I'm an electrician and my work is to maintain boats.
I have a problem. On the boats are sailing lights, and for each sailing light is another light inside the boat that tells you if the sailing light is working or not.
This was no problem with the old 40W incandescent light bulb but now all the lights are LED and the trick doesn't work any more.
This is the sailing light I'm using:
- Hella marine. NAVILED PRO TOP. 2LT 959 908-5.
- Operating Voltage 9 to 33V DC
- Power Consumption Less than 2W (0.14A@12V / 0.08A@24V)
So this is what a have on the drawing on picture 1:
- H1 is the sailing light.
- D1 is the indicator light and will only shine if the sailing light is okay.
- D2 is only to drop the voltage be 0.7V.
Picture/designs 1 and 2:
And here is how I ended up doing it, and it seems to be working 🙂
Thanks for all your help!
One comment on Neil's answer: If the load does draw a current surge the LED is still at risk because there is a low resistance path through the base, a resistor is needed in series with the base.
An alternate circuit would use a PNP to sense the current and drive a LED connected to ground for minimal voltage drop but some extra current usage. Looking at the question again, picture 2 shows this but with base and emitter reversed and a resistor needed in series with the base.
simulate this circuit – Schematic created using CircuitLab