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I can't understand this circuit

Discussion in 'General Electronics Discussion' started by circuit_breaker, Sep 19, 2011.

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  1. circuit_breaker

    circuit_breaker

    2
    0
    Sep 19, 2011
    here are two diagrams of what purports to be the same circuit - the first is floating around online of unkown (to me) origin - the second is taken from the web site of the manufacturers of the device on the left hand side of the diagram -

    [​IMG][​IMG]

    the output of the device is TTL level (i.e. between 0 & +5V) - but to connect to a pc it requires the circuit in the diagram - the trouble is, which one?

    nb. simply connecting the device to a ttl<->usb converter doesn't work as the 100K pull up between pins 1 & 3 of the device is required to put the device into data sending mode -

    I am pretty much a complete newbie to electronics - can anyone explain whether the signal should go to the non-inverting or the inverting input of the op amp? Or just any help with explanations of how the circuit is supposed to work would be greatly appreciated.

    Cheers
    Simon
     

    Attached Files:

    Last edited by a moderator: Jan 11, 2014
  2. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Pin 1 is +5V, pin 2 is ground, and pin 3 is the signal (going to the inverting input).
    R1 is a pull-up resistor as you say. R2 & R3 is a voltage divider providing 2.5V at the non-inverting input. The op-amp then just inverts & amplifies the signal.
    D1 & D2 + C1 & C2 provides an RS-232 compatible dual power supply for the op-amp - from signal lines on the serial port (which needs to be set up correctly).
    Strangely the second attached thumbnail doesn't contain any picture..
     
  3. circuit_breaker

    circuit_breaker

    2
    0
    Sep 19, 2011
    here's a link to the second image
    http://www.combro.co.uk/cb625/m625-3.gif
    notice that the signal is going to the non-inverting input in that diagram - :confused:

    software sets appropriate voltage on the serial port pins - that part works at least.

    Cheers
    Simon
     
  4. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Hm, well, yes, the signals does indeed seem to be inverted between the two diagrams.
    Quote: "In brief the output is 9600 baud, 8 bit ASCII, no parity, one stop bit as 0-5 volts that needs inverting and converting to the RS232 for the computer. The output is an ASCII text number followed by representing the number of 4MHz counts to cover the 2.353 inch gap so "1000" represents 784ft/sec."
    Going by that it seems the manufacturers own diagram is not correct. I'd try to make the circuit in such a way that I could easily swap the signal polarity.
     
  5. sly666

    sly666

    1
    0
    Dec 15, 2011
    Hi
    I too am wanting to build an pc interface for this Combro cb625.

    I am thinking about using a TTL to USB CP2102 uart interface available for a couple of quid from evilbay.

    I also have limited knowledge on these things so would appreciate a little guidance.

    Would the output still require inverting?
    and could the 100k R1 "pull up" resister still be retained to put the device in to data send mode?

    Many thanks.:)
     
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