Arduino GPS communication problem

Hi to all,

I have a project that include GPS module and Arduino uno.

I use SIM28 as a GPS module, the problem is Arduino can not read any information from GPS (GPS module itself is working.).

Communication between Arduino and SIM28 is hardware serial RX,TX. When i measure this signal with an oscilloscope

SIM28 TX pin signal is like below;( 3V– 0V square wave)


When i applied this signal to Arduino RX pin square wave has changed to 4 V to 2.8 V.

I figured out that Arduino needs logic level converter which converts 3V to 5V so i used the circuit below:

After that, SIM28 TX signal has changed to 5V-0.6V square wave as expected.(see below)



However, when i applied this signal to arduino RX pin, signal changes to 5V-3V square wave as below, and arduino can not read that signal.


How can I communicate GPS module (SIM28) and Arduino, OR How can i convert my signal to 5V-0V, which will make it meaningful signal for my Arduino.

Thanks in advance.

A 5V Arduino can usually read a 3.3V serial signal correctly. However, your scope traces don’t make much sense. Do you have the grounds connected together?

Please post a wiring diagram (hand drawn is fine, avoid Fritzing).

Thanks for the reply,

Circuit schematic is like below.


Any help?

If you bought this from Grove, follow their directions for how to connect it.

Show me video.