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I am trying to use HC-05 (5V) with Arduino Pro Mini (3.3v) but I have had little success.

I am using a coin cell to power the pro mini which powers it on and is working fine.

But, my HC-05 doesn't power on. I am trying to power it externally using two 3.7v Lipo-batteries that gives a total of 7.4v but I have tried numerous changes and nothing seems to work.

Circuit Diagram attached below. enter image description here

Also, HC 05 has an onboard regulator for levelling it to 3.3v enter image description here

Please Help!!!

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It seems like 7.4V DC is greater than the 5V regulator input range on the HC-05.

For example, in the Core Electronics tutorial for the HC-05 (the tutorial PDF calls it a JY-MCU. I can't find any major differences), the pinout table says the voltage should be from 3.6V to 6V DC:

enter image description here

These same values are confirmed by a back-of-the-circuit-board photo from arduino-info.wikispaces:

enter image description here

Components101 says the range is 4V to 6V.

Unfortunately your coin cell voltage is too low. That leaves you with the following options to drop your voltage (if you must use the 7.4V DC battery). Your choice will probably depend on the Wh capacity of your battery, and how efficient you need the voltage drop circuit to be.

  1. Zener diode (few components, inefficient use of power)
  2. Voltage divider with resistors (few components, inefficient use of power)
  3. Linear regulator (few (IC) or many (discrete) components, inefficient use of power)
  4. Switching regulator (many components, efficient use of power (because it does not use a resistor to drop the voltage)

Further details on each of these options are at Lower Voltage Methods Other than Voltage Dividing.

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    My module is 5~7v version unfortunately which is the cause of all of my problem. I have added the module image to the original question. – Hardik Sharma Mar 30 '18 at 9:38
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    Also note that 3.7 Volt is the nominal values. It's more like 4.2 when fully charged. So you'd end up with 8.4 Volt when both batteries are full. – Gerben Mar 30 '18 at 14:03
  • @HardikSharma did you choose a technique to reduce your voltage, or are you just going to use a 5V power supply? Also, please upvote my answer if useful or vote it as the answer if it answered your question (stackoverflow.com/help/privileges/vote-up) – MechtEngineer Mar 31 '18 at 23:16
  • My GND was loose and using a single battery worked after fixing that. So thanks for the information anyway. – Hardik Sharma Apr 1 '18 at 12:01

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