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Will this circuit work? what am i missing?

  1. Apr 1, 2015 #1
    hi,
    so i want to be able to run a motor depending on whether or not an IR detector senses IR light. so the basic idea at the start was to connect the motor with the detector and a resistor in series and therefore if there is IR the current will flow turning on the motor. I then learned that the current is limited by the low current device, ie:- the detector.

    so will the correct way to go about this be to connect the detector and a transistor in series then connect a motor in parallel and make it depend on the transistor ?
    here's a quick diagram of what i mean.
    oqzjfd.png
    will this work ?

    the math i used to determine the resistance of R1 is as follows.
    needed voltage drop = 9 - 1.2 = 7.8
    r = v/I = 7.8/0.001
    r = 7800 ohms
    again im not completely sure that this way of doing it is correct. you can probably tell I am very much a newbie....

    also what kinda stuff will i have to consider for the transistor ? like is there a voltage limit ? does it provide considerable Resistance ? will it cause a voltage drop ? will the voltage from the inner circuit be added to it ?
    the motor is a 9 v DC motor.

    thank you
     

    Attached Files:

  2. jcsd
  3. Apr 2, 2015 #2

    davenn

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    hi there

    welcome to PF :smile:

    No it wont work the way you want because the transistor is hard biased on all the time

    here's a quick cct I put together for you
    see how you go
    diode across the motor a 1N4007
    the variable resistor experiment with something ~ 4k7 to 10k .... it will adj the turn on sensitivity
    make sure the transistor can handle the current required by the motor

    motor drive.GIF

    cheers
    Dave
     
  4. Apr 2, 2015 #3
    Can you explain to me why you put a diode across the motor ? How does it affect current flow ?
     
  5. Apr 2, 2015 #4

    donpacino

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    a motor is basically an inductotor. When you shut the transistor off there will be a voltage spike due to the motor inductance. That diode will clamp the voltage and allow current to flow. Otherwise you can/will damage the transistor.
     
  6. Apr 2, 2015 #5

    sophiecentaur

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    When you interrupt the current through an inductor (including motors) you get a voltage spike that can damage transistors etc.. The diode will not conduct in normal operation because it is reverse biased but a voltage spike in the other polarity will make the diode conduct and dissipate the magnetic energy in the inductor when the transistor turns off. It's a very effective form of protection (and quite smart, imo).
     
  7. Apr 2, 2015 #6
    Hmmmnn that is extremely interesting. Thank you for explaining. So the voltage spike that the motor will produce once turned off is AC?
     
  8. Apr 3, 2015 #7

    davenn

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    thanks Don and sophie for answering that Q in my absence :smile:

    ummmm ... not sure if I would deem it AC or DC .... its just a large spike

    one of the others may clarify it better :smile:

    cheers
    Dave
     
  9. Apr 3, 2015 #8

    donpacino

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    sulicat look at it this way. It is a DC system, due to the fact that it runs off DC voltage.
    Most systems have 2 main modes of operation, steady state and transient. Transient operations occur when something is a system changes (like a motor shutting off).
    It is a transient pulse caused by changing conditions. When that pulse goes away, the circuit will be in steady state again (but the motor will be off)
     
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