Ok, this line blows my mind. If we have a 100K ohm resistor connected to ground, and a 1k resistor connected to an LED connected to ground, you're saying the current will get used up on the 100K ohm resistor before going through the 1k resister and LED?The 4.7K resistor ensures that a small current through D1 is passed to ground instead of entering into Q2's base during very dim lighting conditions.
To me it seems a small current passed through D1 should for sure go through Q2's base because it's less resistant. I read some of the descriptions on Wikipedia on how a resistor works, but none of the explanations specify just what happens here.
To help me clarify, do the following two assumptions sound correct to you?
1) When passing 500k ohms of current through a circuit with a choice between a 100k resistor and a 1k resistor, most of the current will pass though the 1k resistor, but some of the current will be soaked up testing the resistance of the 100k resistor, although no current will pass fully through the 100k resistor while there is a less resistant path available.
2) When passing 10k ohms of current through the same circuit, the 100k resistor will soak up so much of the 10k current by testing it's resistance that not enough current will be left to pass through the 1k resistor to activate the LED load.
( We might be off topic now - but this seems like extremely good information for newbs like myself, maybe this tangent should be on it's own question thread to properly assist others.)
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