Conversion of -15V to TTL LOGIC

Discussion in 'Embedded Systems and Microcontrollers' started by VENKATESHN, Jul 20, 2012.

  1. VENKATESHN

    Thread Starter New Member

    Jul 20, 2012
    1
    0
    Dear sir,
    I need to design a circuit with the following requirements,
    1) I will get 2 types of inputs to my card which are +15V,-15V.
    2) When i will get +15V, micro controller should read 1,if i get -15V it should read 0, can u please suggest me how to Sense these two Signals.



    Regards,
    Venkatesh N
     
  2. bretm

    Member

    Feb 6, 2012
    152
    24
    A diode would turn +15V into about +14.3V and -15V into 0V. Then a voltage divider (resistor ratio of 5.7 to 8.6) would turn 14.3V into 5.7V. Then a voltage follower to buffer the output wouuld turn 5.7V into 5.0V.

    An op-amp circuit could be simpler depending on what supply voltages you have available.
     
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    The following works only if the input impedance of the uC is high, if it isn't put a noninverting voltage follower at the output.

    [​IMG]

    EDIT: Just saw it was already posted by Bretm
     
    Last edited: Jul 20, 2012
  4. bretm

    Member

    Feb 6, 2012
    152
    24
    He specified TTL, and TTL requires a lot smaller impedence on its inputs. For logic-low level the input circuit will need to sink current from the TTL's input pin, and a 5k resistor may drop more than allowed.
     
  5. panic mode

    Senior Member

    Oct 10, 2011
    1,319
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    what speed?
    you can use also use devices designed for that like MAX232
     
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