I have an application where I will be taking serial signals will need to level shift the signal to 3.3V. The input signal will be from 15 to 5V for a high input and I need them all shifted down to 3.3V.

I understand how to do this when there is one constant input voltage but in this situation, depending on what is connected to my RS232 connector I could have..

-10 to 10V

0 to 5V

0 to 15V

on the input side. Does anyone know a good option to make sure that all of these 3 different signals can be shifted down to 0-3.3V?

I'm struggling to find a solution.

Thanks in advance

  • \$\begingroup\$ is it just input or do you need it to transmit aswell? \$\endgroup\$ – Wesley Lee Apr 13 '16 at 15:45
  • \$\begingroup\$ Its single directional -- meaning that its always a higher voltage to 3.3V \$\endgroup\$ – mitch33 Apr 13 '16 at 16:18

A simple clamping circuit would do the trick. The diodes will make sure the voltage seen by the buffer is never more than 3.3+Vd and never less than 0-Vd (where Vd is the diode forward voltage). Additionally, the 1K resistor provided current limiting.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Would something like ti.com/lit/ds/symlink/sn74lv1t34.pdf be a good option for the buffer? \$\endgroup\$ – mitch33 Apr 13 '16 at 16:31
  • 1
    \$\begingroup\$ If you are going to use this circuit you should increase the value of R1 to be as high as possible. 10k would be a minimum value, 47k would be better. Having R1 as 1k means that you are dumping up to 11mA of current onto your 3.3V rail where it will appear as noise in phase with the signal. Whether you can get away with this will depend on the load on the 3.3V rail from the rest of your circuit, but as a general rule you should aim to keep your main 3.3V rail as clean as possible. Increasing the value of R1 reduces the current dumped onto the 3.3V rail and reduces the noise. \$\endgroup\$ – Steve G Apr 13 '16 at 16:39
  • \$\begingroup\$ @SteveG is correct, I meant to make the value of R1 larger. \$\endgroup\$ – Brendan Simpson Apr 13 '16 at 17:17
  • \$\begingroup\$ I can do 47K no problem. Thanks for the clarification. Can someone explain why I would want to use a standard diode vs a zener? \$\endgroup\$ – mitch33 Apr 13 '16 at 17:44
  • \$\begingroup\$ Zeners are slower than conventional and schottky diodes \$\endgroup\$ – Brendan Simpson Apr 13 '16 at 18:02

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