: - Current Limiting: Resistor prevents excessive current that would destroy LED - Ohm’s Law: R = (V_supply - V_LED) / I_desired - Brightness: LED brightness proportional to current (up to max rating) - Power Dissipation: Resistor dissipates excess power as heat

Resistor Calculation:

V_supply = 5V
V_LED = 2V (red LED)
I_desired = 20mA = 0.02A

R = (V_supply - V_LED) / I
R = (5V - 2V) / 0.02A = 150Ω

Use standard 150Ω or 220Ω resistor

For ESP32 (3.3V GPIO):

R = (3.3V - 2V) / 0.02A = 65Ω
Use 100Ω resistor (common value)

Sensor Indicator Applications: - Status LEDs: Show sensor state (active, error, standby) - Threshold Indicators: Light up when sensor exceeds limit - Multi-Color RGB: Different colors for different conditions - Brightness Feedback: PWM LED brightness based on sensor reading

Common Mistakes: - No resistor → LED burns out immediately - Too small resistor → LED overheats, shortened life - Reverse polarity → LED doesn’t light, may damage LED - Correct calculation → Long LED life, proper brightness

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      question: "You need to drive a blue LED (forward voltage 3.2V, rated current 20mA) from a 5V supply. What resistor value should you use?",
      options: [
        {text: "90 ohms - calculated as (5V-3.2V)/20mA", correct: true, feedback: "Correct! R = (Vsupply - Vf) / If = (5V - 3.2V) / 0.02A = 90 ohms. Use the nearest standard value like 100 ohms."},
        {text: "250 ohms - calculated as 5V/20mA", correct: false, feedback: "You forgot to subtract the LED forward voltage. The resistor only drops the remaining voltage (5V - 3.2V = 1.8V)."},
        {text: "160 ohms - calculated as 3.2V/20mA", correct: false, feedback: "You used the LED voltage, not the voltage across the resistor. The resistor drops (5V - 3.2V) = 1.8V."},
        {text: "No resistor needed - the LED has internal resistance", correct: false, feedback: "LEDs do not have significant internal resistance. Without a current-limiting resistor, excessive current flows and the LED burns out."}
      ],
      difficulty: "easy",
      topic: "led-circuits"
    }));
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525.7 Knowledge Check: Circuit Fundamentals

Knowledge Check: Circuit Fundamentals Test Your Understanding

Question 1: You connect an LDR (200Ω in bright light, 10kΩ in dark) in series with a 10kΩ resistor to 3.3V. If the LDR is on top, what happens to the voltage at the midpoint when it gets darker?

Click to see answer

Answer: The voltage decreases when it gets darker. Using voltage divider formula: V_out = 3.3V × (10kΩ / (LDR + 10kΩ)). When bright (LDR = 200Ω): V_out = 3.3V × (10k / 10.2k) = 3.24V. When dark (LDR = 10kΩ): V_out = 3.3V × (10k / 20k) = 1.65V. The output voltage drops as darkness increases LDR resistance.

Question 2: An RC low-pass filter has R = 10kΩ and C = 100nF. What is the cutoff frequency, and what type of noise does it remove?

Click to see answer

Answer: Cutoff frequency = 159 Hz. Calculation: f_c = 1 / (2π × R × C) = 1 / (2π × 10kΩ × 100nF) = 1 / (6.28 × 0.001) ≈ 159 Hz. This filter removes high-frequency noise above 159 Hz (like electrical noise, switching transients) while allowing slow sensor changes to pass through. It’s ideal for smoothing noisy temperature or light sensor readings.

Question 3: You need to control a 500mA DC motor with an ESP32 GPIO pin (max 20mA). Why do you need a transistor, and how does it help?

Click to see answer

Answer: A transistor acts as a current amplifier and electronic switch. The ESP32 can only source 20mA, but the motor needs 500mA—25× more current! An NPN transistor allows a small base current (~2mA from GPIO) to control a much larger collector current (500mA from external power supply). This protects the microcontroller from damage while enabling control of high-power loads. The transistor also provides voltage isolation between the 3.3V logic and potentially higher motor voltages.

Question 4: Calculate the current-limiting resistor needed for a red LED (V_f = 2V, I_f = 20mA) powered by a 5V supply.

Click to see answer

Answer: 150Ω resistor is needed. Using Ohm’s Law: R = (V_supply - V_LED) / I_desired = (5V - 2V) / 0.02A = 3V / 0.02A = 150Ω. Use a standard 150Ω or 220Ω resistor. The resistor drops the excess voltage (3V) and limits current to safe 20mA. Without this resistor, the LED would draw excessive current and burn out within seconds.

525.8 Quick Reference Card

TipReference Card: Component Values for Basic Sensor Circuits

Circuit Type	Component	Value	Purpose	Calculation
Voltage Divider (LDR)	R1 (Fixed)	10kΩ	Reference	Match LDR mid-range
Voltage Divider (Thermistor)	R1 (Fixed)	10kΩ	Reference	Match thermistor @ 25°C
RC Low-Pass Filter	Resistor	10kΩ	Filter	f_c = 1/(2πRC)
RC Low-Pass Filter	Capacitor	100nF	Filter	Example: 159 Hz cutoff
LED Current Limit (5V)	Resistor	220Ω	Protect LED	R = (5V-2V)/20mA
LED Current Limit (3.3V)	Resistor	100Ω	Protect LED	R = (3.3V-2V)/20mA
1-Wire Pull-up	Resistor	4.7kΩ	Signal level	DHT, DS18B20
I²C Pull-up	Resistor	4.7kΩ	Signal level	SCL, SDA lines
Button Pull-down	Resistor	10kΩ	Default state	Prevent floating input
Transistor Base	Resistor	1kΩ	Current limit	I_base = V_GPIO/R

Common Calculations:

Voltage Divider:

V_out = V_in × (R2 / (R1 + R2))

RC Filter Cutoff Frequency:

f_c (Hz) = 1 / (2π × R × C)
Example: 10kΩ × 100nF = 159 Hz

LED Resistor:

R = (V_supply - V_LED) / I_LED
Example: (5V - 2V) / 0.02A = 150Ω → use 220Ω

Transistor Base Resistor (NPN):

R_base = (V_GPIO - 0.7V) / (I_collector / β)
Example: (3.3V - 0.7V) / (100mA / 100) = 2.6kΩ → use 1kΩ

525.9 Summary

This chapter covered the essential circuit building blocks for sensor interfacing:

• Voltage Dividers: Convert resistance changes (from LDRs, thermistors) into voltage changes readable by ADCs
• RC Filters: Remove high-frequency noise from sensor signals with simple passive components
• Transistor Switches: Allow low-power GPIO pins to control high-power loads like motors and relays
• LED Circuits: Properly calculate current-limiting resistors to protect LEDs and indicate sensor states

These fundamental circuits form the building blocks for more complex signal conditioning chains covered in the next chapter.

525.10 What’s Next

The next chapter, Signal Conditioning, covers advanced topics including the complete signal conditioning chain, Wheatstone bridges for precision measurement, instrumentation amplifiers, and worked examples for real-world sensor interfacing challenges.

NoteRelated Chapters

Circuit Fundamentals: - Electricity Fundamentals - Voltage, current, resistance basics - Electronics Fundamentals - Semiconductors and components

Next Steps: - Signal Conditioning - Advanced signal processing - ADC and Noise Reduction - Digital conversion challenges - Sensor Types - Temperature, motion, environmental sensors