Oh, that is an interesting consideration! Do you think it would get hot enough to cause potential ignition? Geez, I hope that's not why the flyer stopped posting.
Whenever I'm not in a class that actively uses circuits, I get a bit fuzzy on electrical stuff...
I found a post on another forum
http://www.diystompboxes.com/smfforu...prev_next=prev that explains R4
Quote:
I don't know what that specific circuit does, but I do know what mounting an LED on a voltage divider does.
Assume a circuit with two 1K resistors in series, and an LED parallel with the 1K to ground.
As you increase the voltage across the two resistors from 0V what happens?
Initially, the LED does not have enough voltage to conduct, so it's effectively not there. The voltage at the middle of the two resistors goes up linearly at 1/2 the voltage applied to the top resistor.
Then the voltage at the divider gets to the LED turn-on voltage. Now the LED starts sucking current. But how much?
The LED forward voltage is roughly constant. Not exactly, but close. So with a constant voltage across the lower 1k, its voltage remains constant by the same "roughly". Now as you increase voltage, all of the increased voltage appears across the top 1K resistor, and the current is ((Vsupply-VLED)/1K) + (VLED/1K) The first term being the variable current in the top 1K resistor, the second being the constant current in the bottom resistor.
It's a turn-on delayer. The LED turns on at twice the voltage it otherwise might.
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I think that when the NTC is submerged (temp down, res up), the current will run primarily through the R1/R2 leg (and I figure there are two used just to get the right value?) so actually, that leg is also a turn on delayer (right?).
The combination of R3 and the unsubmerged NTC must be supposed to draw the appropriate amount of voltage for the LED, though I'm a bit confused why R3 wouldn't be before they LED, as that's where I understand voltage limiting resistors are supposed to go, but then then again, as I said, I'm pretty fuzzy on this. I did fail my circuits class the first time around.