Recently I've measured output voltage of my 3V3 Pro mini clone that I got from eBay. It has 5V VCC output voltage. Is it normal? Obviously the voltage regulator is LG33, it means 3.3V. I would like to use it with nRF24L01 transceiver but I'm afraid to burn it.
Is it nRF24L01 compatible?
How are you supplying voltage into the pro mini?
Did you see something with '33' on the voltage regulator ?
There has been a few topics in this forum that Pro Mini boards have the wrong crystal, the wrong voltage regulator, the wrong pcb design, and so on.
It is running at 8MHz ?
I think you have to buy another one.
Thanks for reply.
Power is supplied by serial USB UART.
Crystal marking is not visible so I'm not sure about it. But voltage regulator looks to be 3.3V if it's not fake.
Can you make a photo of the Pro Mini and the USB-serial and the wiring ?
The labels are not always the same, but I think use USB 5V to 'RAW' and the 'VCC' should be 3.3V.
The Pro Mini does not have USB onboard. If you use a 5V external USB adapter, then the Pro Mini will be a 5V Pro Mini while you have the adapter attached. It is ok to program it at 5V when you don't have other devices attached to it that need 3.3V. You can also get a 3.3V external USB adapter if you like. When you remove the adapter and start actually using the onboard regulator by supplying unregulated 5 to 9V to Vin, the Pro Mini becomes a 3.3V Pro Mini (if it is the type with a 3.3V regulator onboard).
So I need to detach all connected 3V3 devices when I program Arduino Pro mini via USB?
Here is the photo of my Pro mini and nRF24 setup. It does work actually without any problems. But VCC has still about 4 to 5 volts.
Now I'm thinking maybe have to try with some other multimeter. 
You connect the 5V from the USB to the VCC of the Pro Mini. That will increase the voltage to 4 or 5V, which is indeed bad for 3.3V devices.
I see a 3.3V output on the usb-serial, can you try that instead of the 5V ?
I have wondered about that style of adapter for a while now. If the output of TX signal from that adapter is above 3.3V, supplying 3.3V to the Arduino would cause you to exceed the VCC on the RX pin of the Arduino, which would be bad. It would probably be OK, but something to keep in mind.
nebulae, to answer your question, yes you would detach 3.3 V devices when programming with a 5 V USB to serial adapter.
When you get around to it, attach 5V to Raw pin, and then you should be able to measure 3.3 V on VCC. This is done without the USB to serial adapter attached. Then you will confirm that you have a 3.3 V pro mini.
Here is the photo of my Pro mini and nRF24 setup. It does work actually without any problems. But VCC has still about 4 to 5 volts.
If the NRF24L01 was being powered by Vcc when you made this measurement, it is probably damaged.
Thank you guys. Yes, I did a stupid mistake.
Peter_n:
You connect the 5V from the USB to the VCC of the Pro Mini. That will increase the voltage to 4 or 5V, which is indeed bad for 3.3V devices.I see a 3.3V output on the usb-serial, can you try that instead of the 5V ?
You have right. I will try tomorrow again using 3.3V. I have connected it directly pulling out from 5V version of Pro mini.
dmjlambert:
I have wondered about that style of adapter for a while now. If the output of TX signal from that adapter is above 3.3V, supplying 3.3V to the Arduino would cause you to exceed the VCC on the RX pin of the Arduino, which would be bad. It would probably be OK, but something to keep in mind.
I see there is special usb-serial adapter for 3.3V Arduino: https://www.sparkfun.com/products/9873
If my usb-serial has a 3.3V power output, is it still not good to use it with?
jremington:
If the NRF24L01 was being powered by Vcc when you made this measurement, it is probably damaged.
It does work so far, but still I made a mistake.
Most of us have made that mistake, or similar ones.
nebulae:
If my usb-serial has a 3.3V power output, is it still not good to use it with?
I've done some thinking about this. I suspect the adapter you have is a 3.3V adapter which happens to have a 5V power supply output. If the adapter is not doing anything and is just connected to your computer, the TX line should stay high. So, measure the voltage between TXD and GND on the adapter and see if it is 3.3V. If so, all you need to do is stop using the 5V output of that adapter and just use the 3.3V output instead, when you are programming a 3.3V Pro Mini.
dmjlambert:
I've done some thinking about this. I suspect the adapter you have is a 3.3V adapter which happens to have a 5V power supply output. If the adapter is not doing anything and is just connected to your computer, the TX line should stay high. So, measure the voltage between TXD and GND on the adapter and see if it is 3.3V. If so, all you need to do is stop using the 5V output of that adapter and just use the 3.3V output instead, when you are programming a 3.3V Pro Mini.
I have measured the voltage between GND and other pins, they are all around 3.3V except +5V.
Problem solved!
Thanks, folks!
