hi
thanks Dennis for taking the time to breadboard the circuit, i wonder if a zener could be used somehow as a steering diode,
at the moment im tempted to go with the dual comparator approach, dosent take up any extra space, a minimum of additional circuitly,
will explore both options over the next few days, and get back,
thanks again
dougal

 

I had to draw this out because it didn't make sense. Anode and cathode above are swapped.

So...do we agree? Your circuit looks a lot like mine.

Yes, the circuit depends on Vf differences, which are not precise. Some LEDs have a sharper knee in their response; for this circuit, the sharper the better. It is dependent on the exact LEDs being used, but it does illustrate a technique. I've used it several times with 3-lead bicolor LEDs to have a single indicator be red, "yellow", and green with only one driver.

ak

 

thanks Dennis for taking the time to breadboard the circuit,

No problem... I already had my breadboard setup for some experimenting with a slope detector.

i wonder if a zener could be used somehow as a steering diode,

If you're referring to AK's circuit, adding another signal diode would give a large enough voltage differential to ensure that the Off LED wouldn't light. The circuit I proposed shouldn't have that problem, but more power is consumed.

at the moment im tempted to go with the dual comparator approach, dosent take up any extra space, a minimum of additional circuitly,

Success will depend on how the comparator inputs are connected. If both inputs are used for active signals and there's a chance that the input signals will be changing slowly, have noise, or the bias currents are unequal, one or both of the comparators might oscillate (if you don't already have some hysteresis to prevent it).

 

So...do we agree? Your circuit looks a lot like mine.

To be painfully truthful, I considered your solution and discarded it because, for one thing, of the dependency on the forward voltages of the diodes. I didn't realize that that was the circuit you described in your first post until I drew it.

The OP didn't give many specifics, so I assumed he might want to use different colors or want to adjust the current in the Off LED so it was the same as On to match brightness.

All three solutions offered can work and each has it's limitations. You pointed out the limitation in mine and I pointed out the ones in the others. The OP can choose the one he prefers, ask for more, or roll his own...

 

To be painfully truthful,

Hmmm...interesting... When I admit that I'm right it isn't painful at all...

 

Hmmm...interesting... When I admit that I'm right it isn't painful at all...

It might have given me some sense of satisfaction when I was younger, but I'm old now and have nothing to prove (at least as far as this forum is concerned). I'm retired and can rest on my laurels...

Your circuit works if you understand the interdependencies; this is a Peer reviewed forum and I just pointed them out for the OP. I would have pointed out the limitation in the solution I proposed; except I didn't see it as one...

 

HI
i would like to have two leds indicating the on /off states of a comparator. eg red for on, green for off.
can it be done, without adding an additional output (transistor) stage?
thank you
dougal

Something like this? An op amp or a voltage comparator with an output that pulls high.

 

Something like this? An op amp or a voltage comparator with an output that pulls high.

Really?? Could you find something better to do than bump old threads?