The High/Low input of Arduino's digital I/O is driven by the FlashAir's GPIO H/L output. Another digital I/O pin of the Arduino will act as a PWM output to the drive motor of model railway vehicle through a full-bridge driver.

We can also control the electromagnet at the switch point through a full-bridge driver.

Controlling Model Railway with FlashAir GPIO


Flashair CONFIG setting

To enable the GPIO of FlashAir, add 「IFMODE=1」 to the CONFIG file of SD_WLAN in a folder in the FlashAir.

ref. IFMODEof command.cgi

Connect FlashAir and Arduino

The GPIO terminal of the FlashAir is connected to the digital I/O pins of the Arduino.

Bit Assignment FlashAir Arduino
0x01 CMD digital I/O pin No.2(D2)
0x02 D0 digital I/O pin No.3(D3)
0x04 D1 digital I/O pin No.4(D4)
0x08 D2 digital I/O pin No.5(D5)
0x10 D3 digital I/O pin No.6(D6)

In order to use an SD Card with Arudino, we used a SDcard DIP module from Akizuki .

ref. Akizuki SDcard DIP module


FlashAir in SD card slot DIP module


Full circuit diagram

GPIO output of FlashAir

From a browser that can connect to the to FlashAir you can pass GPIO commands by accessing the appropriate URL such as http://flashair/command.cgi?op=190&CTRL=0x1f&DATA=0xXX, which set H/L output of GPIO pins. To make this less combersome, I made a JavaScript HTML file List.htm and put it on the FlashAir which has buttons a user can press to set the GPIO register and thus train operations accordingly.


List.htm displayed on the iPhone

Button Behavior and Roles
STOP Emergency stop button. Such as when the vehicle model train was derailed, in order to stop immediately.
DOWN Slow down. Arduino drops speed gradually at once pressed down.
RUN To maintain the current speed.
UP Speed up. Arduino increases the speed gradually at once pressed down.
Left Switch the running direction to the left.
Right Switch the running direction to the right.
SW_L Switch Point( rail open direction ) to the left.
SW_R Switch Point( rail open direction ) to the right.
Off All of the GPIO to Low.
Clear To reload the screen.

JavaScript for Train Operation Webpage

<script language="javascript" type="text/javascript"> <!—
//Function:HTTP communication to FlashAir
function flashair_get( param ){
  var request = new XMLHttpRequest();
  request.open("GET", param, false);

//Global variable definition
var send_mess = "http://flashair/command.cgi?op=190&CTRL=0x1f&DATA=0x"; 
var p = 0x00;  

//Function:to any port in the H/L
function gpio_onoff(port){
  switch( port ){
    case 1:  //Emergency stop
             p |= 0x01;
             document.getElementById('RUN_STOP').style.backgroundColor = '#aa0000';
             url = send_mess + p.toString(16);
    case 20: //point control High: opposite direction
    case 21:  //point control Low: default direction
    case 30:  //running direction High: right
    case 31:  //running direction Low: left
    case 4:  //running control High: speed up
    case 5:  //running control High: slow down
    case 6:  //running control Low: coasting
    document.getElementById('RESULT').value = p.toString(16);

Definition of GPIO input status and behavior

In this case, five operations shown in the following table has been achieved so as to correspond to each GPIO. The operation when each GPIO has become HIGH defined as shown in the table below.In the case all of the GPIO is LOW, it has to maintain the current status( speed and direction ).

Bit Assignment FlashAir HIGH/LOW Behavior
0x01 CMD HIGH Running stop of the model railway vehicle
0x02 D0 HIGH To switch the direction of the point to the left.
LOW To switch the direction of the point to the right.
0x04 D1 HIGH To the running direction of the railway model vehicle to the left.
LOW To the running direction of the railway model vehicle to the right.
0x08 D2 HIGH Increase the running speed of the railway model vehicle.(speed up)
0x10 D3 HIGH Reducing the running speed of the model railroad vehicle.(slow down)

Arduino Flowchart

Check GPIO input status on the Arduino

In Arduino, always check the GPIO input status of FlashAir in a loop function. Check the status of No.2~6 pins of H/L of digital I/O, so that you designated operation is performed.

const int sdcard_gpio_pin[5] = { 2, 3, 4, 5, 6 };
const int sdcard_gpio_pin_num = 5;
void loop() {  
  for(int idx=0; idx < sdcard_gpio_pin_num; idx++)
    if(digitalRead(sdcard_gpio_pin[idx]) == HIGH){
      if(idx == 0){
        status_stop = 1;
      else if(idx == 1){
      else if(idx == 2){
        if(status_LR != 1){
          status_LR = 1;
      else if(idx == 3){
        funcspeed(0); //slow down
      else if(idx == 4){
        funcspeed(1); //speed up
      if(idx == 0){
        status_stop = 0;
      else if(idx == 1){
      else if(idx == 2){
        if(status_LR != 0){
          status_LR = 0;

Let's Get Moving

The output of the full-bridge driver for the PWM output that is connected to the railway input and the output of the full-bridge driver for point conversion to connected to that point. We are using a Kato Precision Railroad Models (KATO) set but you can connect other sets in the same way, such as Tommy Tech (TOMIX).

In the photo, a personal computer is connected for debugging with the serial monitor, it is not needed for the operation of the model railway. In addition, the iPad connected to the FlashAir displaying the operation web page described above.


Operating the model train with a Flashair can be seen in this video:


In this case you can operate the model railway as driving the motor by the PWM output of the Arduino. For example, it would also be possible to operate a model car with a FlashAir.

This article was contributed from Hiro Ayase(@ayasehiro).(2015/08/01)
For all inquiries, please contact to the author.

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