Hey all,
I had a quick question regarding what resistor values I should use for my HM-10. I am using an Arduino Pro Mini (5V, 16 megahertz) and realized that the 5V of the Arduino tx pin could destroy my 3.3v Bluetooth module. I’ve seen many people use no resistors at all, but I want to stay on the safe side for now. Also, I have seen people use 1k/2k dividers, 4.7k/2k, 1.1k/2.2k, and didn’t really know which ones to use. Also, could someone explain to me how to wire the divider up why we need a voltage divider here? Couldn’t we just use one single resistor that could change the 5V to 3.3v instead of of using another one to pull it to ground?
A voltage divider scales the 5 volt to 3.3 volt. A serial resistor will still send 5 volt to the 3.3 volt device. This overvoltage will use the eventual protective diode in the 3.3 volt device. How much current does it manage? No easy question.
There are always jungle designers violating specifications but they are loosers in the final end.
Any of the values will probably work. I would pick 1k2 / 2k2 (voltage 3.24) or 2k7 / 4k7 (voltage 3.18V) unless I know the spec of the HM-10.
One thing is that you must not exceed the maximum output current of the Arduino pin (that is 20 mA) so a 1 Ohm and a 2 Ohm resistor will not work. On the other hand, you need to be able to supply enough current to satisfy the IIH specification of the input pin (RX pin) of the HM-10 (I have no idea what that specification is).
And the last requirement is that you need to be able to exceed the VIH of the RX pin of the HM-10.
The voltage drop over the single resistor depends on the current through the resistor. When using a properly configured voltage divider, the IIH of the HM-10 input pin wil have no effect on the output voltage of the divider.