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...

The shcematic for one of it's four channel is shown below

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The circuit design in Multisim is shown below

enter image description here

in this design im trying to simulate the situation of using 3.3v sensor with 5v MCU by make the switch on to send the 3.3v from sensor to logic level circuit then the circuit convert 3.3v to 5v and send it to MCU i use Multi meter to read the final Voltage, but as you see the voltage is still 5v either switch in circuit is "ON" or "OFF" why?

Q2/ why use resistor in the design of chanel one and each channel in sparkfun logic level converter what is the purpose of register also when I compare the Bi-Directional logic level converter to the one direciton

So can someone explain the schematic and how to use the bi directional chip and how it's work without forget the purpose of resistor.

Thanks for everyone shearing his knowledge in this site,

  • \$\begingroup\$ The circuit with the MOSFET in your first picture is incomplete and does not show what else is connected so it cannot be determined from that schematic how it works. So you trying to simulate that incomplete schematic is pointless. It is also utterly pointless to try and simulate a circuit without understanding how it works. The proper use of a simulator is to confirm what you think is happening. \$\endgroup\$ Feb 11, 2016 at 10:58
  • \$\begingroup\$ I read this question electronics.stackexchange.com/questions/143724/… \$\endgroup\$
    – Haider
    Feb 11, 2016 at 11:04
  • \$\begingroup\$ Maybe my circuit didn't simulate because I connect the TX to multi meter, I thought it's the output signal from converter after convert it from 3.3 to 5v but in the other part of schematic they connect it to ground but what the RX connected to ground not TX which used in converter? \$\endgroup\$
    – Haider
    Feb 11, 2016 at 11:07
  • 1
    \$\begingroup\$ The thing is your TX_LV is either 3.3v or floating. You never pull it to ground. Try to apply a square wave in your TX_LV and see if it works \$\endgroup\$
    – brtiberio
    Feb 11, 2016 at 11:51

1 Answer 1


That kind of logic level converter circuit requires an active pull-down on the input when you're intending to feed it a 0 level.
Simply floating the input will not work since (as you can see in the schematic) it has a pull-up resistor to the supply.
To get your simulation to work there are 2 things you can do:

  1. remove the 3.3V source on your input and simply tie the switch from the input to ground
  2. replace the DC voltage source & switch with a square-wave source
  • \$\begingroup\$ I make some change and here is the results in Two photos postimg.org/image/mlvsory5j postimg.org/image/42you0rrn \$\endgroup\$
    – Haider
    Feb 11, 2016 at 15:45
  • \$\begingroup\$ Looks like its working. Switch closed = 0V in -> virtually 0V out. Switch open = 3.3V in -> 5V out. \$\endgroup\$
    – brhans
    Feb 11, 2016 at 15:58

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