Voltage Divider Basics: Principle, Application, and FSR Integration

What is a Voltage Divider?

A voltage divider is a simple circuit that uses two resistors in series to produce an output voltage that is a fraction of the input. It’s commonly used in sensor circuits or microcontroller analog inputs to scale down voltages.

Formula:

V_out = V_in × (R₂ / (R₁ + R₂))

Where:

• V_in ​: Input voltage
• V_out ​: Output voltage (measured at the junction of R1 and R2)
• R₁, R₂​ : Series resistors (R2 is typically connected to GND)

Why Use a Voltage Divider?

• Converts resistance change to voltage change: Many sensors like FSRs, LDRs, and thermistors don’t output voltage directly — instead, their resistance varies. A voltage divider converts this into a readable voltage.
• Simple and low cost: Only two resistors are needed.

Example: Voltage Divider Calculation

Let’s assume:

• V_in = 10V
• R₁=10kΩ, R₂=10kΩ

Then:

V_out = 10V × (10kΩ / (10kΩ + 10kΩ)) = 10V × 1/2 = 5V

Now, if:

• R₁=10kΩ, R₂=5kΩ

Then:

V_out = 10V × (5kΩ / (10kΩ + 5kΩ)) = 10V × 1/3 ≈ 3.33V

This shows how resistor ratios affect the output voltage.

Using Voltage Divider with FSR

Basic Circuit:

5V ── FSR ──┬── Rfixed ── GND
            │
         Analog In (Vout)

• With no pressure, FSR resistance is high → Vout ≈ 0V
• When pressure is applied, FSR resistance drops → Vout rises

This allows the microcontroller’s analogRead() to measure pressure changes.

How to Choose the Fixed Resistor

‍

Tip: Choose a resistor value that matches the mid-range resistance of your FSR to get the most usable voltage swing.

Practical Tips

• Match FSR range: Choose a fixed resistor that matches the expected resistance range of your FSR during operation.
• Maximize ADC resolution: Adjust the resistor ratio so that the voltage swing covers as much of the 0–5V range as possible.
• Avoid extreme resistor values: Using very low or high resistance may flatten the voltage curve, reducing sensitivity.
• Test and tune: Try several resistor values (e.g., 4.7kΩ, 10kΩ, 47kΩ) and observe how they affect analog readings with light and firm pressure.

Conclusion

The voltage divider is one of the simplest but most essential circuits in electronics. It allows easy signal conversion from resistive sensors like FSRs to voltages that microcontrollers can read. With proper resistor selection and layout, you can quickly build a force-sensing input circuit for your Arduino or embedded system.

In the next article, we’ll introduce the inverting op-amp circuit setup — a great way to amplify and linearize the signal from your FSR sensor.

‍