I have a couple things like a sonar sensor and a relay module that need a 5V input and the Spark Core only gives off 3.3v. Is there a easy way to use these things without using another power supply?
When the relay module has optocouplers, it’ll probably just work on 3.3 volt though you might need to supply the relay board with 5v for the coils.
The digital inputs on the core except for D2 are 5v tolerant.
A lot of sensors will run on 3.3v as well (double check of course).
If it’s an analog sensor that requires 5v, a simple voltage divider might do, if it’s digital and requires 5v comms you can use a level shifter.
5V inputs to the core pretty much work. It’s often the output to a 5V input that causes a lot of headaches. In these situations I would run with level shifters. Those little boards from Sparkfun are worth having lying around. I bought a bunch of them a few months ago when I got my first Spark Core and sure glad I did. Solves a lot of issues.