RFID readers work only when Arduino supplied over USB

Projects General Guidance
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Hello community! I made a system based on two Arduino Nano boards. I use both Arduinos for RFID purpose. Simplified block diagram is shown on picture

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Problem occurs whenever I try to supply system with something else than USB. I tried all possible combinations:

  1. 9.5v 3 amp power supply to DC DC step down which provides 5v and 3.3v outputs and then its outputs to Arduino 5v pin and RFID readers
  2. 9.5v 3 amp power supply to Vin Arduino and then 3.3v output from Arduino to RFID readers
  3. 9.5v 3 amp power supply to Vin Arduino and to DC DC step down which provides 3.3v output and then its output to RFID readers
  4. 5v 2 amp phone charger to USB Arduino inputs and then 3.3v output from Arduino to RFID readers.

Nothing works! Only case that works is when I power Arduino with USB from my laptop and then power RFID readers with 3.3v output from Arduinos.

I checked the wiring, everything is good (p.s. grounds are properly connected). Any ideas?

Thanks in advance!

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Show circuit diagram and code.

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I will, thanks! Any ideas in meanwhile? Are you suspecting to something and you need code and diagram to confirm your suspicions or you need them as input to get an idea? @bluejets

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The powering is not good enough. Please post a wiring diagram, schematics. Most interesting is the powering.
Writing and sending You a book of all possible errors is not the way we work.

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Add decoupling capacitors on the supplies to the readers and space them at least 1 meter apart.

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Thanks! Any suggestion for capacitance? Regarding spacing, unfortunately I don't have enough wire. The max what I can do is half of meter of spacing. But, I don't see it as problem, since it works on same distance when powered over USB.

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Here is schematic. Sorry for it is not the prettiest one, but I think it is readable (just double click on it to make it really large). Pay attention that it also contains physical implementation (for example, if you take a look at the ground connection of relays, it is drawn from DC step down module, not from breadboard. In theory, it should be the same, but in practice, sometimes is not). Also, I led the ground directly from power supply to breadboards (i.e. I avoided leading it through DC step down)

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Thanks. That wiring show some not good things. The 9.5 volt power is stepped down to 5.0 resp. 3.3 volt. So far, so good.
But.... This regulated voltage can't be connected to Vin. It should be connected to the Vcc pin, 5 volt pin or a 3.3 volt pin.
Okey, some 3.3 volt controllers accept 5 volt into their Vin.

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A tip. The breadboard brings nothing to the logic in the wiring. You can drop that.
A general rule is: Never let motor currents pass a breadboard.

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That lock needs a kick back diode if it's inductive....
9.5 volt feeding the relay modules looks odd. A common voltage would be 12 volt.

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Just the standard ceramic 0.1uF and a parallel 100 to 470uF across the supply, and on each Arduino, and reader.
This is why it is needed http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html

A step down regulator causes a lot more noise on the power supply which in turn can interfere with the reader’s working. When I worked in the access control industry the perceived wisdom was not to use them. I once had to visit a site where the readers were not working so well and it turned out to be high frequency oscillations on the power supply output. The power supply was not a switched one but it was a new design we subcontracted in and we found out that in certain circumstances it could oscillate.

It isn’t, the wire going from the step down into the Arduino is labelled as “input pass through”, so I am assuming it is 9.5V.

It would be good to know exactly what step down module you are using.

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Really good tip. Thank you!

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You mean like this? I added capacitors to existing schematic. Can I add one of large capacitance (i.e. 100uF or 470uF) instead 4 of 0.1uF on RFID readers?

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@Railroader Grumpy_Mike is right, Arduino Vin is fed with 9.5v, it is only passed through one channel of DC step down.
P.S. here is DC step down which I use https://www.alibaba.com/product-detail/HW-228-Multi-output-DC-DC_62306359590.html

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I agree. I initially started with 12v, but power supply has potentiometer which allows decrease of voltage down to 9.5v. I did this in order to increase life of DC step down modules. I expected that lock and LED strip wouldn't work on that voltage (since they are declared to work on 12v), but as the matter of fact, they work :slight_smile:

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No.
The large and small capacitors need to be in parallel with each other and be placed as close to the interfering item as possible.

No. Did you not read that link I posted? Or did you not understand it?

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Hi,
Welcome to the forum.

Do you have a DMM?
If so, what do the 5V and 3v3 power lines measure when in fault mode?

Thanks.. Tom... :smiley: :+1: :coffee: :australia:

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Sorry, I read in rush. I read once again and did like it is suggested. So far, so good :slight_smile: I will perform couple of more tests and provide feedback, but it seems it will work :slight_smile:

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Hello. I checked the current consumption in well operating mode and in fault mode. In operating mode, current consumption was around 50-60 mA. In fault mode, around 20 mA. But, it seems that decoupling capacitors will solve the problem :slight_smile:

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Hi,
Okay, but check the 5V and 3v3 lines, their voltage.

Tom... :smiley: :+1: :coffee: :australia:

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I soldered 0.1 uF and 100uF capacitors in parallel. I created 3 pieces of that junction. One I added to the input of DC step down module (i.e. betweeen GND and 9.5v line that comes from power supply) Second one I added at the 5v output of stepdown module and the third one at the 3.3v output of stepdown module. I checked datasheet of integrated regulators in stepdown module and I concluded that one step down is enough, so second step down is removed from circuit. Also, I decided to power Arduinos with 5v output from step down module (i.e. I led that line to 5v pin of Arduinos). After all of that, my circuit started to work! Special thanks to @Grumpy_Mike :heart: thanks to the all other guys that offered their help! :blush: