Electronic circuit question

[Paul G]

Hello all,

I am working on some annunciators which will be driven by a Phidget LED 64 board. I was thinking that it would be really cool to have annunciators that have an 'on' state and 'off state. For example, bright green when on, and dull orange when available but off. That way you know where indicators are, and it doubles as a fail safe (i.e. no light = circuit failure)

Is there any way to do this electronically? Basically I'll draw power from the phidget board to light the green LEDs. But when the output is switched off, I'd like a circuit that detects this and applies power to the orange LED. I'm pretty sure this is all possible, but have no idea where to start. Something tells me that transistors are involved in this somewhere, but I'm guessing.

Thank you

Paul

[Mike.Powell]

Paul,

After taking a cursory glance at the LED-64 board, I think the easiest approach is to use two outputs and two LEDs in each annunciator; one for the "on" color and the other for the "off" color.

It would be simpler if the LEDs were simply powered on or off by the board, but they are multiplexed in a 2 by 32 array. This means each "on" LED receives a series of pulses rather than a constant current. While you could certainly develop a little circuit which properly interpreted this series of pulses and powered an "off" LED in its absence, the parts count adds up. Off the top of my head, you'd have something like a diode, two or three capacitors, some resistors and a transistor for each LED. Cost per LED channel would be $2~3 once built. Since the Phidgets LED card retails for $100CAD, maybe a seond card is cheaper/easier?

[Paul G]

Thanks Mike for the insight. Your explanation reveals how the LEDs in the 64 can be dimmed, which is normally not a characteristic associated with this type of lighting. Thanks also for laying out the rationale for another board. As you say, at $100 it doesn't take many additional components coupled with time soldering and burning your fingers, to make succumbing to another LED 64 more inviting.

[phil744]

This may sound a daft suggestion, but can you not squeeze 2 different LED's in there, one of these "hyper bright" LED's connected to the phidgets board and on "cheapo standard" LED that is connected to a permanent on circuit that is only supplying half of the voltage required, when the phidgets signals the "hyper bright" LED to on its light intensity washes out the dull permanent on LED?

[Paul G]

Sounds like that would work too. As you say, the brighter one would dominate. While we're on the subject, is it possible to stick two LEDs in parallel in the LED 64 board? I'm looking more for light dispersion than anything. Or do I have to use two of the 64 outputs for this?

Thanks

paul

[phil744]

I dont know the 64 board well enough but check its documentation to see what the maximum load is on its output, it should be OK, if not you can use a PNP transistor like a BC846 and switch both LED's to ground that way

[Mike.Powell]

This may sound a daft suggestion, but can you not squeeze 2 different LED's in there, one of these "hyper bright" LED's connected to the phidgets board and on "cheapo standard" LED that is connected to a permanent on circuit that is only supplying half of the voltage required, when the phidgets signals the "hyper bright" LED to on its light intensity washes out the dull permanent on LED?

!

One excellent idea!

!

[Mike.Powell]

Sounds like that would work too. As you say, the brighter one would dominate. While we're on the subject, is it possible to stick two LEDs in parallel in the LED 64 board? I'm looking more for light dispersion than anything. Or do I have to use two of the 64 outputs for this?

Thanks

paul

It looks to me as though you can parallel two LEDs, however, you may not get the same amount of light from each. If there is a difference you can place resistors in series with each to equalize the light output, but at the cost of a perhaps overall light output reduction.

Another way to get more light spread is to choose an LED in a different package that is designed for greater dispersion. It's also possible to modify an LED with a coarse file. I've ground the lens end flat to spread the light a bit more. Seemed to work just fine.

[Trevor Hale]

Hello all,

I am working on some annunciators which will be driven by a Phidget LED 64 board. I was thinking that it would be really cool to have annunciators that have an 'on' state and 'off state. For example, bright green when on, and dull orange when available but off. That way you know where indicators are, and it doubles as a fail safe (i.e. no light = circuit failure)

Is there any way to do this electronically? Basically I'll draw power from the phidget board to light the green LEDs. But when the output is switched off, I'd like a circuit that detects this and applies power to the orange LED. I'm pretty sure this is all possible, but have no idea where to start. Something tells me that transistors are involved in this somewhere, but I'm guessing.

Thank you

Paul


I may be mistaken, but I don't think so. If you have PM systems, using the LED64 you can specify a brightness on and a brightness for off if you want, for instance, there are LED's on the 737 Overhead, that when the Valve moves it goes really bright, then when the valve is in place, it goes dimmer. Or something like that.

In essence you can use the PMsystems code to manipulate the intensity of the LED in the state that you specify.

Just a thought.

Trev

[rprather]

... is it possible to stick two LEDs in parallel in the LED 64 board? I'm looking more for light dispersion than anything. Or do I have to use two of the 64 outputs for this?

Thanks

paul

Yes it is possible. I use the Phidget boards on my overhead and it works fine.

If Es in this image is the output from 1 port on the Phidget board, then Vr1 = Vr2 = Vr3

http://www.tpub.com/neets/book1/chapter3/32NE0082.GIF
Copied from (http://www.tpub.com/neets/book1/chapter3/1-24.htm)

Like Mike said, the thing to watch for is total current. However, if you need to switch higher currents or voltages, you can use the output to switch a low voltage relay, then you can use as much voltage as you need!

Robert
777simulator.com

[Paul G]

Just as a follow up, I tried the method with one bright green LED as the 'on' state and two very dim orange leds to indicate 'available'. When on, I can still see a bit of the orange on the edges, but there's a distinct difference between on and off, and that's most important. I will upload some real pics of this when I have a moment.

In terms of wiring, the green led will be wired to an LED64 board. The always on led will simply be hard wired to the power supply.

[AndyT]

Another option I have not seen listed here:

Use a tri-color LED and switch the polarity for ON, OFF or FAIL.
A few more parts, but if you put all the switching circuits up front of the matrix, it won't be that much more complicated.

You could have:
RED = Alert
Blue = Normal
Green = TEST
OFF = Off