RFID read range vastly better when Arduino is plugged to mac USB

This is very strange. I can’t think of any reason why this would happen.

I’m hacking my electronic dog door to use RFID instead of the buggy ultrasound it’s using now. For this I need a fair read range on the dogtag type RFID tag. I figured I need about 4 to 6 inches (10-15 cm) of reliable read range to make sure the dog isn’t there scratching at the door unable to get in.

My setup is the ardunio nano and RFID reader from Priority 1 in Australia and a 16cm loop antenna also from them.

When I have the nano plugged in to the Mac’s USB port and getting power from that, I’m able to get a read range of between 13-14cm. Pretty darn good and I think I’m ready to work on the install.

But when I detach the nano’s usb and plug it in to a 5v power supply (+5v at VIN), the read range drops down to 6cm and sometimes a lot lower.

When I have the nano plugged in to the Mac’s USB port and also plugged in to 5v power, I see the Mac USB amp load drop to 0 - so it should be getting power from external, but is still connected to serial. Like that I get the bigger read range.

If I plug the nano usb into a usb wall wart power supply the range is dropped down.

This rfid reader can work without the arduino and tell me when it reads. And without the arduino at all it’s also short range.

Something else must be going on in the USB connection to a mac that’s different than when it’s unplugged from the mac… something that isn’t about the power. But what? And why would it effect my read range?

My next test was powering the rfid reader with the 5v supply and the arduino with the mac’s usb, but have common gnd. With the rfid reader connected to the same gnd as the usb we have a longer range again of about 10-12cm.

Is my issue still just bad power coming from the external power supply? Should I be using some sort of cleaner power conditioner module?

crickets....

It appears to be the dc power modules I'm using. Which one of them I bought from sparkfun specifically to power arduino projects.

I just tried a step down regulator with a 9v battery and I got 14cm reads. So it's absolutely dirty power.

I will need to find a way to have clean dc power from a wall power supply. Suggestions are welcome...

Are you powering the Nano with 5v at Vin? The docs say if you want to power it with 5v, use pin 27 (5V), not pin 30 (Vin). http://www.arduino.cc/en/Main/ArduinoBoardNano

SurferTim: Are you powering the Nano with 5v at Vin? The docs say if you want to power it with 5v, use pin 27 (5V), not pin 30 (Vin). http://www.arduino.cc/en/Main/ArduinoBoardNano

Thanks I will try that. See if it changes anything.

Ok.. solved.. it is RF interference from the AC power.

How I solved it was by connecting the ground from the arduino circuit to the ground of the actual ac. When I do that my read range goes way up. If the power adaptor had 3 prongs it would probably just work out of the box.

I tried several different power regulators and such, but none of them helped at all. I don't have a scope but I suspect that there is RF noise on the dc line and that grounding it clears that up.

If anyone knows of a wall wart type power source that does use the ground wire to clear that up let me know and I'll order it. Thanks!

You might find better power filtering would help - RFID sensing circuits are looking for tiny voltages superimposed on very large voltages, its important to preclude power supply noise. In fact you probably are best using a 5V linear regulator to power the thing, not a 5V switchmode supply - so something like a 9V wall wart into a 5V linear regulator to clean it up.

I think I was having a similar problem with IR emitters. I was trouble shooting for hours before I realized (or at the very least, suspected) that part or all of the problem was coming from the fact that I was powering my Arduino with a 9v battery through Arduino's DC input. My results were much more reliable once I switched to power from the USB from a Mac.