OK. So this is probably electronics 101, but I missed this lecture! 
So up to now, when I have wanted to say power an Arduino (now Spark, of course) which controls a 12v device and a 24v device and maybe a 9v device using relays, I would have used three wall warts powering the different devices.
But surely there is a less cumbersome way. Can I just use one power supply at the highest voltage and then reduce the voltage for the other components with resistors? Or is it more complicated than this?
thanks!!
Yes, you could have 1 24v device and then regulate that down to the various voltages you need. Unless you application is extremely simple you will need to use something more complicated than resistors however.
Can I get a Bill of Materials?
What are you using that uses 24v? 12? 9?
Dear Harrison, you certainly may have a bill of materials.
I am wanting to power and/or switch:-
- Rain Sensor - DC 12V (just power)
- Bell (doorbell) - AC 8V OR 12V (power & switch)
- Strobe light - DC 24V (power & switch)
- Electric Door Strike - DC 12V (power & switch)
Ideally I’m wanting to do the above as cheaply, simply and compactly as possible - even if this means a bit of extra work on my part either soldering or procuring.
I was interested in using a MOSFET because wiring a relay up to an Arduino/Spark seems to involve soldering more components than hooking up a MOSFET. I may be wrong. I’ve only done the former, I’ve never touched a MOSFET.
@daneboomer, you can use the excellent Spark Relay shield you can find in the store. As for your power, except for the AC doorbell which is supplied by its own transformer, you can use 24V and a couple of dc-to-dc converters like this one on eBay. They can supply up to 2 amps at the voltages you need. The relay shield has 4 relays which is exactly what you need. 
Thanks peekay. So you are saying I can power the spark core AND my various peripherals from one 24v PSU with DC-DC converters inserted where necessary to provide other voltages? Thanks for your help.
@daneboomer, absolutely as long as your 24VDC supply is rated for the total current needed in your project. Be sure to add filter capacitors (0.1uf and 100uf) near the pins where you supply power to the Core (Vin or 3.3v),
Thanks. I was actually looking for official documentation on powering the Spark Core when it’s not plugged into USB but could only find forum posts. I will have another look because I did not know about the requirement for caps. Thanks!