What lights does the Cessna 172 light rheostat control? I am part of a team developing a Cessna 172 P model for FlightGear, an open source free flight simulator. We are now programming how the panel and radio lightning rheostat should work, but we found a lot of unclear and/or contradictory information about it online. Our main problem is to understand how the radio light knob (labelled as RADIO LT) work in these older models of the 172.

The POH from 1982 states the following:


  The engine instrument cluster (if post lights are installed), radio equipment, and magnetic compass have integral lightning and operate independently of post or flood lightning. The intensity of this lightning is controlled by the inner knob on the light dimming rheostat labelled RADIO LT; rotate the knob clockwise to obtain the desired light intensity. However, for daylight operation, the compass and engine instrument lights may be turned off while still maintaining maximum light intensity for the digital readouts in the radio equipment. This is accomplished by rotating the RADIO LT knob full counterclockwise. Check that the flood lights/post lights are turned off for daylight operation by rotating the PANEL LT knob full counterclockwise.


This makes it look like the RADIO LT knob controls both the brightness of the radio digital readouts (i.e. the frequency digits) as well as other types of lights (radio cluster buttons and compass), but I found out that other renowned simulations of the 172 for other simulators do not dim the digital readouts with that knob. It also seems strange to me that one would be able to dim the radio digits under any condition, let alone turn them completely off.

So does anyone here have experience with these models of the 172? Would anyone be able to describe how to best model the behaviour of the radio lights?
This depends more on the radios installed than the plane its self, older era planes may have had King radios with analog numeric readouts. These radios are generally back lit and controlled by what ever rheostat controls the panel those radios generally look like this 



(source) 

later planes may have had digital readouts which in some cases had another control to dim the intensity. This is often because in a sense the lights are always on you may need to control them with out turning on or dimming the panel. As a matter of fact you generally want them to work opposite, during the day when the panel is dim or off you want the radio lights to be brightest they would then be dimmed in the evening as you bring the panel lights on. Those radios often looked like this, 



(source)

The dim function may appear on units like the DME and Auto Pilot if they are similarly lit. 

Its been a while since I have night flown with one but if memory servers the newer GPS units like the GNS430/540 have backlit buttons that come on when the panel lights are activated. These units also tend to have separate ways to control the screen brightness/contrast


(source)

You may have more luck referencing the manuals for the radios them selves than the POH for the plane. You should design for the types of radios you are putting in (looks wise)