I am trying to make a simulation by using Multisum 14 for i bi-directional channel for Logic level circuit from SparkFun which use Philips AN97055 IC as a Bi-directional level shifter...

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My circuit design

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The UN-clear part in this simulation is I'm using the low side or TX_LV to transmit serial data signal from 3.3v MCU to 5v MCU over this LLC, if I send "Logic 1" 3.3v from MCU over serial pin to the mosfet the last will read it as open circuit and don't send 5v in the other site and when I stop sending data "Logic 0" 0v the mosfet will read low pin as open circuit then drive 3.3v from pull up resister to source pin in mosfet and this make the mosfet drain pin connect to ground which the last cause drive 5v from pull up resister to TX_LV pin?

Hope to find some explain about that.

Thanks for everyone shearing his knowledge in this site,

  • 1
    \$\begingroup\$ I am not quite sure what is unclear, you have a simulation showing the circuit working and sending low and high states from the 3.3V side to the 5V side. The circuit works as expected, when the 3.3V side is low the MOSFET is turned on and pulls the 5V signal low. When the 3.3V side is allowed to go high, the MOSFET is off and allows the 5V side to go high. \$\endgroup\$ Commented Feb 11, 2016 at 18:33
  • \$\begingroup\$ You'll have to add some periods, punctuation, and paragraphs to that block of text. It's very unclear. Your circuit seems to work as expected though. \$\endgroup\$
    – uint128_t
    Commented Feb 11, 2016 at 18:50
  • 5
    \$\begingroup\$ I explain what is unclear, instead of switch I'll use serial pin RX or TX and the last send either 3.3v or 0v this difference than using switch connection to ground, I tried to add switch connected to 3.3v supply to source pin in MOSFET and use it as a RX or TX pin to generate signals but this don't work fine please see this question electronics.stackexchange.com/questions/216612/… \$\endgroup\$
    – Haider
    Commented Feb 11, 2016 at 19:01
  • \$\begingroup\$ This will also work with a TX pin delivering 0 or 3.3V. In the other question you have the 3.3V side pulled up to 3.3V with a resistor and you were switching it to 3.3V, so it was always at 3.3V. \$\endgroup\$ Commented Feb 11, 2016 at 19:07
  • \$\begingroup\$ How about this article , it say difference and what I try I'm implement hobbytronics.co.uk/mosfet-voltage-level-converter \$\endgroup\$
    – Haider
    Commented Feb 11, 2016 at 19:32

1 Answer 1


An output will act the same as a switch to ground, as you have drawn it, rather than a switch to 3.3V (in reverse, of course).

When the output is low it will conduct to ground like the close switch. It won't do much when it is high. Your switch has to mimic the output. If you want to get closer to the real situation, you can do this:


simulate this circuit – Schematic created using CircuitLab

There are actually two MOSFETs in a CMOS output, one of which acts as the N.C. contact and the other as the N.O. in the above mechanical switch.


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