Controlling a Solenoid Valve from an Arduino. Updated.

A very short post I made sometime ago has been more popular that it should have been. It wasn’t particularly detailed and it was in desperate need of an update. So here is the update, this time giving more details and adding mosfets.

Arduinos are limited in the current and voltage they can supply. A typical 5V Arduino can provide 5V and 3.3v at a maximum 40mA from a single pin. 40mA is the maximum and ideally the current draw should be kept to around 20mA. 20mA is fine for a LED but not so good for motors, solenoid vales or long strings of RGB LEDs. For these we need a separate power supply and a way of controlling it.

One of the problems many beginners have is the Arduino does not give a warning when another device wants more power than it can supply, it simply tries to provide the current the device wants. This leads to over heating and mini explosions (or at least a dead Arduino and the smell of burnt plastic).

In this post I am using DC voltage solenoid valves only. Valves that use AC have very different requirements and the below does not apply. Most DC solenoid valves tend to use a voltage any where from 6v up and 12V and 24V are very common. The one I am using below is 24v. The Arduino cannot handle these voltages and so cannot (or should not) be connected directly to the valve. Instead, we use another device as a middleman, one that can accept the 5v signal from the Arduino as a control signal and also handle the higher voltage and current required by the valve. There are various different devices that can fulfil this role. I am using transistors. The Arduino controls the transistor and the transistor controls the valve. A simple way of thinking about this is we are using the transistor as a switch. The Arduino is controlling the switch and the switch is controlling the valve.

There are many transistors you can use and you pick one depending on the voltage and current you need to control. Here I am using the good old TIP120 and the IRFZ44N mosfet. Why these 2 devices? Because they are what I have. They were purchased sometime ago purely because they are suitable for use with 5v Arduinos.

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Arduino Serial Part 4: ASCII data and using markers to separate data

In part 3 we sent and received single characters to control LEDs using a fairly simple technique. If all you need is to remotely turn a few things on and off then this method is probably the best. It is simple, easy to program, and reliable. Sometimes though single characters are not enough and we need to use more complex commands or we may want to send sensor data that comprises more than one character.

In this post I look at a few different techniques for sending complex data and commands; starting with functions that are built in the Arduino language and moving to our own functions that, IMHO, perform better and allow for better code.


Multiple Characters

A common mistake a lot of beginners make is to test data before you have it. When receiving more than one character via serial, it is easy to assume that all the data arrives at one time. It does not. When a device sends “HELLO” it is sent one character at a time and received one character at a time. The receiving device then has to put all the characters together to form the word “HELLO”. By Arduino standards serial is very slow and the Arduino is capable of performing thousands of tasks in the time it takes to receive all the characters. This means if you are not careful your code can start checking the received data before you have actually received it.

Another problem I have seen is thinking a reads all the available data. It doesn’t. It reads one character or byte only and it is up to you to read all the data and put it together.

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Arduino Serial Part 3: Getting started with serial communication

In the last post I briefly talked about different data formats and how I recommend keeping things as simple as possible. With this is mind for a first project let’s create a simple blinking LED. We will have one Arduino controlling an LED on a second Arduino. Controls to turn the LED on or off will be sent via serial from the first Arduino to the second Arduino. This is as basic as it gets. Arduino Blink by remote control. The LED has only two states so simple control codes can be used and to start I am using 1 of on and 0 for off.

In these examples I am using Arduino Nanos but any kind of Arduino can be used and for this series I am using Arduino to Arduino communication. The techniques are exactly the same for any UART to UART device. For example, in Arduino to Arduino by Bluetooth I use exactly the same serial communication techniques wirelessly over Bluetooth.

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Arduino Serial Part 2: Serial Data

In the previous post I went through the basics of using serial on an Arduino and ran through the different commands. In this post I want to talk about different types of serial data and some of the things you should consider before starting to create code. The type of communication you use or can use will depend largely on the project but there are things that can be considered before starting.

  • Type of communication? 1-way or 2-way
  • Type of data? Values or strings? Simple or complex?
  • How much data and how frequent? A couple of values every few seconds or a high rate continuous stream.
  • Is the data critical? Must you be sure you receive all the data or can you afford to lose some of it.

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Arduino Serial Part 1

Updated 26.01.2019

Here we look at using serial communication on the Arduino. Serial UART is one of the various ways an Arduino can communicate with other devices. This includes a host PC and using the Arduino serial monitor is communicating with the PC using serial UART.

Arduino Serial Monitor
End Of Line Characters
Formatting output using the tab command
How fast is serial
Different Arduino Serials
   Hardware Serial/Serial
Using a software UART and usb adapter to talk to a PC
Buffer Size
Serial Commands

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Arduino with RN4870/1

RN4870_01_800The RN4870/1 is a small (only 12mm wide) BLE module from Microchip. What makes this a little bit special (when compared to modules like the HM-10) are the advanced features that allow you to create your own services and characteristics. This opens up true BLE functionality. It has been available for a while now and I am surprised it is not more popular in the hobby area.

The RN4870 is very different to common hobbyist modules like the HM-10, AT-09, and BT05 and if this is all you have used you may need a refresher on BLE. Especially if you want to use your own services and characteristics.

This is a first look / getting started guide that I will add to as I get time.
Latest update: May 27, 2018

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ESP8266 and the Arduino IDE Part 6: JavaScript and AJAX

ESP8266_6_01_360If you have followed the previous posts you will have a working, fairly robust, LED control. If you haven’t gone through the previous posts you can find links just below.

The webpage works well but the whole page reloads to update the button and controlling a single LED is not ground breaking. In this post I address the page reload by adding AJAX/JavaScript and in the next post I will add more controls.

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ESP8266 and the Arduino IDE Part 5: adding wifiManager


In the previous guides we connected the ESP8266 to a local network using hard coded credentials. It is fine for messing around with examples and when developing sketches but not very convenient or practicle for final projects.

What happens if you want to move the ESP8266 to another network or if you buy a new router? You need to change the sketch and re-upload. It would be better if we could pick the network to use at run time. This is exactly what WifiManager allows.

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Arduino, HM-10 and App Inventor 2: Adding a slider

This post continues the Arduino, HM-10 and App Inventor 2 guide. A few people asked about adding a slider so here it is. Please beware; this is not a basic guide for using a slider in App Inventor 2. I am adding a slider to an existing app and have certain things (semi advanced) that I want to achieve.

Arduino_HM-10_AI2_Adding_Slider_33 - updateApp_04

In the previous guide we created a basic Android app to control 3 LEDs. The app was developed in such a way to make adapting it as easy as possible. So let’s see if that is true and add a slider to control the brightness of one of the LEDs. We start with extending the app and then update the Arduino sketch.

Using the same command method as before, we will create an ascii command and send it to the Arduino. The command will be in the form “[Snnn]”. Where S is used to denote slider and nnn is a value from 0 to 255, or more specifically “000” to “255”. The Arduino will read the command and set the LED brightness accordingly. Using ascii for this keeps things fairly simply but it is not the fastest way to do it. The slider value will be from 0 to 255, this is actually the same value range as an 8bit byte. Using the ascii command we need 6 characters (including the square brackets). To send the actual value only would require 1 character (2 with a label/marker character). If all you are using is a single slider then there is no reason to use ascii.

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Create A Bluetooth Joypad With App Inventor 2

BluetoothJoyPad_draftLayout_01_800Here we create a basic Classic Bluetooth joypad for use on an Android device to control a microprocessor (Arduino) connected to a Bluetooth module. The app is fairly simply. There are 2 screens; the first is the main control panel and the second is a connection page.

When the CONNECT button on the main screen is clicked the second screen is opened to allow the user to connect to a Bluetooth device. The direction buttons, when clicked, transmit codes to the Arduino.

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Arduino, HM-10 and App Inventor 2

Hopefully this guide will give you a good introduction to using the HM-10 with App Inventor 2. I also hope that this takes you beyond the usual starter guides that do not go past very basic information.

Although I am using an Arduino the principles will be the same for any other microprocessor or indeed for using the HM-10 on its own. Warning: This is going to be a very long post.


To use this guide you should be somewhat familiar with App Inventor, have a BLE enabled Android device, and of course have an Arduino and a HM-10.

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ESP8266 and the Arduino IDE Part 4: Connecting to an ESP8266 with unknown IP address using mDNS

In the earlier parts we got the ESP8266’s IP address by displaying it in the serial monitor. This is OK for examples and development but not practical for real life projects. There are a few ways to get the IP address, here I look at mDNS. mDNS allows you use to connect to the ESP8266 using a name or url.

This sounds great, and it is unless you are an Android user. More on that later.

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Switching Things On And Off With An Arduino

One of the first projects many people new to the Arduino do is blinking an LED and there many many guides on line. Unfortunately, many of the guides never go beyond the very basic first sketch. In this guide, I hope to help new users take the next step.

Besides the obvious fact that blinking an LED is cool in its own right it is a good exercise because switching an LED on and off is the same process for switching any digital device on and off. Once you can create the code to blink an LED you can create code to turn anything on and off. Of course, you do not need to control an LED, you can use the same methods to do almost anything that is controlled in the same way. For example, I use similar techniques when setting up remote controls using Bluetooth and wifi connections and instead of setting a pin state I send control codes.


Polling vs interrupts
Connecting Arduino pins directly to vcc
Polling. Example 01: Very simply press for on, release for off
Polling. Example 02: Press for on, release for off. Slightly refined
Polling. Example 03: Toggle switch
Polling. Example 04: Multiple states from a single push button switch
Polling. Example 05: Start and stop an action
Interrupt. Example 01: Turning an LED on and off
Interrupt. Example 02: Turning an LED on and off with debounce

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HC-05 with firmware 2.0-20100601

There are now many cheap and no brand HC-05 modules that use the 2.0-20100601 firmware and rather than keep making the same post for different modules I will have a single main post which, as I update and add to it, will become a single point of reference.

The 2.0-20100601 firmware is originally by HC/Wavesen and as the version number may suggest it is from 2010. I am not sure if other manufacturers use it under license or simply copy it since the genuine HC/Wavesen modules now use version 2.1 firmware.

HC-05 with 2.0-20100601: AT Command Mode
HC-05 with 2.0-20100601: Connecting to an Arduino
HC-05 with 2.0-20100601: Initial Communications Test
HC-05 with 2.0-20100601: Main AT Commands
HC-05 with 2.0-20100601: AT Commands Dealing With Connections
HC-05 with 2.0-20100601: Making A Connection

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