I'm trying to build a simple ±12 (and switchable to 24)V power source for use in audio apps. Just as an experiment, I'm trying to make a led light up when I power something from it. I'm not sure how to do this. I was thinking of connecting over one power line a resistor, and on the resistor more positive supply, a base of a NPN transistor, on the other end, the transistor's emitter.
This way, when current will be applied, it will go through the resistor and there will also be a potential between Base and Emitter, allowing current to pass from the collector to the emitter, and with the led connected in the emitter, it would theorethically light up.
Re: Detecting a load on a power source without modifying vol
You got the right idea, but a simple transistor needs too much voltage to work (≈0.6V). That means a large voltage loss.
Use a low-resistance shunt and monitor the voltage across it with an opamp or comparator. That will work with just a few mV across the shunt.