Bluetooth HC-05 level conversion schematic - Help!

Discussion in 'General Electronics Chat' started by jerseyguy1996, Nov 25, 2012.

  1. jerseyguy1996

    Thread Starter Active Member

    Feb 2, 2008
    206
    9
    Hello! I am trying to connect a 3.3v bluetooth module to a 5v arduino and I need help with the level conversion. Certainly I could just use a voltage divider on the arduino side to bring the 5v down to 3.3v and I believe the 3.3v is high enough on the arduino receive side to register a high level, but I wanted to explore some more sophisticated ways of doing it. I saw the schematic on page 9 (Figure 6 Circuit 2) on this pdf:

    http://cxem.net/arduino/download/HC Serial Bluetooth Products 201104.pdf

    In the bottom right hand side of the schematic, I feel like there should be a 3.3v pull-up on the UART_RXD to RX_TTL circuit in front of the diode on the UART_RXD side. Why would the pull_up not need to be on there as is shown on the schematic?
     
  2. jerseyguy1996

    Thread Starter Active Member

    Feb 2, 2008
    206
    9
    As long as I am being confused by level shifters, perhaps someone can explain this one to me as well:
    ftp://imall.iteadstudio.com/IM120417010_BT_Shield_v2.2/SCH_IM120417010_BTShield.pdf

    It seems to me that if the mcu sends a zero on RXD, the Vgs of MOS2 will be 3.3v and MOS2 will conduct causing the voltage at RX to equal zero volts. So far so good. If the mcu sends a one on RXD, the Vgs will be zero I think (5 volts on MOS2 gate (through R3), and 5 volts on MOS2 source) and MOS2 will stop conducting. It seems to me that this would cause RX to rise to 5 volts through R6 which on a 3.3V device would be a problem. I also can't help but think that the gate of MOS1 would rise to 5 volts as well (also through R3) and would that cause 1.7 volts to try to flow backwards through the 3.3V supply? I get really confused trying to put all of this together. I haven't even started trying to figure out what is going on on TXD and TX yet.
     
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