I have just got a HC-05 Bluetooth module for a new project - that is running on 5v. I did some searching and concluded that the Rx pin is not 5v compatible. There is no level switcher, and the RXD pin connects directly to the Bluetooth chip. The chip specification sheet says level 1.8 to 3.6v. So a voltage divider is required.
I initially thought of adding the resistors to the connecting lead plug. This would not look neat and the resistor leads would break after a while. I then noticed that it would be possible to add the resistors directly to the circuit board. Fortunately I have surface mount components available. However conventional resistors (preferably 1/8 or 1/4W) could be used.
You will need:
- HC05 module
- solder paste (if using surface mount resistors) or thin solder wire
- 3k3 0603 SMD
- 6k8 0805 SMD
- soldering iron with 1mm tip
Note that solder paste usually comes in larger syringes than the one in the picture. I find it easier to transfer around 0.5ml into a 1ml syringe (using a 10mm length of silicon tubing (model engine fuel tubing)).
Step 1: Checking the Board
The photo above shows the module. The red line shows the track from the RXD pin to the chip. The larger area of copper is the ground plane. I decided to use a 3k3 and a 6k8 resistor. The 3k3 needs to be in series with the RXD pin and the 6k8 needs to connect between the track and the ground plane. Other resistor values could be used. I felt these were a good compromise between lower values that would take more current and higher values that might limit higher baud rates.
Step 2: Prepare the Board
The picture above shows the board ready for soldering. The insulation over a 2.5mm length of the RXD track has been scratched away and a small piece cut out to make a break - between the RXD and TXD lables. This is where the 0603 resistor goes. Then two areas for the 6k8 resistor have been cleared on the track and the ground plane close to the V of 3.3V. There is a track in between the Rx track and ground - that an 0805 resistor nicely bridges.
Check with a meter that there is now a break in RXD track. You don't want to find it is still connected after the resistor has been soldered!
Step 3: A Quick Solder and All Done
Soldering surface mount components seems a bit daunting at first but I now prefer these to leaded. I use a pair of tweezers to place the component, and hold it in place using a 3mm flat screwdriver tip. Then I place a dot of paste on the connection and then apply the iron. It does need a steady hand - but becomes second nature with a bit of practice.
As a final check I measured the resistance between the RXD pin and GND. This should be close to 10k.
If using leaded resistors I would clear around 3mm of the RXD track and make a break between this and the RXD pin and then solder the 3k3 resistor from here to the RXD pin and the 6k8 resistor from the same track connection to the GND pin.
As a further check I also set the HC-05 to 115200 baud and it received and sent the data fine.