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I have a bluetooth HC05 module on a breakoutboard w/pins for STATE/RX/TX/GND/+5V/EN.

Some tutorials say I need to use a voltage divider with 2 resistors, like these tutorials - http://www.martyncurrey.com/arduino-with-hc-05-bluetooth-module-in-slave-mode/

Others say I don't - https://alselectro.wordpress.com/2014/10/21/bluetooth-hc05-how-to-pair-two-modules/

(I only posted 2 tutorials but there's many others..)

I am using it without a voltage divider, and it appears to be working fine. Do I need to use a voltage divider? Why do some say to use it, and others not? Thanks!

edit - hc05 baud rate is 9600, it's being used to transmit very little data.

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    \$\begingroup\$ You better read the specs about the TX/RX pins on both your arduino and the HC05 and see if they can work in the range 3.3-5v. If they can't - the guys who use without voltage divider are wrong, and potentially going to break their HW.. \$\endgroup\$ – Eugene Sh. Aug 22 '16 at 18:29
  • \$\begingroup\$ Yes, the breakout board the hc05 uses can be powered w/5V. And I'm not using an arduino, I'm using a tty cable and communicating through my PC (although eventually I will use arduino). Are you saying even if I powered the hc05 breakout board w/5V I still need a voltage divider? \$\endgroup\$ – WOnderingWhy Aug 22 '16 at 18:34
  • \$\begingroup\$ It's not about power, but about the signal levels on the interface pins \$\endgroup\$ – Eugene Sh. Aug 22 '16 at 19:39
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It looks they do that as a simple form of level shifting between 5V and 3.3V. If you have a device with 5V tolerant inputs, you don't need to do anything special. Check your data sheet for details on what is supported -- it's usually a nice selling point for 3.3V devices and is often called out on the first page / marketing material.

Check out the tips here in Chapter 8: http://ww1.microchip.com/downloads/en/DeviceDoc/01146B.pdf for more ways of doing level conversion.

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  • \$\begingroup\$ Yes the hc05 supports and I power it w/5v, but in those tutorials I posted it's being powered w/5v too and they still use a voltage divider. Does not using a voltage divider make it consume more battery or something? \$\endgroup\$ – WOnderingWhy Aug 22 '16 at 18:31
  • \$\begingroup\$ It's not the HC-05 that matters, it's what what you connect it too. If you mate it to a 3.3V-tolerant micro, you need to level shift. \$\endgroup\$ – Krunal Desai Aug 22 '16 at 18:32
  • \$\begingroup\$ I don't know if you've worked with the hc05 breakoutboard as shown in those tutorials, but the chip is on a breakoutboard. The breakoutboard has some kind of voltage converter, allowing 5v's for a normally 3.3v only chip. The breakout board is being powered w/5v.... is that clear? \$\endgroup\$ – WOnderingWhy Aug 22 '16 at 18:37
  • \$\begingroup\$ Got it -- the tutorials are worried in the other direction, about the Arduino 5V I/O affecting the 3.3V I/O on the HC-05. In this case, the only time you need to be concerned is if the device outputting into the HC-05 is not 3.3V. \$\endgroup\$ – Krunal Desai Aug 22 '16 at 18:39
  • \$\begingroup\$ OK, so lets say my arduino is powered w/3.3V and the hc05 breakoutboard powered w/5v (or vice versa) - if I understand right - then I have to use a voltage divider on the UART? However, if both my arduino board & hc05 breakoutboard are powered w/5v, then I can connect the rx/tx lines directly to each other w/no additional resistors/voltage dividers, and no negative side effects? This isn't going to make it consume more battery or anything like that? \$\endgroup\$ – WOnderingWhy Aug 22 '16 at 18:43

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