So we’re working on controlling an 12v electric door strike from the Spark.
Plan will just be to use a transistor (and diode) to control the power to the door strike.
Ideally the project will just require a single power supply, so wondering if anyone can put me onto the right path about what we’d need to do to convert the 12v from the power supply to the 3.3v the spark wants ?
You can use a voltage regulator to go to either 5V or 3.3V. LM7805/LM7803 springs to mind. You could also crack open one of those car USB adapters, since they go from 12V to 5V as well.
Great idea on the car charger. I have a solution for 24 volts AC or DC to 4 volts DC but nothing for 12 to 4
4V is a bit odd, why would you want that? You could do 12V to 5V and then use a voltage divider, if it isn’t anything critical.
4volt is perfectly fine for the spark core powered through vin/gnd.
I have a potentiometer that I can dial to any voltage I want. I chose 4 volts arbitrarily because its in the middle of what spark can support.
Photo of said device below.
Thanks for the reply. So would it be as simple as just passing the 12V+ through the LM7805 to the VIN on the Spark ? Or would I need to use this in some larger circuit ?
Although 4V is indeed perfectly fine, I personally wouldn’t use it. The Core is perfectly capable of handling 5V. More importantly, a lot of peripheral devices/sensors/actuators run on 5V, which can then be reused. If your only goal is to ‘make it easy on the spark’, then you should go directly to 3.3V, and avoid the onboard regulation entirely. Anyhow, this is a case of personal preference, so either way is okay 
As far as I’m aware, a voltage regulator should suffice for going from 12V to 5V/3.3V. Although we could ask @peekay123 or @kennethlimcp for some confirmation.
Looking at the datasheet , it seems that you require 2 additional capacitors (figure 1, page 7). I think they are used to filter the output signal. I don’t think it will be a big problem if you use it without them (spark core has its own filter) but I recommend you do.
@Herner, the picture of the step up/down converter you posted can handle an input from 3-40V DC so from 12V to 4V using that converter is possible. However, I do not see the requirement of the converter, a simple voltage regulator will do fine at much lower costs.
A regulator works great but I will go for a SMPS IC like the one on the photon for better efficiency and not wasting energy on heat loss. 
SMPS sounds conplicated but they are actually not. Nowadays, all it takes is an IC, inductor and a couple of passive components to do that!
Ok great thanks.
I ended up getting an LM340T-5 as it what was available locally, which looks to do a similar job as a LM7805.
It's datasheet Power management | TI.com looks to be suggesting mainly an capacitor on the input, so I'll probably do that.
Thanks for the tip, I'll look into it once I've got my head around using this voltage regulator 
the 7805 will get pretty warm a 7v to drop is a fair amount for a linear regulator, and it will need the caps - without them the output may oscillate and will kill the core.
5v BEC (battery eliminator circuit) buck converters for remote control things are super cheap ~$4 and easy to find on ebay. most will handle 2s - 6s battery packs so 5.5v - 26v. and 3A so more than enough for the core and a fair few accessories.
I have a 24volt AC signal and for $2 it was well worth it
I completely agree. Since my voltage is adjustable when I need 5 volts I will have it If the voltage was fixed I agree start with 5 volts for other accessories. Great comment