connect mobile to microcontroler

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Hy all!

I have a question!

I can connect Siemens C35i or Ericsson T10s to Atmega8 microcontroller.
The phone communicate with the pc fine.
The M8 comunicate with the pc fine.
The M8 can't communicate with the phone.
I attachement the schematic for my project!
With Max232 or without the problem is one.
Can someone idea what I do wrong?
 

The problem is due to voltage differences of Phone and Microcontroller. Normally phones operate with 3.6V and Microcontroller operates with 5V. Hence high and low logic-levels in one system can’t be identified by other correctly.

Please read
http://www.interfacebus.com/voltage_threshold.html



The attached simple schematic can solve the issue. This circuit worked for 115k baud-rate serial communication.
(Please note in 74LS05, pin-7 is GND and pin-14 is VCC +5V from Microcontroller circuit)
 

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