Comment: How is this going to be powered?
Response: A 5v battery is what I plan to use. Although it would be nice to shrink the power needs down more if it could handle it. But 5v is fine.
Comment: What is the power draw of the light (volts/amps)
Response: I haven't really decided on which light to use. I'm more at the conceptual phase of whether I'm even on the right track. I'd probably build a test circuit out of a single led just to test it. They I could adjust things from there.
Comment: do you want it simply to turn on/off once a threshold has been met?
Response: Yep. When the temperature gets hot enough I want a light to turn on.
Comment: Old style mercury units simply close a contact whenever a set point is reached and modern replacements don't need the mercury.
Response: I hadn't considered that although I probably should have clarified that I was looking for a small and cheap circuit. My understanding is mercury units are larger. But I'll definitely look into that as well.
Comment: You need a "comparator" for this.
Response: Thanks I'll look into this.
Comment: Transistors like the 2N22222 don't turn on because of a voltage, they turn on because of current. Trying to manually tune a circuit to turn on at a specific voltage is doomed to failure.
Response: Ah. Good to know. Glad I'm running it by you gurus.