Keeping on a motor

Discussion in 'The Projects Forum' started by Voltboy, Oct 23, 2007.

  1. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
    197
    0
    Hello.
    Im making a project in which a IR phototransistor detect a fire, and turn on a motor (the fan) the extinguish the fire. The problem is that when the fire is almost extinguished, the motor turns off, so the fire come up again, and then the fan almost extinguish it, but the PT turn off. And again, and again.
    So I cant extinguish the fire.
    The normal answer will be "use a more sensitive PT". I thought that too, but I have to give out this project tomorrow, and I can only make something with what I have in my house (Resistors/Capacitors).
    I thought of using a RC circuit in there.
    The RC circuit I'm trying to add is circled by a red circle (just added one cap and when resistor) in the picture.
    If you have any opinion on how to do it with capacitors/resistors just post it.
    I know I could use IC's, but the project is for tomorrow and I cant get IC's now.

    Thanks
     
    • RC.bmp
      File size:
      201.1 KB
      Views:
      20
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    You might give this circuit hook up a try. By using a current mirror you can hold the Vce across the phototransistor constant and squeeze some additional sensitivity out of the phototransistor.

    hgmjr
     
  3. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
    197
    0
    I dont got any transistors :mad:
    I only got capacitors (well I just took out from a radio) and resistors.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Do you have some aluminum foil?

    You could make a parabolic reflector with it. Shiny side in, of course.

    Even if it isn't a perfect parabolic shape, you could focus a lot more of the IR into the PT. You'll have to experiment.

    Careful that you don't short out your circuit with the aluminum foil.

    Just about any moderately-sized spherical shape would work as a form - even a tennis ball.

    Put the PT's lens at the focal point of your reflector, pointing at the rear of it.

    Cooling the IR sensor makes them much more sensitive - but unless you have some liquid nitrogen around, forget that approach for now. Don't try using stuff like "Dust-Off" or propane/butane refills or any other recently-manufactured spray cans; the propellants are generally flammable. However, if you have a can of R134a refridgerant sitting around... ;)
    Oh, CO2 cartridges work well for cooling, too. They will also extinguish small fires.
     
  5. niftydog

    Active Member

    Jun 13, 2007
    95
    0
    Perhaps a large value cap from the base of the motor transistor to ground?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    A larger cap would keep T1 turned on longer, thus keeping the motor on longer, but it would delay the fan turning on as well - might have a roaring fire by then!

    R1 could be reduced quite a bit; that would certainly help. Wouldn't want to eliminate it, because charging the cap will draw a LOT of current through the PT, frying it. Ohm's Law and the PT's datasheet would need to be consulted.
     
  7. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    As an alternative to the use of the current-mirror to enhance the sensitivity of the phototransistor, I suggest you add a high valued capacitor to your existing circuit with the positive terminal connected to the junction of the 15K and the phototransistor's emitter and then connect the negative end of the capacitor to negative side of the battery.

    This will overcome the concern that Sgtwookie correctly identified with using a capacitor added in the manner suggested by niftydog.

    By connecting a capacitor of around 100 microfarads as I have suggested, you should get an asymmetrical charge/discharge behavior. In other words, the charge current for the capacitor will be supplied by the phototransistor when it is subjected to the light from the the flame. This is likely to be on the order of few milliamps. The dV/dt charge rate is approximately I/C. The discharge path for the capacitor will be through the 15K base current limiting resistor. This current is much lower so the discharge rate will be correspondingly lower.

    You may need a base return resistor. That is a resistor from the base of the motor drive transistor to the negative side of the battery. This will aid in turning off the resistor when there is no flame. A value of 10K would be a good value to start with. You may be able to get by with a larger value say 22K.

    hgmjr
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Wait a sec, hgmjr!

    You just suggested that he (basically) connect a cap between the emitter of the PT and the battery negative terminal. When the PT turns on, IT will be the load as the cap gets instantly charged, right up to the point where the PT fries!! :eek: That's just not good!

    Let's figure this out...

    The QSE133 has a max current of 100mA, battery is 3.7v
    R1 = 3.7v / 100mA
    R1 = 37 Ohms
    For a safety factor, I suggest you use anything larger than 37 Ohms; use a couple in parallel or even three if you don't have anything close.
    Resistors in parallel:
    Rt = (R1 x R2) + (R1 + R2) for two, or
    Rt = 1 / ( 1/R1 + 1/R2 + ... 1/Rn ) for 3 or more

    Now that we have the new value for R1, connect ANY electrolytic capacitor with at least a 3.7v rating from the base of T1 to battery negative. Anything over 1uF would do the trick.

    You can use the original R1 of 15k from the base of T1 to battery negative to ensure T1 turns off as hgmjr suggested.
     
  9. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    I don't think there is any danger of frying the PT since it can't develop enough current to damage itself. It's maximum current is on the order of 10 milliamps or less and so the maximum instantaneous power is around 40 milliwatts.

    hgmjr
     
  10. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
    197
    0
    Srry I didn't said that the acual PT is a QSE113, and the T1 is a power transistor.
    And to any who want to know, I did the presentation today and to make the fan going on I added a bump switch (made at the moment) so that when the car crashes with the candle the is made a conection from the negative battery to the motor.
    But thanks everybody in trying to help!
    Still I will try to do what you guys told me, and I will inform you on what happened.
     
  11. Salgat

    Active Member

    Dec 23, 2006
    215
    1
    A simple solution may have been just an SCR that automatically shorts/shuts off after a certain duration(that duration being equal to at least the length of time that the motor needs to stay on after it normally shuts off). If you have another transistor, an SCR, and a 555 timer I'm sure you could accomplish this.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The trouble was, is the OP didn't have anything like that available; just some caps and resistors they were salvaging from an old radio besides what was already available in the circuit.

    But almost every household has aluminum foil of some sort lying around ;)
     
Loading...