Help with simple LED switching circuit

[Oziam]

Hi all,
I need help with a simple LED switching project, I have 2 LED's, 1 RED & 1 BLUE connected to a 6VDC power source, the BLUE LED acts as a standby indicator & the RED LED indicates full circuit saturation, what I need is for the BLUE LED to turn OFF when the RED LED ANODE receives power. I need it to be as simple as possible & I am thinking using transistors but not quite sure how to do it, any relevant suggestions would be appreciated. Thank you

 

[MikeMl]

Here are a list of forward voltages for different color LEDs. If yours are as published, this simple circuit should work.

Red is the current through the Red LED; Blue is is the current through the Blue LED as a function of the input voltage sw6V.

 

[Oziam]

That's perfect, doesn't get much simpler than that, I initially failed to see the LED as a (light emitting) diode, but now I see what you've done. Thank you so much, I really do appreciate your help MikeMl. Cheers, Beers

 

[Tony Stewart]

The only catch is the threshold for switching is ~5V and not a logic level like 1.3V for TTL or V/2. for HCMOS.

The switch threshold depends on the difference in Vf of the LEDs below the value of the 6V supply.

Using HB LEDs Red is around 2~2.2 and Blue 3~3.2 so the difference is 1V, hence the threshold will be ~5V +/-0.3V with overlap and both ON during transition.

For margin the Vsw must be equal to the other supply.

 

[MikeMl]

...
For margin the Vsw must be equal to the other supply.

Which is exactly what the simulation shows and the OP said he wanted, and is going to do!

 

[Tony Stewart]

Yes very good Full saturation <1V below V+

For Saturation, keep in mind BJT must have 10% base current to get rated Vsat.