I have an in-wall switch that is wired to 20V and controls the illumination of the house number plaque outside. I’m wondering if there’s a way to connect the spark to the 20V line? The idea would be to either remotely control the light (and perhaps other outdoor lighting) and to program it to automatically turn on at sunset/off at sunrise. Is this possible or a dumb idea?
Very possible but you need to step down the 20V (and I assume this is 20V dc?) to +5V to power the core.
Using a normal voltage regulator tends to need a good heatsink so consider the little TO220 replacements that are DC-DC and won’t generate much heat. The Recom range is easy to use. You should be able to locate these locally. This is Element14 from Singapore.
http://sg.element14.com/recom-power/r-78e5-0-0-5/converter-dc-to-dc-5v-5w/dp/2078564
This is a 500mA output so same as you would get from a PC USB port. I would then go for either a simple relay to switch your lighting or maybe use a MOSFET based switch (again assuming the output is DC). It all depends on your electronics skills but is very doable. 
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Thanks Dave for the quick reply! The 20V is AC - so I think I’m looking for an AC to DC “transformer” that isn’t a wall wart (ideally this would be in the wall in a double gang box. My skills are novice (which is why I’m wondering if this is a good idea or not). Any additional insight is much appreciated!
OK AC, is not difficult. You just need to include a bridge rectifier before the DC-DC regulator.
As a novice, it will actually depend on how good your soldering skills are and being able to follow datasheets and schematics. As the voltage is safe and low, the worst that can happen is that you blow up the core or the other parts but it might be a better idea to try and find parts that you can wire up together to do this.
It can all squeeze into the space you have but as I said, it will require some soldering and wiring skills and maybe not something you might want to tackle as a first project but then again, how do you gain skills if you don’t try
AC-DC modules are available but they tend to be designed for 110/220Vac and not low voltages like you have so your only option is to use a bridge rectifier before the DC-DC module I linked to earlier.
I came across this…looks like it would do the trick: http://www.amazon.com/gp/product/B00JEXJJKQ/ref=ox_sc_act_title_2?ie=UTF8&psc=1&smid=A2OCOGC9B25845
Nice find. That will do the job nicely. Now all you need is the ability to switch the 20V ac to your lights.
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Or do I need something more complicated?
Perfect. With the power supply board and this you have saved yourself any soldering so it should all just be a wiring exercise now. You can even test it on the bench as the AC-DC will take in DC power too. User +12V to test it.
Make sure you power up the supply without your core connected so that you can adjust the voltage output as it is variable. Hopefully you have a multi-meter handy (if not, another Amazon search may be required) 
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