guys, I am new to arduino. I am trying to power my HC-05 (5v with regulator to 3.3v) from Arduino Pro Mini 3.3v but I can't get it to power on.

There is a problem: The site I bought HC-05 from says that it needs 3.3-6 volts to power on but the back of HC-05 shows only 5v pin. To confirm that it runs only on 5v I need to connect it to external 5v supply.

But how do I make the connection? What do I connect to negative of the battery if I choose to externally power it?

Second Problem: If my HC-05 turns out to need only 3.3v input voltage, how many coins cells do I connect in series to power both pro mini and HC-05 module.

I am using a coin cell (3.3v) for powering the pro mini.


  • If you can't figure out the battery connection (it goes to ground) then I doubt that you connected the module correctly in the first place. Have a look at this question ( electronics.stackexchange.com/questions/280500/… ) to make sure you got the wiring correctly. And as far as I know a coin cell usually is 3.3 V, so if you don't specify what cells you have it's hard to tell. edit: if 5 V is printed on it I would trust that over the vendor specs. I found images of HC-05 modules that have 3.6 - 6 printed.
    – idkfa
    Mar 21, 2018 at 13:26
  • 5V is within the valid range of 3.3V to 6V. What makes you assume that it only works on 5V?
    – sa_leinad
    Mar 21, 2018 at 14:18
  • @sa_leinad Who do you mean? I meant that I found modules that have 3.6 to 6 V printed on them. If his module has 5 V printed on it I would suspect it needs 5 V and not the range, since they could have printed 3.3 V to 6 V then.. I don't know these modules so I don't know if all work with 3.3V, but I also mentioned that wiring needs checking. If it works with 3.3 V and OP did everything correctly it should have worked when powering with 3.3 V.
    – idkfa
    Mar 21, 2018 at 14:34
  • @idkfa I have a regulator in my HC - 05 which levels it to 3.3v. Should I still use a voltage divider? You are probably right about that I should trust what is printed on the board and consider my HC-05 to be 5v input and a regulator to 3.3v (which is mentioned as level 3.3v on the board). If I don't require a voltage divider? Where do I connect the negative of the power supply? Mar 21, 2018 at 16:16
  • As I mentioned I don't know the module. I'm sure if you read the link / do some further research you can find out if you need a voltage divider or not. I would have to read that myself, too. As I already wrote: it goes to ground. @chrisl wrote in his answer "In this case be sure to connect the grounds of both power supplies, so that they have a common ground."
    – idkfa
    Mar 21, 2018 at 16:24

1 Answer 1


The HC-05 (the PCB that is mounted on the PCB of the module) itself runs on 3.3V. Many microcontrollers run on 5V, so people are selling the HC-05 mounted on a PCB with extra components, for example a voltage regulator from 5V to 3.3V.

Voltage regulators have a minimum input voltage for which they can hold the output voltage at the specified value. Refer to this question for more information about this. So supplying the module with 3.3V instead of 5V will result in a supply voltage for the HC-05, that is significantly lower than 3.3V, so it most likely won't work correctly.

If you want to use the HC-05 module as it is, you have to provide 5V supply voltage from another source. In this case be sure to connect the grounds of both power supplies, so that they have a common ground.

If you don't want to use an extra power supply, than you may bridge the voltage regulator on your modules PCB and provide the 3.3V as supply voltage, that are used by the Arduino. Doing so would not be an easy task for a beginner, since you would have to solder small SMD components. Also the outlay of the modules PCBs are very different (depending on manufacturer). Note that in this case you should never ever put 5V on the Vcc pin of the module again, since it is then directly passed on to the HC-05 chip, which would be destroyed from the 5V supply.

  • I don't want to use a divider so I should connect the negative of the battery to GND on the pro mini. RIGHT? Mar 21, 2018 at 16:23
  • Yes, negative battery pin to ground of HC-05 and to ground oft arduino. Positive battery pin only to HC-05. This way the current for HC-05 only comes from the battery, but the HC-05 can also evaluate digital signal Form arduino, since they have a common ground
    – chrisl
    Mar 21, 2018 at 22:20

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