I am building a simple sensor device that's using Adafruit Fona to be connected to the Internet. I want to power up RFduino from the Fona USB 5v power supply. There is a pin on Fona that supplies 5 voltage as far as I understand and I am using voltage regulator to step it down to 3.3v There is a schematic: RFDuino with Fona

The problem is: When Fona starts sending data over gprs RFduino gets rebooted. I guess there is a voltage drop or current spike or something. I've tried to put 100uF capacitors around the regulator, but it didn't really help. Although sometimes it works fine... but in most of the cases it does not.

How to fix the circuit that it works and stable?


  • \$\begingroup\$ I dont see a 5V output on the datasheet. What pin are you using for 5v? also can you provide part number and/or datasheet for 3V regulator please. \$\endgroup\$ – I. Wolfe Nov 30 '15 at 17:40
  • \$\begingroup\$ There is a pin close to the usb plug. From the Fona description: 5V (left middle) - this is the USB 5V from the microUSB connector when its in and powered. Good if you need to know when the microUSB is plugged in and/or want to recharge the battery from an external plug. I am using AZ1117T voltage regulator \$\endgroup\$ – azimvi Nov 30 '15 at 18:11
  • \$\begingroup\$ From what I can tell, that pin is not meant to supply 5V. It states that it is used to either tell if USB is plugged in (5V will be present on the pin), or it can be used to input 5V to charge the battery from source other than USB. I would guess that the pin is able to output some small amount of current at 5V in order to be used as a check for 5V USB, but has a limit in place to keep it from drawing excessive current. An easy way to check would be to measure the voltage at the 5V pin and the voltage at USB input. If the 5V pin drops when the duino reset, but USB stays at 5V, the board is \$\endgroup\$ – I. Wolfe Nov 30 '15 at 19:18
  • \$\begingroup\$ continued - limiting that pin so it won't be misused as output. I agree with Chris below that using a regulator straight off the battery (or usb input) would be best, instead of relying on that pin. His #1 solution could work also depending on if that 50mA is enough \$\endgroup\$ – I. Wolfe Nov 30 '15 at 19:18

The FONA does not have a 5v output - there is no boost converter to provide one.

I see two possibilities which would be worth contemplating.

  1. The SIM800 module has an internal regulator providing a 2.8v output on VDD_EXT (Adafruit seems to simply label the pin 2.8v), which is within the allowable range of the RFduino. The data sheet specifies that this should be good for up to 50mA, which is more than the claimed transmit power draw of the RFduino. In theory it seems like this should be a workable power source, but you will have to check out all of the of the details. You will also have to see if the 2.8v supply is active at the times you want it to be.

  2. Since the RFduino claims to work down to 1.9v, you could presumably use a small low dropout regulator configured for a bit over 2v to derive a power supply directly from the lithium battery which the FONA uses. Select your regulator for low quiescent current and an enable if needed, but mostly make sure that the combination of your configured voltage and the regulator's dropout offset add up to something less than the minimum cell voltage you want to work at.

You could also consider a small switching regulator for efficiency, or even a boost converter if you feel you need a higher voltage, but it doesn't seem clear that this would be required.

  • \$\begingroup\$ Hi Chris, Thank you for your response. Option 1 doesn't work, 2.8v pin is low when I need it. I will try option 2 when I get 2.5v regulator \$\endgroup\$ – azimvi Dec 1 '15 at 0:26
  • \$\begingroup\$ Hi Chris, I've tried the second option and it didn't help. I am using 025EF02 2.5v regulator, but still the same behavior. I noticed that the blue PWD LED on the FONA is blinking a little(not fully on/off, but like voltage drop or something...) synchronously with the red network LED. Why is that might be happening? The power goes from the RFDuino pin 2 that should be stable since the regulator is stable and there are 100uF capacitors as well. \$\endgroup\$ – azimvi Dec 2 '15 at 5:25
  • \$\begingroup\$ (Continue) I've tried to connect Vout from the regulator to Vio on the FONA directly and it fixed the blinking problem, but not the whole system and rfduino is still rebooting. I am clueless why does it make any difference at all what is connected to the Vio pin. Do you have anything in mind that I can try? I am planning to get an oscilloscope to debug it, but I need to find one first. I am also using very low 105mAh battery, but the system is always connected to usb. Might that be a reason? \$\endgroup\$ – azimvi Dec 2 '15 at 5:25
  • \$\begingroup\$ It sounds like you have several problems, but yes your battery is probably far too small. The whole reason you need a battery is that a USB port can't deliver the burst current required for transmit, and it sounds like your battery may also be too small to do that unless it has a very high "C" rating. \$\endgroup\$ – Chris Stratton Dec 2 '15 at 7:00

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