Controlling the LM3914
The datasheet mentions a 1.25V reference source. Is that built into the IC so that no matter how I configure it there is a 1.25V source there?


From that same datasheet:



So yes, that's built in.




  Assuming pin 8 is connected directly to ground, what does this do?


You might simply want to actually read the datasheet :) it says:


  The reference is designed to be adjustable and develops a nominal 1.25V between the REF OUT (pin 7) and
  REF ADJ (pin 8) terminals. The reference voltage is impressed across program resistor R1 and, since the
  voltage is constant, a constant current I1 then flows through the output set resistor R2 giving an output voltage of:
  
  
  


you can assume Iadj to be very small.

 


  The next two pins I'm confused about are 4 and 6 (RHI and RLO). They seem to be connected in series with the 10 1k resistors. Assuming RLO is connected to ground, as I decrease/increase the voltage at RHI, what happens?


Look at the block diagram at the top of my answer. That's actually the resistive "ladder", ie. what your device does. It's hard to explain it better in words than with the schematic show: You divide the Voltage between pin 6 and 4 into ten equal voltages, and then have a comparator the "partial sums" of these voltages to the input signal.