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Keeping on a motor

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Voltboy

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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.
In the picture, the actual circuit is all. The RC circuit I'm trying to add is circled by a red circle.
If you have any opinion on how to do it with capactiors/resistors just post it.
I know I could use IC's, but the project is for tomorrow and I cant get IC's now.
 

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I blow air on a fire to make it a stronger fire, not to put it out.

You need a huge capacitor to power the motor for a while and a huge current to charge it.

Instead, add another transistor to make the motor driver a darlington transistor with a very low input current. Then the photo-transistor can quickly charge a little 22uF capacitor that will delay the motor turn off for 22 seconds, but the motor will be slowing down. Use a higher capacitance for a longer delayed turn off.
 

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Thnx by the help.. And about the fire. I'm not extinguishing a big fire, its just a candle.
 
Voltboy said:
And about the fire. I'm not extinguishing a big fire, its just a candle.
Hee. hee.
I must remember that these electronic chat sites are full of little school kids with their teacher's quizzes, not real people with real problems.
 
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