Comparator Question (and more general question)
This is not a complete answer - yet.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. OP's schematic with + and - inputs swapped.

Everything you have reported indicates correct understanding of the circuit operation. Switching the + and - inputs should have worked.


  Another possibility is that I don't know if the comparator is sinking current from my voltage dividers or not, and/or messing with their operation by providing a low-load path for electricity. Does it matter how stiff I make my divider? 200-ohm or 10k ohm dividers?


If the LDR goes &lt; 5k when lit then you have set this up correctly. The input impedance of the LM339 will be very high in comparison with your 5k resistors so that shouldn't be the problem.

Next steps:

Using the battery negative as reference, measure the voltage at A; measure the voltage at B and report readings for dark and light. We're expecting:

  | Dark | Light |
==================
A |  4.5 |  4.5  |
B | &lt;4.5 | &gt;4.5  |


Report back.



Tip: The problem with your connection between 'A' and the comparitor (on the CircuitLabs schematic) was that you were trying to cross the node of R2. That will cause it to connect. I pulled R3 and R2 down one square and the lines then cross without connecting.