I have a circuit which basically have to link up to MFRC 522 module, Arduino Nano and also the relay 2 channel module.

See below for the basic schematic.


simulate this circuit – Schematic created using CircuitLab

Apologize for some of the miss out pins as I wanna use the schematic as illustration only.

AC - DC module is using SMPS power supply which come out as 15V. So i use two voltage regulator 12V and 3.3V. 12V to power on Relay module and VIN to Arduino NANO. Another separate 3.3V to serve power to MFRC522 as it is a 3.3V module.

2 Channel Module is opto-isolated Relay module. However, I only uses power supply with all common ground.

All is working as usual at first, MIFARE card is able to be detected via the MFRC522 module and relay is able to turn on.

However, after few round of trial out, the MFRC522 is not responding. Even hitting reset on Arduino Nano, there is no respond. I need to turn off the SMPS 15V supply then turn back on, then it is able to operate back.

As i suspected on the 12V relay module, i remove the 12V to power the relay module. MFRC 522 is working as expected which dont have the intermittent case anymore.

My operation mode is as follow: 1. Scan a MIFARE card, turn on relay to drive a motor then after 3 seconds, the relay will turn back off.

I did not connect any load to the relay yet with only the relay coil energize which causes the problem.

It seems like only with the power off and then it will work back on. As if the relay coil generate noise to the power supply and then the SPI signal does not work.

I understand that we could use separate power source to power on the relay module with separate grounding. If i would like to do with one power source, how can i shield the relay coil noises to not impact the MFRC 522.

This is the module I am using. Change it to 12V relay instead of the 5V.

Take note again that if i remove the 12V to the Relay module that flows from transistor Collector - emitter to turn on the relay coil, it works perfectly as expected.

Any recommendation to help shield the noises from the relay coil ?

I have tried to add capacitor to the AC-DC 15V output and 3.3V output as well but it is not working. Need more advise here.

  • \$\begingroup\$ It sounds like you are trying to draw too much current from the 15V supply and it is shutting itself down. Another possibility is that you are drawing too much current from the 7812 and it is overheating. You haven't provided enough information to be more specific. \$\endgroup\$ Jun 11, 2019 at 13:36
  • \$\begingroup\$ Power supply is rated at 1A but i only turn on 2 relay with 12V. wont think that is going to overheat. \$\endgroup\$
    – Nick
    Jun 11, 2019 at 14:07


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