Since you have a processor there it would seem strange not to use it to the full extent - particularly on something as "wormy" as an old radio like this whose threshold voltages are likely to walk all over the place with temperature. (I'd bet that there's a thermistor in that radio's squelch circuit for that reason!)
Sorry if I'm preaching to the choir, but it's pretty easy and painless to use the many A/D channels and forget that there's a MUX involved. One possible way to do this is to have a state machine called by the endless loop that does something like:
(Using the "static" variable "State" initialized to 0):
- State 0: Read A/D, store in variable "ADC0", then change MUX to A/D 1 (MUX will be at 0 after the first time through this)
- State 1: By this time, MUX has settled, Start A/D conversion
- State 2: Read A/D, store in variable "ADC1", then change MUX to A/D 0
- State 3: By this time, Mux has settled, Start A/D conversion - Set back to state 0 for next time through
Global variables "ADC0" and "ADC1" will always be up-to-date (albeit a bit delayed) and containing most recent A/D values: If you wanted to know when a value was updated you'd put something like "9999" in it (e.g. a value too large for a 10 bit A/D, small enough for a 16 bit word) and when it changed to something else, you'd know that it was just updated
If this is made to be a separate function, you can have it check the hardware bit to see if an A/D conversion is complete - but RETURN if not, so that your main program never has to wait: The MUX usually settles so quickly that by the time you come back again, it will have already met the settling time, anyway! The other ways would be to have it called by a timed interrupt at a rate of a few hundred Hz . You could also have a "soft" interrupt in which you look at a hardware timer and advance to the next state whenever a counter rolls over or a particular bit changes, etc. - in either case, if your timing was long enough (e.g. slower than a few kHz!) you'd never even have to wait for a MUX to settle or wait an A/D conversion to finish (or even look to see if it did) as you'd be guaranteed to never get back to it before it had!
You may have to dig around a bit in the library to find the way to do an "A/D Start-Only", an "A/D Read-Only" and an "A/D Still-busy-converting", but I'm sure that these are already out there somewhere - and once you use the above functions, you'll keep them around and never think twice about using as many A/D inputs as you want.
As for using LEDs as voltage droppers. About 20 years ago I was working on a commercial project for the automotive environment and I needed a regulator that would operate a PIC microcontroller that operated from 12 volts, but it had to consume no more than 30uA while the unit in which it was placed was "sleeping" (with the PIC operating at 32 kHz) but when it was awake, it had to source 10-20mA. At the time there were really no off-the-shelf options for a regulator that would supply the 3.6-4.2 volts needed for this processor and draw practically zero current over the disparate voltage/current range required.
Anyone who has used a Zener knows that they are useless over this sort of current range - at least over a wide temperature range and with variance from manufacturers and production runs. 3.2-4.5 volt Zeners were tried but at a few uA it was dropping only a few hundred millivolts. Several series diodes were tried (1N4148, 1N4001, etc.) but these had a rather soft "knee" as well.
The strange, yet useful, solution was two ordinary red LEDs in series: These particular Liteon devices changed only about 0.4 volts drop from about 10uA to 20mA for the pair, so a simple emitter-follower was used as the regulator for this processor. When in the "standby" state, the LEDs were biased with a resistor from the +12 volt line somewhere in the 1Meg range, but when "on", this bias was diode-anded to a few mA . The end result was that the unit drew 10-15uA in "standby" mode with the Vcc on the PIC changing only about 0.5 volts between the two states - a change that it could easily "ride out".
The biggest worry when I mention this is that of the well-known sensitivity of LEDs to light, but we thought of this: The forward drop voltage changed only a few 10's of millivolts between darkness and the brightest light that we could muster (Sunlight or my 10 mW HeNe Laser tube) and environmental and in-situ (e.g. production!) testing validated this.
To be sure, the "wrong" sorts of LEDs (super high-bright, green, blue, white - anything that was devised since the mid 1980's) will not work for this, but these were the plain old red, T1-3/4, non high-brightness indicator types that exhibited the 1.7-1.9 volt drop, still cheap and readily available.
Since that time I've used this trick a number of times to get a quick and dirty "3 diode drop" with a nice, sharp knee and it has always worked with no surprises.
(Just something to stick into your mental toolbox.)