Sensors & Measurement · Study deck

Sensor Circuits: Dividers and Filtering

A motor can make a sound sensor value jump even when the sensor is healthy.

Physics Phoebe is your guide for this deck.

sensorcircuits

After studying this chapter

Learning objectives

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

Major section

Start With the Measurement Story

This runway does not prove every sensor or circuit is accurate.

The sensor may be healthy while its wiring, supply, filter, or input circuit changes the value.
A microcontroller means the small computer that reads inputs and controls a device.
A sensor signal usually reaches the microcontroller through a circuit before it becomes data.

Major section

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.
With R1 = 10 kΩ and R2 = 20 kΩ, the shown relation gives 5 V × 20/(10 + 20) = 3.33 V.

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

Major section

Learning Points

Variable Resistance: Changing R1 or R2 changes V_out proportionally.

Real-World Application:: These resistive sensors (LDR, thermistor, flex sensor, potentiometer) commonly use a voltage divider to interface with microcontroller ADCs.
The circuit makes the changing LDR resistance and the fixed resistor part of one measurable ratio.

LDR Voltage Divider: Light-to-Voltage Conversion Circuit

Major section

Putting Numbers to It

LDR Voltage Divider Design: An LDR varies from 200Ω (bright sunlight) to 10kΩ (darkness).

With the same fixed value, swapping the sensor between top and bottom reverses the direction but preserves the swing magnitude.
A fixed value near the geometric mean of the sensor range is a useful starting choice: $\sqrt{200 \times 10{,}000} \approx 1{,}414\Omega$; 1.5 kΩ is a closer standard value than 1 kΩ.
The worked Option 2 still uses 1 kΩ.

Deck summary

Key takeaways

This runway does not prove every sensor or circuit is accurate.

Theory:: A voltage divider is the most fundamental circuit for interfacing resistive sensors (LDR, thermistors, potentiometers) with microcontrollers.
Variable Resistance: Changing R1 or R2 changes V_out proportionally.
LDR Voltage Divider Design: An LDR varies from 200Ω (bright sunlight) to 10kΩ (darkness).

Retrieval practice

Recall check 1 of 3

Physics Phoebe says: answer from memory, then check your reasoning.

Q1A 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?

ADecreases from 1.65V to 1.1V

BIncreases from 1.65V to 2.2V

CStays constant at 1.65V

DIncreases from 1.65V to 3.3V

Show answer

Answer: B Correct!

Retrieval practice

Recall check 2 of 3

Physics Phoebe says: answer from memory, then check your reasoning.

Q2Your 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?

ASwitch to SPI protocol instead of I2C

BAdd stronger external pull-ups

CReduce cable length to under 10cm

DLower the I2C clock speed to 10kHz

Show answer

Answer: B Correct!

Retrieval practice

Recall check 3 of 3

Physics Phoebe says: answer from memory, then check your reasoning.

Q3You need to filter 60Hz mains noise from a temperature sensor that updates every 2 seconds. What RC filter cutoff frequency should you choose?

A100Hz - just above 60Hz to block the noise

B1 Hz — well below 60 Hz, while still passing slow temperature changes

C0.001Hz - as low as possible for maximum filtering

D30Hz - half of 60Hz following Nyquist

Show answer

Answer: B Correct: a 1 Hz first-order RC attenuates 60 Hz by about 35.6 dB. If readings are sampled only every 2 seconds, add adequate anti-alias filtering or sample faster and filter before decimating.

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Answers

Answer key.

B · Correct!
B · Correct!
B · Correct: a 1 Hz first-order RC attenuates 60 Hz by about 35.6 dB. If readings are sampled only every 2 seconds, add adequate anti-alias filtering or sample faster and filter before decimating.