Autonomo & Grove - 125KHz RFID Reader

Just tried to connect the Grove - 125KHz RFID Reader to the Autonomo Groove shield ( at D0-D1). The red LED is continuously on, the reader is in Uart mode. However on my serial port I don’t receive any data and the LED does not blink when I hold a tag before the antenna what I would expect. The serial port has been tested with a loopback and works fine.

Closer investigation of the reader specs shows one possibility: The reader requires 4.75-5.25V power supply and the Groove shield only supplies 3,3V. Could this be the reason that I don’t receive data?

Other suggestions?


The Autonomo has one 5Volt pin. The grove only suports 3,3V.
You can solder the powerpin to the 5volt pin on the autonomo or shield.

I believe, the 5V pin is only active when the USB cable (with the 5V source) is connected.

You also need to be careful about the UART logic levels coming from the device as these might be
at 5V. The Autonomo will likely be damaged by 5V levels.

Are you sure you have a working 125Khz tag?, once it recognizes the tag a little yellow led next to the red one will light up briefly

I used the Sodaq / Grove tags: RFR103B2B. They are 125kHz. I did not see the yellow LED flash.

I was assuming the 3.3V was the reason it is not working (the reader requires 5V), but at your Autonomo it seems to work. Or did you connect the reader to the 5V pin?

The module’s documentation specifies 5V so I’ve used 5V
I must confess I didn’t think about the implications for the 3v3 serial of the autonomo, but it seems to work, i might add a resistor divider on the receive end of the autonomo, but as it survived so far i’m not sure it’s necessary.
Usual disclaimers apply :smile:

Thanks Jen,

However I just wanted to play around a bit with RFID.

On the other hand, this might be handy in the future!


hi axel

you are welcome.
anyway, it is my pleasure to help you.