Whether you are a hobbyist tinkering with an Arduino, or a professional engineer developing industrial monitoring systems, there is a good chance that at some stage of your engagement with electronics, you will need to interface sensors with microcontrollers.
Many common sensors, such as pressure, level, or temperature transducers, output voltages that can go well beyond the safe input range of typical analogue-to-digital converter (ADC) pins. Microcontrollers like the Arduino and Raspberry Pi Pico are often limited to 3.3V or 5V.
There is, however, a simple-but-effective way of scaling these voltages down: using a voltage divider circuit. Provided that you get the design of yours right, it will protect your microcontroller at the same time as preserving measurement accuracy.
Below, then, we will take a closer look at what voltage dividers are, the all-important matter of safety, and how you can design yours to deliver the right results.
A Quick Introduction to Voltage Dividers for Sensors
A voltage divider is a simple passive circuit that produces an output voltage (Vout) that is a fraction of its input voltage (Vin). It achieves this by using two resistors in series (or other components such as sensors), effectively “dividing” the total voltage across them in accordance with their resistance ratio.
The scaled voltage (Vout) is taken from the junction between the resistors and fed to the ADC output. The basic formula can be expressed as: Vout = Vin x (R2 / (R1 + R2)), where R1 is the resistor between Vin and Vout(top resistor), and R2 is the resistor between Vout and ground (bottom resistor).
With regard to sensor inputs, a voltage divider allows you to map a higher sensor range (for example, 0 to 10V from an industrial transducer) to the ADC’s full scale (for instance, 0 to 5V). This enables you to maximise resolution, at the same time as keeping voltages within safe limits.
5 Steps of Designing a Sensor Input Circuit
So, let’s take you through the “real-world” stages of putting together an effective circuit:
Define Your Parameters
This will entail determining the sensor’s maximum and minimum output voltage range (Vin), as well as identifying the maximum allowed input voltage range for the receiving device (for example, a microcontroller’s ADC). The latter will be your target Voutrange.
In addition, you will need to figure out the input impedance (Zload) of the receiving device, and the current requirements of the sensor.
Calculate The Resistor Ratio
Turning to the voltage divider formula we cited above, Vout = Vin x (R2 / (R1 + R2)), rearrange it so that you can find the required resistor ratio based on the maximum input and desired output voltages: R2 / (R1 + R2) = Vout / Vin.
Select Resistor Values and Consider Loading
Choose a value for one resistor (such as R2) and solve for the other (R1).
Be sure to adhere to the “10% rule”. This is about making sure the total current through the divider is a minimum of 10 times greater than the current drawn by the load (Zload). It means the load resistance should be at least 10 times larger than R2 to minimise the loading effect and maintain accuracy.
Incorporate Safety and Protection
By calculating the power (P) that each resistor dissipates, using the formula P = V2 / R or P = I2 x R, you can ensure it is rated appropriately. I, of course, refers to current, the flow of charge through the specific resistor, while R is resistance.
The addition of external components, such as Zener diodes or clamp diodes in parallel with the microcontroller input pin, can limit voltage to a safe level and protect the device from unexpected spikes in voltage.
Build and Verify
It is important to be mindful that resistors come in standard values (such as the E12 or E24 series). Given that the ideal ratio rarely matches exactly, you will introduce a small error.
Nonetheless, when you build the circuit, it will be crucial to measure the actual output voltage with a multimeter. This will verify that it mirrors your calculations and stays within the safe range under expected operating conditions.
Finally, make sure you test with the sensor connected across its full operating range. This will enable you to check that the output remains safe and provides sufficient resolution for your ADC.
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One More Thing: Bookmark The Right Digital Tool to Help with Your Calculations
When you are designing sensor input circuits, a key challenge will be selecting standard resistor values that closely match your calculated ratio, at the same time as minimising error.
For this reason, it might make a lot of sense for you to plan values using the precision voltage divider calculator for sensor circuits on the website of electrical component supplier, RS.
This tool enables you to rapidly compute the actual output voltage from any combination of R1, R2, and source voltage. So, it can be well worth keeping a link to it saved in your browser, for the next time you take on a circuit design project that requires you to divide voltage safely.
