I'm starting out with resistors and would really use some good guidance here. I've researched a lot but still the knowledge is quite dark in my mind and there's things I don't finish understanding.
For instance, I'm working on a project now which consist in converting a set of lights so that they'll work with a USB connector instead of a battery.
The set works with a single AA battery. So the input voltage is 1.5vdc. I've measured the resistance of the lights and the circuit and, unless I measured very wrongly, its resistance is of 178 kohms.
So using a Voltage Divider Calculator I get that if I need to input 5v and the 1.5v and the 178 kohms resistance and I get 76 kohms.
Now, I'm sure I'm doing something wrong. My idea is to get a resistor to bring down the USB 5v to the 1.5v the circuit needs and, when researching, I was pointed toward the Voltage Divider formula as the way to do this but I'm confused as to how it works.
I was playing around with another set of lights that use 3 x AA batteries and found it only had a 200 ohm resistor that connected the positive side to the light series. However, I wasn't able to measure the resistance of the light series itself. Still, both sets I'm talking about have just 10 white leds, so I'm imagining their load to be about the same.
This second set I'm also planning on converting to 5vdc but first I need to understand how to go about doing it; that is, how I can calculate what I would need in order to get a circuit that takes 1.5vdc and another that takes 4.5vdc and make each work with 5vdc. If anyone could point me out in the right direction and/or clarify certain concepts that I fear I've got wrong, I would appreciate it greatly!
Thanks for all your responses! And I really an VERY sorry that my question (and in fact all my knowledge on electronics) is very much all over the place! I am, as I said, starting out; I'm a very excited amateur that wants very much to learn but hasn't learned anything formally. So please bear with me!
After a couple of the comments, I went back and (yes, I forgot to say there was a circuit on one of the streams of lights) found a capacitor in the circuit, which I'm assuming is used in order to allow the stream to be powered by the single AA battery (1.5 v).
Now, the second stream only has a resistor after the 3 x AAs. As the number of LEDs there was the same as in the first stream and they were both warm white, I went under the assumption that both worked with about 3.5 vdc and a current that a AA can provide. So I tested the stream and found that, yes, it works with 3.5 vdc. So my target voltage is that. And I believe the current out of a 5v USB output will be OK if the original power source was a (or 3) AA battery(ies).
I may be wrong, though, and would very much like your help. But I've since read a couple of things (here and elsewhere) that may be of use to me. What is seemingly the easiest solution, although I don't know if it'll work:
trying to power with 3.5V and how much current you need. If all you want is just to get replacement for pair of AA batteries, just put two 1N4001 diodes in series.
Will that work, do you think?