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This is a value that is defined by the software itself, and has been given an easy name. Constants will have a blue color. By default, the Arduino's pins are set as an input, but by using the pinMode function, we've now set pin 13 as an output.

We haven't told it what value we want to output yet, so it will just be 0. This means that, inside the chip, pin 13 is connected to 0v, this is the ground. If you would connect the 5v pin to pin 13 now, you would create a short circuit! Be careful! Arduino reference: pinMode. Arduino tutorials: Digital Pins. In the Arduino IDE, and other C-like programming languages, every statement is ended with a semicolon ; , as you can see in this Blink example.

When you get cryptic errors when trying to upload, one of the first things to check are the semicolons. Most of the times, you'll get an error like this: Blink error: expected ',' or ';' before Arduino reference: ; semicolon.

Arduino reference: loop. The next function is ' digitalWrite 13, HIGH ' This 'writes' pin 13 high, in other words, it connects it internally to the 5v pin.

You can see the same function being used with the LOW constant as well, this will connect pin 13 to the ground inside the chip. Now there's no voltage difference across the LED, and it will go out. Arduino reference: digitalWrite. The last function is a very intuitive one: delay It just waits for a certain amount of time, in milliseconds. In this case, it will wait ms, or 1 second, before executing the next command. Arduino reference: delay. As an example, we'll upload the Blink example to the Arduino, but since it's already installed by default, change the value of the delay functions to instead of , otherwise we won't see any difference.

Now connect your Arduino to your computer, if you haven't already. In Windows, this will probably be a COM port. If there are multiple ports available, unplug your Arduino, then plug it back in, and see which of the ports disappears and reappears, to know the port of your Arduino.

In the bottom right corner, you should now see the name of your board, and the port it's connected to. Now click the right-pointing arrow in the top left corner to upload Blink to the board. If all goes right, it should start compiling, and then upload. While it uploads the TX and RX lights should flash. When it's complete, your led on pin 13 should now blink twice as fast. Compiling means that the human readable code you wrote is 'converted' to a series of binary numbers that the Arduino can understand.

This binary file is then copied into the Arduino's memory during the upload. Well, let's just face it: flashing one little LED is not that impressive So in this step, I'll show how you can drive more LEDs.

Before you begin, we'll have to calculate the resistor to use in series with our LED. But the problem here is that we don't know the voltage and the current of the resistor yet Let's find it! We connect the resistor in series with our LED, and we know that the voltage of the Arduino's output pin is 5v, so if we add the voltage of the LED to the voltage of the resistor, this should equal 5v. Since they are in series, this also means that all electrons that pass through the LED, will go through the resistor as well.

Since current is defined as the amount of electrons per unit of time, we know that the current through the resistor is equal to the current through the LED. Note that if the current is given in milliamps mA , you'll have to divide it by , to get Amps, the SI unit of current. Otherwise, you won't get your answer in Ohms but in kilo-ohms instead.

This holds true for every physics formula: if you enter your values in their respective SI units, you'll always get an answer in an SI unit. If you are using a 3. This is just the supply voltage, it can be anything.

V L is the LED's forward voltage drop. LEDs that emit lower energy light, with longer wavelengths, like red and infrared have a much lower voltage drop than light with higher-energy photons, like blue or ultraviolet light. You probably won't have a resistor of the exact value you just calculated, so round up to the next value in the E series. This series only has the values 1. It is better to take a resistor with a higher value, so you don't kill your LED.

Like I explained earlier, an LED is basically a diode, so it lets the current pass in only one direction. If we connect the LED the other way around, nothing will happen.

An LED has a cathode and an anode: in schematic representation, the direction of the arrow is the conventional current flow direction, so it points to the negative side: the cathode.

When you take a close look at an LED, you'll see that one side is flat, and the other is round. Also, one leg is shorter than the other: the flat side, with the shorter leg is the cathode, and we'll connect this side to the negative, or ground of the Arduino. It doesn't matter where you put the resistor, as long as it's in series with the LED, so you can connect it either to the cathode or to the anode. Take a look at images , then connect the resistor to the ground and one of the horizontal rails of your breadboard, and an LED with the short leg into the same rail, and the long leg in pin 12 on the Arduino.

Note that you have to calculate the specific resistor for your LED. You can use the formula above, or use an online tool, or an app I use this tool and this app.

If you click the question marks next to the fields, the tool will give you some extra info, and the app has a drop-down menu for selecting the color. This only holds true if you use it with a 5v Arduino!

If you want to use an LED with a higher voltage, you'll need to calculate it. When you have connected the LED and resistor to the Arduino, you can replace pin 13 in the Blink example by Now upload the sketch to your Arduino: the LED you've attached should now blink.

Still not impressed? Well, you're right But now you've got the basics, we can do something more spectacular If you remember the Blink example, it shouldn't be too hard to predict what this program does. Upload it to your Arduino, and check if your hypothesis was correct! If you take a closer look, you'll see that some pieces of the code are just 6 times the same commands, with just a different pin number Well, there's a way to solve this: we call it the for-loop.

Open the blink6-b sketch from the. ZIP file. When you upload it to the Arduino, you'll see that it does exactly the same as blink6-a, but it has much fewer lines of code. This is all thanks to the two for-loops that are used. But before I can explain the for-loop, I'll first have to explain how variables work:.

A variable is just a piece of memory, where you can store pretty much anything you like. A variable has a data type , a name, and a value: in this case the data type is 'int': an integer, a whole number and can positive or negative.

The name is 'delayTime', this is just an arbitrary name you give the variable, it can have letters and numbers in it, and some symbols like an underscore. If the name consists of more than one word, capital letters are used for every new word, because you can't use spaces. Choosing appropriate names for your variables will make a huge difference in the readability of your code. With 'int delayTime;', the variable is correctly declared , but doesn't have a value yet.

Assigning an initial value to the variable is called initializing. In this case, we initialize variable delayTime to Now every time we use 'delayTime' in our code, it will be replaced by the number If you read a variable without first initializing it, it will just give you 0, in the best case, but probably just a random value that happened to be in that place of the memory, possibly causing errors in your program.

You also have to know that a variable has a so-called scope. In this example, delayTime is declared at the top, so it is a global variable.

This means that you'll be able to access it inside every function, every loop, in the whole program. A variable can also be local , meaning you can only use it in the scope in which it is defined.

For example, if we were to move the first line to the setup, we would get the error. But local variables can come in handy, when you don't have lots of memory to spare, for example, since the local are 'deleted' when you leave the scope they are declared in.

Most of the time, the boundaries of a scope are curved brackets. But why use this variable? Well, instead of having to change all the delay ; functions, we can just change the first line. In this case it's not very useful, but if you have really large program, it really matters. Arduino reference: variables.

Arduino reference: variable scope. Another useful thing about variables is that you can change them, while your code runs. And this is exactly what happens in a for-loop. The first part between the brackets is just the declaration of a certain variable 'i', and we initialize it to 2.

All this means that it will first execute the code inside the brackets with a value of 2 for i, sets pin 2 as an output. Until i equals The program exits the for-loop.

Every pin with an LED attached is now set as an output, with only two lines of code, instead of six. If you really want to know the actual power the resistor dissipates, you'll have to measure the current or the voltage across it, or in the best case, both.

In the case you can measure both the current and the voltage case A , you can just use the same formula. If you can, however, only measure one of these values, you can use Ohm's Law to substitute either the voltage or the current in the formula for power. The value written on the resistor has a certain tolerance, so you'll have some deviation in your wattage as well.

On line 3, you'll see a new structure: we call this an array. An array is basically a list of variables, in this case, it is a list of variables of the type int. We use square brackets to indicate that it is an array. To initialize the array, we use curved brackets, and the values are separated by commas. When we want to use a value from an array, we'll need to specify which place in the array we want the value of.

Let's use the array in blink7 as an example :. The zero between square brackets is called the index. Arrays are zero-based , this means that the first element in the array has index zero. This can be confusing at first, for example, array[5] will return 6, where you'd expect it to return 5. You can imagine that this can cause a lot of problems That's why this error has its own name: an off-by-one error , or OBOE for short. When you try to read a value that isn't inside the array, array[10] for example, the Arduino will just read the next place in the memory, without realizing that the value it's reading isn't a part of the array anymore.

It will just give you the value it finds at that particular spot in its memory. But things get even worse when you are writing to an index outside of the array, because you may be overwriting other data, like variables or pieces of code that happen to be at that place in memory! A bad idea If you initialize it on the same line, like in blink7, the Arduino IDE will count the elements for you, and you don't have to specify anything between the brackets.

If you want to know the number of elements in the array, you can use the sizeof array function. This however, isn't the amount of elements, in blink7, sizeof array will return This is because every element is an int, and an int is 2 bytes long. We use this to exit our for-loop when we've read the whole array: As long as the index is less than the number of elements, we can safely read the array. This time we only increase i with 1. Arduino reference: Array.

Arduino reference: Sizeof. Arduino reference: Increment. Arduino reference: Compound Addition. You can change the values in the array to make your own sequence, and you could even add or delete elements, since we made our program independent of the length of the array, by using the sizeof function. In an array, the elements can not only be data types, like ints, but also other arrays.

This allows you to have so-called 2-dimensional arrays, that can be compared to matrices. Note that we have to specify all dimensions except the first one between the square brackets, when declaring a multi-dimensional array. This example won't do anything if you upload it, it doesn't have any outputs, it's just for learning purposes.

It creates 4 matrices 2D arrays , two of which have values. Then it just calculates the sum of the two matrices, and the transposed matrix for the first one. This is done by using nested for-loops. You can follow this explanation on the image above. The numbers are the order the for-loop will go over them. They start at the first column and the first row 1,1 in matrix notation, [0][0] for array notation, then the column is incremented 1,2 or [1][0], the column is incremented again 1,3 or [2][0] when j is incremented again, the j-loop exits, because j is no longer less than three.

Now j exits, and i exits as well. Inside these arrays, you can use more arrays, basically creating multi-dimensional spaces. In theory, he number of dimensions is unlimited, but once you use more than three dimensions, it gets really complicated, so other methods are preferred.

In this step, we'll just use push buttons as input devices, but of course, you can use any switch. To make our button output these voltages, we'll use a pull-up or a pull-down resistor. The leg with the resistor connected goes to the input pin on the Arduino. This way, when the button is not pressed and doesn't connect the 2 legs , the input is at 0v, because it's connected to ground through the resistor.

When you press the button, the input is at 5v, because it's directly connected to 5v through the switch. The resistor doesn't matter when you press the button, it just makes sure that the input is at 0v when the button is not pressed. A pull-up resistor image 2 works in exactly the same way, but everything is swapped: the first leg is connected to ground, instead of 5v, the second is connected to 5v, through a resistor hence the name pull-up resistor, since it pulls it up to 5v.

The input pin still connects to the side with the resistor, but now, it is high when the button is not pressed, and goes low when the switch is closed. Now, connect a push button with a pull-down resister to pin 3 of the Arduino, and a push button with a pull-up resistor to pin 5. Then connect two LEDs with their appropriate resistor to pins 10 and Now open example button2, and open it.

This program just reads the two inputs, and sets the outputs in the same state. There are only two new things, and they are really obvious: instead of the OUTPUT constant, we use INPUT to set the pins of our buttons as inputs, and the digitalRead pin function just returns the state of the given input pin. Note: using pinMode pin, INPUT is actually unnecessary, since all pins on the Arduino are inputs by default, but it's often done anyway, to make the code more readable.

When you upload the sketch, press the buttons, and you'll see that the table above is indeed correct: the LED on pin 12 is always on, until you press the button on pin 5, this is because it has a pull-up resistor. If you want the LEDs to light up only when you push the button, you can use the Boolean not-operator: this just changes a 'true' into a 'false' or a 1 into a 0 and vice versa. It would be really inconvenient if we had to use an extra resistor and an extra piece of wire, every time we want to use a normal switch.

That's why the chip on the Arduino has a built-in pull-up resistor on every pin. Now connect the push buttons without the resistors, just the connection to the ground as shown in image 5 You can see that we don't need to use the 5v pin of the Arduino anymore, and if we were to produce this at a large scale, those two resistors we saved would make a significant difference in production cost.

Open the example button2-b. As you can see, I used the two ways to enable the pull-up resistors. Also note that I used the 'not' operator, so the LEDs are on when the button is pressed. DigitalRead, digitalWrite and pinMode are great and simple functions, but they are relatively slow. Also, you can't turn on 2 pins on or off at exactly the same time, and writing 8 bits simultaneously for parallel communication is not possible either.

Sometimes, when you're running short on memory, these 3 functions can use a lot of the available space, too. The solution to these problems is direct port manipulation. In the image above, you can see the entire pin mapping of the Arduino Uno, the port numbers are in the yellow fields next to the pins.

Since every bit of the byte represents one pin, it is easier to write the values in binary notation. Similarly, you can use a leading 0 to use octal notation , or 0x for hexadecimal notation , however in this case using these two notations doesn't really make sense.

When counting bits, the rightmost least significant, LSB bit is bit 0, so it corresponds to the first pin of the port, while the MSB most significant bit corresponds to the eighth pin of the port. However, using it like this can cause some problems: e. To change only one pin at a time, we can use some bitwise operators.

To set one bit high, without changing the other bits, we can use the bitwise or-operator. Note: this is only one , whereas the boolean or-operator is. Bitwise means that it is applied to every bit separately. We use a mask to set the right bit high: the bit we want to set high is 1, and all other bits are 0. You can check out the truth tables and some examples in the images above. Now we have to invert our mask: the bit we want to set low is zero, and all the other bits are one.

This notation works just fine, but we can make it a little more readable. If you just want to flip one or more bits, you can use the exclusive or-operator. This returns 1 if one of the input bits is one, and 0 if both inputs are the same.

The same goes for bitwise operators. We won't need the temporary variable anymore. To get one specific bit from the input register, for example, you could use the bitRead byte, bit function:.

The bit we wanted to check is now the rightmost bit. Modulo gives you the remainder of a division, e. In binary notation, the last digit rightmost bit of an even number is 0, and the last digit of an odd number is 1. Another way to get only one bit of a number is using the conditional operator? From these examples, you can see that the result is zero if bit 6 was 0. We use the conditional operator: condition?

If the condition is true, the operator will return the resultIfTrue , if it's false, it will return resultIfFalse. Note on compatibility : the port-to-pin mapping depends on the chip. This means that you have to change your program if you want to use another Arduino.

For personal use, this isn't so much of a problem, but if you're writing programs or libraries for the community or to share online, you should take this into account. Arduino reference: Integer Constants. He killed my wire: direct port manipulation tutorial. Arduino reference: Port Manipulation. The Atmel ATmega p datasheet p. Arduino reference: bitRead.

Up to this point, we only used the USB connection to upload new programs to the Arduino. But we can also use the connection to send data from and to the computer. Some Arduino boards like the Leonardo have a microcontroller that handles the USB communication all by its own, but most boards have a second, smaller microcontroller, solely for the communication.

The communication between this chip and the main microcontroller is done using a serial connection, then the second chip tells the computer 'Hey, I'm a serial port', and then simply converts the serial data from the main chip to USB format, and converts the messages from the computer to serial for the main microcontroller.

Serial means that bits are sent over the same wire, one after the other, whereas parallel connections send 8 bits or more simultaneously, on separate wires like the large parallel printer port on the back of some older computers.

These letters are written next to pin 0 and 1. This means that these pins are in use when you have the connection to the computer running, so you can't use them for LEDs or buttons etc. If you want to connect the Arduino to another device using serial communication, you also have to use these two pins. Open example serialButton, and upload it. The breadboard configuration is the same as in the previous step. Make sure autoscroll is enabled, and the baud is set to You'll see a bunch of ones.

Now press the push button connected to pin 3, and you'll see zeros. This is just the raw input from the pin. This will plot the values into a graph. In the setup, we add a new command: Serial. This is just to start the communication, and the is the baud rate , the number of pulses per second, so the speed of the serial connection. If you set this to a different value, you'll have to change it in the serial monitor as well. Otherwise, it will be out of sync, and give you very strange characters.

The opposite of Serial. In the loop, you'll see the same Serial keyword, this time used with the println This just prints out the value that's specified between the brackets, followed by a new line ln. Note that this doesn't just convert it to binary to send it over serial, instead, it converts it to ASCII , and then sends it over serial, so that the computer can print it out. If you want to send binary bytes over serial, use the Serial. To get a new line, you could also use Serial.

This is example serial2Buttons. Upload it, and open the serial monitor to see the result. Arduino reference: Serial. The communication also works the other way around: let's write a program to control LEDs from the computer. Open the example serialLEDs. Upload it, open the serial monitor, and try sending values between 0 and 3. Now send 4. Get how it works? Let's check out the code:.

The setup should look quite familiar, except one command: this is a while-loop. A while-loop repeats, as long as the condition between the brackets is true you could compare it to a for loop, but without the first and last argument, only the condition.

I explicitly used the curved brackets to indicate it is a loop, but you could also just use a semicolon instead: while! Serial ; Serial returns true when the serial communication is active when you open the serial monitor, for example. So if we add a not-operator! So the while-loop keeps on repeating while the communication is not active, and essentially waits for it to become active. Then we know we can start sending data with the println If we wouldn't use the while-loop to wait, some boards like the Leonardo and other boards that have USB capabilities in the main chip will just lose the first data.

It does try to send it to the computer, but nothing there is listening for serial input. In the loop, there's another new statement: the if-statement , this is probably the most important statement in computer science.

It does exactly what the name implies: it executes a piece of code, only when a certain condition is true. Otherwise, the code between the curved brackets is ignored. Now I'll take a moment to explain how the Arduino receives serial messages: As I explained earlier, serial sends one bit after the other, byte by byte. This means that your message, "test", for example, gets split up in pieces of 1 byte.

A character is one byte in size, so it will look something like 't' 'e' 's' 't' when you send it over serial. When the Arduino receives these bytes, it will store them in a buffer just a small piece of memory to temporarily store it. It receives it byte by byte, so the buffer will look something like this "" empty "t", "te", "tes" "test". When the buffer is empty, the Serial. If you call the Serial. If you call Serial. You can keep on reading until Serial.

If you want to know the first byte without deleting it while doing so, you can use the Serial. So the first if Serial. If so, it will read the first byte, and store it in the variable serialValue. Then it will check if the value it just read is '0'. If the value is '0', it will turn off both LEDs.

If the value is not '0', code in the ' else ' section will execute: so it will now check if the value is '1', if so, it turns on the first LED and turns of the second. If it's not '1' either, it will check if it's '2', if so, it turns on the second LED, and turns off the first one.

If it's not '2', it will check if it is '3', if so, it turns on both LEDs, otherwise, it executes the code in the last else section, and prints what values you should enter. Note that a double equality sign is used to check if two values are the same.

This is a very common mistake. You can try to figure out how it works yourself, and then check with the truth tables in the image above. Arduino reference: While. Arduino reference: If and comparison operators. Arduino reference: Else. Arduino reference: Boolean operators. In the last example, we used a lot of if Although this is the fastest way to do it, it's not easy to read. If you want to compare a variable to some given values, you can use a switch.

As you can see, the switch starts with the 'switch' keyword, followed by the variable you want to check between brackets. Between the curved brackets, our cases are defined. They use the syntax ' case value:', followed by the code you want to execute if the given variable equals the value in this 'case'. After the case-specific code, the ' break ;' keyword is used to close the 'case'.

Note that the value you enter can not be a variable. If you want to compare 2 variables, you'll have to use if-statements. The code in the ' default :' case is executed if the variable doesn't match any of the values of other cases.

This case is optional and can be omitted. Most Arduinos have between 6 and 12 analog input pins. They can measure voltages ranging from 0v to the input voltage 5v or 3v3 An the board, these pins are labeled A0-A5, and in case of the Leonardo, some of the pins on the digital side can also be used as analog inputs.

They are marked with a dot, and the name A6-A11 is written on the back. Note that reading an analog input is relatively slow, compared to reading digital pins. This is because the Arduino measures voltages using an internal reference voltage generator DAC, Digital-to-Analog Converter , and then comparing the input voltage to the reference voltage, then changing the reference voltage, comparing again, changing the reference voltage, comparing The resolution of the internal DAC is 10 bits.

A potentiometer, variable resistor or pot for short, is just a knob with a wiper that slides over a round strip of resistive material. This way, it varies the resistance between the wiper and the endpoints. We're going to hook up our potentiometers and faders as simple voltage dividers. You can read more on this Wikipedia page if you're not familiar with this principle.

If you take a look at the schematic, you can see 2 resistors. R1 is the resistor between the right pin of the potentiometer and the wiper center pin , and R2 the resistor between the left pin and the wiper. Take a look at the formula as well. This voltage can be read by the Arduino's ADC analog to digital converter , and represents the position of the potentiometer or fader. So basically, you connect the left pin to the ground, the right pin to the 5V pin on the Arduino, and the center pin to an analog input.

The only new function is analogRead pin. It's quite self-explanatory, it just gives you the bit value, representing the voltage on the given pin. You should see values ranging from 0 to Setting the delay to 10 or more, instead of 1ms may give you better results in the serial plotter. You can map the values from the range to any other range, for example from 0 to This can be done using the map function.

Take a look at example AnalogReadSerialMap. The syntax of the map function is: map value, lowerLimitInput, upperLimitInput, lowerLimitOutput, upperLimitOutput value is just the value to map, in this case our sensor reading. The range of this input is 0 to , so these are our input limits. We want the output to range from 0 to , so these are our output limits. This means that if the sensor reads , the output value will be To convert it to a ratio, we have to use a data type other than int: float floating point.

If we divide our sensor value by , we get a ratio between zero and one. We can either change the data type of the sensorValue variable to float, or convert the value from an int to a float. If we don't convert our value to a float first, the result will be treated as an int as well. Normal numbers like '1' will be treated as ints, unless you add a decimal point 1.

If one of the factors in a calculation is a float, the result will also be a float. To convert from a float to an int, you can do this in the same way: int number or int number , but note that this will just truncate the number at the decimal point, it will just ignore the values that come after the decimal mark. For example, int 1. To round the number, use the round number function, For example, round 1.

Since an analog value of corresponds to 5v or 3. This is used in the AnalogReadSerialVoltage example. To read voltages higher than 5v 3. Any values between 10k and k should work fine. If the resistance is too low, it will draw too much current, and influence the reading a lot. This gives me a maximum voltage of Note: if R2 is - for whatever reason - disconnected or interrupted, the high voltage will be connected to the Arduino directly through R1 and will probably damage or destroy it.

So don't change the resistors if a high voltage is applied. At the top of the file, there are some constant declarations. Like a variable, constants can store values of all sorts, but unlike variables, constants can not be changed while the program is running. This means they are stored in the program storage space, instead of the dynamic memory RAM , leaving more space for variables and arrays etc. Change the values of these constants according to your setup.

The ratio is calculated automatically, so you don't have to change it. You could use the color codes of your resistor to get a theoretical value, but you'll get a much better approximation if you measure the resistors using a multimeter.

The same goes for the operating voltage. The Arduino can't output analog voltages, it can only output either 5v or 0v. The Arduino creates a square wave, and then varies the on- and off-times of the wave. For example, 2ms on, 2ms off; or 1ms on 3ms off. This is referred to as the duty cycle of the square wave. You can now calculate the average voltage, which is the area under the curve divided by the amount of cycles. Divided by the amount of cycles still gives 2.

In other words, the average voltage can be written as the duty cycle multiplied by the supply voltage. These average voltages are indicated in red in the images. Your eyes are way too slow to see the underlying square wave, so this method is perfectly fine for dimming LEDs.

Turning the potentiometer dims the LED. The only new function is analogWrite pin, duty cycle. It just sets the duty cycle of the pwm on a pin. Another example is analogWriteDimmer. Connect a pushbutton to pin 2, and an LED to pin 5. When you press and hold the button, the LED gets brighter, when you release it, then press and hold it again, its brightness will decrease. Up until now, we only used components that draw very little current.

But if we want to drive things that draw more than 20mA, the current supplied by the Arduino's output will not be sufficient. We'll need some sort of current amplifier, and that's when the transistor comes in. When you want to drive things that run off a different voltage than the Arduino, you'll also need a transistor. A small current from the base to the emitter results in a much higher current from the collector to the emitter.

More information can be found in step 3. Before we can start to build our transistor circuit, we'll need to know the voltage and current draw of the load you want to use.

Connect your load motor, fan, LEDs, bulb, heating element, solenoid Voltage is measured in parallel with the load. Now disconnect the negative wire of the load from the power supply. Then plug the red wire of your multimeter into the amps connector, and set it to DC amps. Then connect the black wire of the multimeter to the ground of the power supply, and the red wire of the multimeter to the negative wire of the load.

Current is always measured in series with the load. Note: remember to plug your red wire back into the voltage connector of the multimeter, if you try to measure voltage with the current input, you will basically create a short circuit , and blow the multimeter's fuse or even completely destroy it.

Some higher end multimeters will beep and show a warning when it's set to voltage while the current connector is used. A transistor has a certain current gain, typically around There should be a graph called "DC current gain".

Note that most of the time, a logarithmic scale is used. Now take the supply voltage of your Arduino, and subtract 0. This is because the base-emitter silicon junction of the transistor has a voltage drop of mV. You don't have to know why this is, just know the voltage drop is there. Now use Ohm's law to calculate the resistance of the base resistor. As you can see in the second graph, at mA, the current gain is about In most cases you don't want to use the formula above however: this is only true in the linear region of the transistor.

It called is linear because the relation between the base current and the collector current is linear. If the base current goes up, the collector current goes up as well. This is also called the active region. However, there comes a time when an increase in base current will no longer cause a further increase in collector current: we say that the transistor is saturated, or in its saturation region.

This means that a maximum amount of current flows from the collector to the emitter, and that the voltage drop across the collector and the emitter is at its lowest. This is what we want if we're using the transistor as a switch. The lower the voltage across C-E, the lower the power dissipation in the transistor, which is a good thing. This voltage is approximately mV, but in practice, this varies widely depending on the transistor model, current, temperature etc.

If you know the base current and voltage, you can now use Ohm's law to calculate the required base resistor.

For example:. To drive a mA motor, for example, we'll need a base current of 0. Using Ohm's law:. Note : When you use an inductive load , like a solenoid, relay, motor etc. This is to prevent the transistor from being damaged by voltage spikes caused by the solenoid. You get self-inductance inside of the coil when it is suddenly turned of.

Note : the Arduino's ground should always be connected to the ground of the transistor circuit. However, at 5v, the drain current might be enough for your specific application. The gate acts like a capacitor, so when turning it on and off, there's a very high current that flows into or out of it. To prevent this, a resistor is connected in series with the gate. To ensure fast switching, this gate resistor should have a relatively small resistance, ohms, for example. Just like with normal transistors, you should also use a flyback diode when switching inductive loads.

To get higher currents, we have to get a higher electric field, so we need a higher gate voltage. We could use transistor to do this, but it is easier to use an opto-coupler, or opto-isolator. This is basically an infrared LED and a phototransistor light sensor in one package. When the LED is turned on, the phototransistor conducts, so the gate is now at a higher voltage.

It looks just like an IC with only 4 or 6 legs. Using an opto-coupler also means that there is no electrical connection between the Arduino and the MOSFET, so if the higher-voltage circuit fails, it is almost impossible that it gets to the Arduino, and destroys it. This is a great advantage. It is possible to use them with 3.

Since the relationship between gate voltage and drain current is far from linear, as you can see in the graphs. The Arduino can't drive a relay directly, so you'll need a small transistor. A relay is an inductive load, so you'll need a flyback diode to protect your transistor. Use the method above to calculate the base resistor. Warning: Wall power can kill you, if you're not careful enough. Never leave V or V connections exposed, and unplug your circuit when you're working on it.

In the previous paragraphs, we only used NPN transistors, that take a positive signal to turn on. PNP transistors on the other hand turn on when a negative voltage is applied to the base, resulting in a "negative" base-emitter current. It is still only the electrons that move, and a positive charge just means the absence of electrons. Take a look at the image above for the schematic. Just like with the NPN variant, the arrow at the emitter indicates the direction of the current.

With the techniques explained in the previous step, you can only spin the motor in one direction. To change the direction, you'll need an H-brigde sometimes called a full bridge. When input A is low, transistor Q1 doesn't conduct. The left terminal of the motor is now connected to ground, via Q3. When input B is high, Q4 conducts. This means that the gates of Q5 and Q6 are low. They are connected to ground through Q4, and R2 doesn't matter in this case.

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