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I need to connect arduino to K-Line in my car's OBDII port.
I used an L7805 to power the atmega but how can I interface 12V car logic with 5V arduino logic?
The problem is, the connection has to be bidirectional- sometimes the arduino
will pull the line high/low and sometimes the car will - the purpose of
this connection is to get throttle, rpm etc. data.
This would be easy if the connection was one directional - I could use a
transistor to steer 12V to going IN the K-Line and voltage divider if I wanted to do the reverse. But how to do both?

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    here they use a voltage divider for the RX and a transistor for the TX. – Gerben Aug 28 '18 at 12:50
  • These bi-directional logic level converters work with 5V and 12V, and cost 0.33CAD: aliexpress.com/item/… – VE7JRO Aug 28 '18 at 14:18
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I found this that may be helpful to you. this circuit allows for bidirectional communication between automotive K-line ECUs and an arduino. K-Line Interface circuit

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