Hello all,

Currently, I have a system consisting of a number of incandescent bulbs which are used as indicating lights. To indicate "ON" the bulbs are displayed at full brightness, and to indicate that the system is "OFF" the bulbs are displayed at low brightness. This way, at all times, I'll easily be able to see if any bulbs have failed (i.e. this would be indicated by a completely unlit lamp).

The way this works right now, is the lamps are provided with full voltage when the system is considered "ON", and are provided approximately half voltage when the system is "OFF".

I would like to replace all of the incandescent bulbs with LEDs, since they are much more reliable, but the problem I have run into, is that even at half the voltage, the LEDs are still fairly bright, and thus it becomes hard to distinguish between the "ON" and "OFF" states.

I've thought of a couple different ideas, such as using a bi-color LED, which would for example display RED to indicate "OFF" and display GREEN to indicate "ON". However in this case, it is still possible for one color to fail while the other is on, and I would not know that it failed (until I tried to activate it, of course).

Does anyone have any suggestions on how I can easily implement LEDs into my current system (with emphasis on being able to distinguish which lights have failed, regardless of whether the system is "ON" or "OFF")?

Let me know if you require clarification on anything.

 

What is the voltage to the lamps at "full" and at "half"?

Ken

 

Just some other ideas I've considered:

1.) Decreasing the current to the LEDs (either by decreasing the voltage even more, or by adding in series resistance). In this case, I'm concerned with the LEDs not giving consistent output, since they're not really designed to be operated well below their rated forward current. According to what I've read around the net, is that with low forward current, LEDs may either be dimly lit or completely off (even if they have the same specs and are from the same manufacturer).

2.) Pulse width modulation. I suppose this would be the ideal way to dim the LEDs, but this solution would be too time consuming and costly for my purposes.

 

What is the voltage to the lamps at "full" and at "half"?

Ken

I do not recall exactly, as I do not have my specs in front of me, but I believe "full" is 24 V, and the lower voltage is about 9-10 V.

 

Are we talking: a couple of lamps?...a dozen?...a hundred?

Ken

 

Ah yes, I forgot to mention, there are 56 lamps.

 

OK, here's a concept. In the 24V FULL level the LED gets 20mA (bright), and in the 9V HALF level the LED gets 2mA (dim). You may have to play with the exact zener and resistor values to get the best bright/dim ratio for the particular LED.

Ken

 

Could you elaborate on the purpose of the zener diode?

 

It provides a fixed voltage drop. At 24V it isn't very much, so you select a resistor that will give maximum brightness...(24v-6v-1.2v)/~20mA=~1000. But at 9V, it's significant and leaves very little voltage to drive current through the LED....(9v-6v-1.2v)/1000=1.8mA. The numbers are just ballpark.

ken

 

I bench tested it. I added D3 to the zener to make the voltage drop a little greater, and got a better bright/dim ratio. This will also depend on the actual half-voltage level in your system.

ken

 

Hmm interesting. I'll have to try this also, as soon as I get home from work.

Thanks for your help so far!