Transistor(s) as an off switch?

[thepaan]

zipdogso: I'm not sure. Hence, the multimeter and my apparent ADD. I am certain I accidentally shorted the probes on the multimeter yesterday when I had it dialed to temperature so I'm back to presuming this is a diode.

I've back-tracked a little lately as I'm getting more details about how I can use this thing. I took the bigger multimeter home last night and took a reading at room temperature (25*C) then from the freezer later (-20*C) and from the fridge this morning (3*C). I jotted them down on some paper on my desk at home so I don't have them just yet. But, iirc, I got .0019V/*C.

Right now I'm trying to figure out how exactly to use the comparator. With a diode the voltage drop is the difference in voltage from either side of it, correct? If I apply 24V to the diode and it has a .66V drop then does the wire past the diode only have 23.34V? Does the voltage drop change with the amount of voltage applied? If all this is correct then what do you use for a reference voltage? If the voltage drop decreases with an increase in temperature, then the voltage past the diode will increase with an increase in temperature? Thus, from what I can tell, I would need a reference voltage such that when the voltage past the diode is greater than the reference voltage then the the comparator passes electricity. The only thing I can think of is building a circuit just for reference voltage.

 

[shambhukumar]

i think it will work...
why does LED lamp glow.it is because in BJT base is forward biased with respect to emitter.so current goes from collector to emitter. if we make base -emitter reversed biased then there will be no current from collector to emitter and LED will not glow.
just apply some voltage at base of BJT to make initially LED on.now connect the emitter with the sensor (heat sensor) output.i.e voltage produced by the heat sensor.initially LED will be glowing. when there will be too much heat then sensor voltage will increase and at some time it will be more than base voltage. at that situation base emitter will be reversed biased and there will be no more current coming from collector to emitter. thus LED will be off. i hope it will work.if you want to stick to BJT.

 

[Roff]

I simulated this circuit. You may have to adjust the resistor values around the pot to get the range you want.
R5 and R7 provide about 1 deg C hysteresis. You can adjust either of them to increase or decrease the hysteresis. There will be some change in threshold temperature when you do so.