How to Use Resistors in a Project

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The Maker’s Toolbox contains a basic selection of essential Maker’s Tools. The tools without which we cannot work. Breadboards, soldering irons, LEDs are important, but resistors are the tiny components that projects depend on.

No matter which edge we choose, whether Raspberry pie, Raspberry Pi Pico or Arduino, we need resistors to protect our LEDs, divide voltages, and provide accurate values ​​for our circuits. But what do they do, why do we need them, and how can we ensure we have the right value? For this, we have to do a little calculation and consult some data sheets.

In this reference, we’ll explain what resistors are, what they do, and tell you how to choose the right resistor for your next project.

What are resistors?

Resistors are components that introduce electrical resistance into a circuit. Generally they are used to reduce current flow in a circuit, for example when used with LEDs they prevent the LED from drawing too much current.

An LED without a resistor will turn off very quickly. Resistors can also be used to create voltage potential dividers, useful circuits that will reduce the voltage in a circuit. Each manufacturer will have resistors in their kits. They come in bandolier bands and can be purchased in single packs or in the thousands.

Why do we need resistors?

The most basic use of resistors is to prevent a component from drawing too much current. Take for example an LED (Light Emitting Diode). LEDs are designed to pass current in one direction and produce a small amount of light while operating. If we give the LEDs as much current as they want, the LED will turn on strongly but turn off quickly. In some cases, we can give it too much current at once, causing the LED to “jump” and then turn off.

We can use the following calculation to determine the exact value of the resistor.

R is the resistance value, Vs is the supply voltage, Vf is the forward voltage (the amount the component needs), and If is the forward current.

Let’s put this into practice. We have a blue LED connected to a 5V supply. The forward voltage of the LED is 3.2 V and the current required is approximately 10 mA. So the calculation looks like this.

(Image credit: Tom’s Hardware)

This means that the value of R is 180 Ohms. In the standard series of resistors we can use this exact value or we can choose a 150 or 220 Ohm resistor instead. For basic tasks the exact value is not essential, but when designing circuits for professional/industrial or high precision devices you will need to use the exact values. The exact values ​​can be found in the component data sheet or on the product page of your chosen store.

For most hobbyist/manufacturer applications, we can choose the closest value we have. We often prefer a 220 / 330 Ohm resistor for our LEDs.

(Image credit: Tom’s Hardware)

Resistors can also be used to pull up or down a GPIO pin. A pull up resistor will pull a pin high by connecting a voltage supply to a pin. A pull down resistor will pull a pin to GND. We used a 10K Ohm resistor with a DHT22 temperature sensor to pull the data pin high using the 3.3V supply.

Resistors can also be used to drop voltages from one level to another. This is called a voltage divider and is commonly used in potentiometers to vary voltage.

To create a voltage divider, we need to use this equation.

(Image credit: Tom’s Hardware)

Vout is the voltage we want.

Vin is the input voltage.

R1 is the value of the first resistor.

R2 is the value of the second resistor.

(Image credit: Tom’s Hardware)

So for our voltage divider, we want to convert the 5V input voltage to around 3.3V. This process is commonly used when we need to change the logic voltage of a component, for example the HC-SR04. The HC-SR04 ultrasonic distance sensor originally used 5V logic, and so the echo pin, which activates when sound bounces off an object, will send 5V to the GPIO.

For an Arduino, it’s ok. For a Raspberry Pi, this can damage the pin or even the Pi. We use two resistors, R1 a 1K Ohm resistor (top) and R2 a 2.2K Ohm resistor (bottom) to create a voltage divider. R1 and R2’s legs go in the same row of the breadboard. In R1 we supply 5V and in R2 we connect to GND. Where the legs of R1 and R2 meet is the output voltage, which should be 3.4375V, well within the GPIO’s tolerance of 3.3V.

(Image credit: Tom’s Hardware)

The calculation works by adding R1 and R2 together (1000 + 2200 = 3200) then dividing the value of R2 by that (2200 / 3200 = 0.6875) and finally multiplying by the input voltage (5 8 0.6875 = 3.4375V) .

How to choose the right resistor?

Resistors have colored bands around their axis. These bands are a code system that we can use to identify the value of a resistor. There are four, five and six bands but the most common are four. In fact, four-band resistors are the easiest to read.

(Image credit: Tom’s Hardware)

Let’s take this resistance as an example. The bands are printed on the resistor but the final band, the tolerance, is printed on one of the “bulges” at the end of the resistor. We can see that the first band is yellow and the second is purple. This gives us a value of 47. The third band is the multiplier, in this case the red is 100. If we do the math, 47 x 100 = 4700. We have a 4700 Ohm resistor, usually referred to as a 4.7K resistor Oh. The final band is tolerance. Our tolerance band is gold which means we have a 5% tolerance, it can be 5% higher or lower than the 4.7K Ohm value.

(Image credit: Tom’s Hardware)

Five-band resistors provide additional precision and use an additional third digit to dial in the precision. The third band of the same 4.7K Ohm resistor is now black, which refers to zero. The fourth band is the multiplier and the fifth is our tolerance.

This chart provides a quick reference that can be applied to four and five band resistors.

(Image credit: Tom’s Hardware)

Checking your resistors

Sometimes it can be difficult to correctly identify a resistor by its color code. It may be old, damaged or misprinted. If so, we can check our resistance using a multimeter.

Multimeters are an indispensable tool for makers. Among other features, multimeters can measure voltages, current, and check continuity in a circuit. There are two common multimeters, autoranging and manual. Autorange tries to detect the reading and place it within a range. For manual, we need to set the range.

For autoranging multimeters

(Image credit: Tom’s Hardware)

1. Turn the dial to the Ω (Ohm) symbol and press the power button. Some multimeters turn on when the dial is turned, while others have a power button.

2. Wrap one leg of resistance around a probe. Resistors have no polarity, so we can connect either leg to the probe.

3. Wrap the other leg around the remaining probe.

4. Read the value on the screen. Give it a few moments to settle down before taking a reading.

For manual multimeters

(Image credit: Tom’s Hardware)

1. Turn the dial to the Ω (Ohm) symbol and select the lowest range. Press the ignition button.

2. Wrap one leg of resistance around a probe. Resistors have no polarity, so we can connect either leg to the probe.

3. Wrap the other leg around the remaining probe.

4. Read the value on the screen. Give it a few moments to settle down before taking a reading.

5. If the reading shows OL or scrambled, go back one range until you see a stable reading. It’s the multimeter trying to tell us that our reading is out of range, normally higher than the manual setting we used.

(Image credit: Tom’s Hardware)
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