Retiredguy
Joined: 13 Nov 2006
Posts: 8
Location: Kent, OH
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 Posted:
Mon Dec 11, 2006 2:16 pm Post subject:
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To figure this out you must first know the value of r1 in your problem. If you can adjust the variable resistor so that Vout is 5 volts, the value of r2 is the same and if the value of the variable resistor is known, say 5000 ohms, r2 is 5000 ohms. I assume that you are trying to figure out how the humidity sensor turns on the transistor in the kit. The sensor is a variable resistor dependent on the humidity and the pot is also a variable resistor which sets the threshold of the moisture you wish to measure. As the humidity increases, the resistance of the humidity sensor reduces which increases the current through the voltage devider ( as total resistances decrease, current increases as long as voltage remains constant: ohms law : V=IR or V/R=I ) there by increasing the voltage across the pot and the base of the transistor which starts to turn the transistor on and allows current to flow through the LED> I hope this helps.
Mike
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