I've successfully implemented Door Unlock system described in this tutorial. At first I used Arduino Uno and RFID RC522 Reader and MG90S Micro servo.

When I used Arduino Uno with Arduino Pro Micro for arduino ATmega32U4 5V/16MHz, the RFID reader stops working after plugging the servo. Since Arduino Pro Micro have only 5V output, I used resistors to downgrade 5V to 3V in order to poser the RFID reader. I used same output 'vcc' to select 5V for the servo and 3V converted to the reader.

The problem is that each component works perfectly independent of each other, however, when I place them together they stop working.

  • 1
    Which voltage did you "downgrade"?
    – Majenko
    Commented Aug 30, 2016 at 14:28
  • 5v to 3v, so I can power the rfid reader
    – Nasreddine
    Commented Aug 30, 2016 at 14:50
  • With resistors?!?!
    – Majenko
    Commented Aug 30, 2016 at 14:51
  • 2
    With a voltage regulator. Resistors cannot do what you want.
    – Majenko
    Commented Aug 30, 2016 at 14:53
  • 1
    You use resistors when using resistors is a viable option and when the circuit demands resistors. They are not, can not be, never will be, nor ever have been, a way of changing a power supply voltage.
    – Majenko
    Commented Aug 30, 2016 at 14:59

3 Answers 3


Use a 3.3VDC regulator for your RFID board a small boxed DC-DC buck converter will be easiest, if linear use an LDO type. Ideally also use a level shifter on your data lines. This could be a resistor voltage divider or zener diode clamp at the most basic, FETs or a dedicated IC is a bit more involved but gets better results.


The code you are using is not complete. You should check the second code given at Interfacing of RFID RC522 with Arduino UNO. In this second code they have a function:

void dump_byte_array(byte *buffer, byte bufferSize)

In this funtin place your servo Motor Routines. Like when First card is detected then move serbo in one direction and when second card is detected then move your servo in opposite direction. I have tried it and it works perfectly fine.


The current may be not enough. Try using extra power supply for Servo Motor

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