I am trying to program a 4DOF robotic arm (using 4 servo motors). I would like to move the servos using an Android app, by means of bluetooth (HC-05).

I developed the app, in whose interface there are 7 buttons (Up, Down, Left, Rght, Forward, Backward, Save) and a field in which you can type using the keyboard (I attached an app interface screenshot).

So far, I tried to manage only Up button (which senses 'U' via bluetooth to Arduino) and Down (which sens 'D'), which would move the servo intended for vertical arm movements. That code works quite well, although the app crashes sometimes, but I still can't figure the reason out. Using the println() function I can see that Arduino reads in the bluetooth input the character '⸮ ' , what does it mean? I use a baud rate of 9600.

Then, what I would like to do is: once I have moved the servo and reached a particular position, I'd like to save the position of the motor into an array. In order to allow multiple positions saving, after Save button is pressed, I want Arduino to wait for another input from the bluetooth (so that, it would wait for another button to be clicked from the app or for something typed into the dedicated bar from keyboard) that defines the position within the array, in which I save the servo position (eg: I type '2' into the dedicated app bar and Arduino, which is waiting for an input, receiving it, saves the last servo position into the array servo01pos[2]). I wrote the "case ('S')" code that is quite long and inefficient but, at least, it works.

Thanks in advance to anyone who will help me.


#include <SoftwareSerial.h>
SoftwareSerial bluetooth(10, 11); //BlueTooth RX ,TX
#include <Servo.h> // servo library

unsigned long timestampSend = 0; // BT Write wait period
String readString;
Servo servo01; // servo name
byte pos01 = 90; 
#define ANGLE_STEP 10 // 10 degree steps
int servo01pos\[50\];
char data;
char button;
int x = 1;

void setup() {
  Serial.println("Ready ...");
  Serial.println ("\n");

void loop() {
  do {
    if (bluetooth.available() > 0) {
      do {
        data = bluetooth.read();
      } while (bluetooth.available() > 0);
      if (data == 'U' || data == 'D' || data == 'L' || data == 'R' || data == 'F' || data == 'B' || data == 'S' || data == 'X') {
        Serial.print("Received value: ");
        switch (data) {
          case ('S'): //SAVE
            Serial.println("Enter the button you desire to set...");
            do {
              data = 254;
              button = bluetooth.read();
              if (button == '0' || button == '1' || button == '2') {
                data = 'S';
            } while (data != 'S');
            Serial.println("SAVING POSITIONS ...");
            if (button == '0') {
              servo01pos\[0\] = pos01;
            } else if (button == '1') {
              servo01pos\[1\] = pos01;
            } else if (button == '2') {
              servo01pos\[2\] = pos01;

          case ('U'):
            if (pos01 <= 180 - ANGLE_STEP) {
              pos01 = pos01 + ANGLE_STEP;

          case ('X'):
            x = 0;
  } while (x != 0) ;

App interface

  • 1
    When your app crashes, it might send garbage data via serial, which then gets interpreted as a reversed auestion mark. You should look into the logs of your app to check, why it crashes. What is your question about the rest of the project? You didn't formulate one there
    – chrisl
    Nov 20, 2020 at 18:24
  • Thank you for your answer. I tried with the catlog but no problems seems to occour (I'll try again just to be sure). My main question is just about the reversed question mark. The second part is just to know if there are others easier way to do what I would like to do in the case 'S'. Many thanks. Giammarco
    – Giammarco
    Nov 21, 2020 at 10:34
  • I just tried using a random app downloaded from PlayStore (and using the same Arduino code) and I still have that problem. In that case I think I can say that it is not a my app problem, is it?
    – Giammarco
    Nov 21, 2020 at 10:51
  • I think I solved the problem using a power supply. It seems that the reversed question mark occurred when there was a power outage. Now it seems to work properly. Thank you very much again for your help! Giammarco
    – Giammarco
    Nov 22, 2020 at 10:24
  • 1
    Please write that solution into a proper answer and accept it as correct. That helps us keeping track, which questions are already answered.
    – chrisl
    Nov 22, 2020 at 14:06

1 Answer 1


The character '⸮ ' was given everytime there was a power outage. So, it is enough to use an external power supply to power bluetooth and servos (Arduino cannot power all those modules simultaniously).

  • As chrisl mentioned, you should mark your answer as accepted.
    – timemage
    Nov 23, 2020 at 12:36

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