Simple led status circuit..

Hello, I hope everyone is well.

I am drawing a blank on a simple circuit I am putting in a power supply. All I want it to have an LED turn on when power is applied, and when power is cut off . I would like another LED to turn on with the previous one turning off. I know I can do it with a relay, but also remember a simple circuit I've seen in the past that does it with a transistor, and I believe a diode or 2. Thanks for the help..

hi R1,
Would you post details of your power supplies.?
I guess you must be using two psu's.? one for the 'powered circuit and 1st led' and second one for the 2nd led.
E

Hi Eric,

I was going to ask the same question because if the first power supply turns off there is nothing left to power up the second LED

A single LED is a binary device. It emits light when powered, and doesnt when not powered. Why do you think you need an indicator to tell you that something is not powered?

he might have a psu like one of mine, the power output switches off but the actual psu still whirs away with fan going etc, so maybe he just wants something that say output active/not active.
or maybe i am totaly wrong

Maybe to indicate AC power to the supply.
I have an older ATX power supply that has no power switch but has an LED on the back that turns on when you plug it into AC power.
A front panel led indicates when DC power is on..

eT

I could see it being like the "Standby" LED in a computer: even though the computer is "off" and the power LED on the front panel is off, motherboards frequently have an on-board LED that stays lit as long as they are connected to power.

The second LED would indeed need its own power supply, unless you're switching the load independently of the supply (but from what I read of the OP, he wants it to light when the power supply is turned off). As far as making the second LED turn on when the power supply is off, you could either use an inverter IC (also powered by the second supply) which takes the output of the first supply as its input -- so when power is on (HIGH), the inverter output is LOW and the LED is off and vice versa, or possibly use a standard NPN transistor as a "pull-down" in parallel with the LED. I'm not sure of the details on the latter option (you'd need a resistor to limit the current flowing through the transistor, and possibly another few bits), but that should get you started

Lets hope that the OP comes back with more information regarding his project, speculation is getting us nowhere.