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How does this circuit work?

  1. Nov 5, 2009 #1
    I have a schematic with a section that I don't understand. I have recreated in LTSpice and simulated it to get a better understanding of how it works. As I understand it, the circuit uses the input from a uController (labeled V3) to open/close a 12V output (P1-13). When I simulate this circuit, if V3 is even 0 volts, then P1-13 is 12V. If V3 is disconnected, then P1-13 goes low. Does this mean that the uController output must be tri-stated for the circuit to be 12V?

    I have attached the LTspice file for anyone interested.

    Best regards,

    http://imagebin.ca/img/KujFJGaz.png [Broken]

    Attached Files:

    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Nov 5, 2009 #2


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    Staff: Mentor


    The circuit seems clumsy at first look -- where did you find it? The input diode and resistor do not appear to be productive...
    Last edited by a moderator: May 4, 2017
  4. Nov 5, 2009 #3


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    Science Advisor

    Some engineers have the knack, an ability to figure out what's going on, when, and how things work (or don't work, as the case may be):
    https://www.youtube.com/watch?v=<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/CmYDgncMhXw&hl=en&fs=1&rel=0"></param><param [Broken] name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/CmYDgncMhXw&hl=en&fs=1&rel=0" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>

    For the rest of us, it's methodically working through the problem, breaking it down into its component pieces, and relying upon intuition/experience to simplify the amount of work that has to be done.

    In electronics, it often comes down to: what possible inputs are there, and what outputs do these produce (think truth table). Thankfully, this circuit is pretty simple, so you can assume that you have 5V and 0V as inputs. Figure out what impact these have upon transistor Q1 (off or on--assume saturation when on), and then what impact this has upon transistor Q2 (off or on--assume saturation when on):

    Code (Text):

    INPUT   Q1_State    Q2_Input    Q2_State    OUTPUT
    HINT: Does a transistor (the connection from Emitter to Base) look more like a short circuit, or open circuit when it is turned off?

    EDIT: Berkeman, I think D1 should more properly be an LED, indicating driving action. To neergmas, are you sure that R3 is not attached to the right of R1? Nevermind, I think that's actually okay just the way it is.
    Last edited by a moderator: May 4, 2017
  5. Nov 5, 2009 #4
    Hi MATLABdude,

    Thanks for the awesome response! I love the Dilbert clip.

    You are right, all of the diodes are LEDs.


    I added a ground on the other side of R6 too. Now I am thinking of R6 as the load that is being powered.

    Below is the truth table. The values are from running a simulation, but looking more into the operation of PNP and NPN transistors, at least the results make sense:

    Code (Text):

    INPUT (V3)  Q1_State    Q2_Input    Q2_State    OUTPUT
    0V             on            5V             on             0
    5V             off            0V             off            3V
    Does this seem correct?

    Attached Files:

  6. Nov 5, 2009 #5


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    Science Advisor

    Yes, the transistor states are correct, and the voltages are probably at least in the right ballpark (note that if you tristated the microcontroller, you'd have basically the same result as an input of 5V). Not sure about the output of 3V, but the current would flow from the 12V supply through the two LEDs and R6. As you can see, you can sometimes save yourself from having to do a lot of work (plugging into Spice) just by sitting down and breaking down the circuit.

    EDIT: You can model tristating by replacing the microcontroller output with a megaohm-ish resistor to ground
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