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Remote Status LED

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jack0987

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PNP-1crop-small.jpg

Hi:

I know little about circuits.

I have equipment fifty or a hundred feet away I would like to see the status of from my desk.

With a little help from friends, I created the above circuit.

It works fine on my first piece, but when I tried to use it on a second piece from a different manufacturer, the LED stays lit all the time.

Please prehaps suggest a better way.
 
Since you haven't shown the nature of the Source Equipment signal to Q1 (the signal level to both sides of the remote LED), it's not possible to know what the problem is or how to suggest a better way.
 
K. In this case it is the Hard Drive LED of a computer motherboard, but I would like to prehaps be able to apply this to any piece of equipment.
 
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Thanks for the reply ronsimpson.
Yes I do know because when I tested the source positive supply pin it was always high.
I am thinking an optocoupler or phototransistor may be a better way of doing this.
What do you think?
 
Hi:

I know little about circuits.

I have equipment fifty or a hundred feet away I would like to see the status of from my desk.

With a little help from friends, I created the above circuit.

It works fine on my first piece, but when I tried to use it on a second piece from a different manufacturer, the LED stays lit all the time.

Please prehaps suggest a better way.
As crutschow noted, if you could give us the voltage levels (with respect to ground) on both sides (legs) of the source LEDs for both the working circuit example AND the non-working (LED always ON) circuit example, we will be better able to suggest a solution. These levels should be checked, obviously, when the source LEDs are ON, but it would also be useful to know what they are when the source LEDs are OFF.

These are needed to properly assess the PNPs' base biasing levels.
 
You would think one end of the LED will change voltage when the light is on/off. Knowing how much the voltage changes I can modify your ciruit.

I have seen a photo transistor used like this. I think they glued the photo transistor over the LED.
 
A1-small.jpg


Thanks for the replies. I will do the voltage readings as soon as I can. (may take a day or so)

However, I am thinking something like the above may work more reliably and we will not have to worry too much about voltages.

I like the idea of the phototransistor as well and will do a quick circuit layout of it later tonight.
 
The idea of using an opto coupler is good, but...

Connecting the LED of the opto in parallel with the status LED is a bad idea.
If the two LEDs operate at different voltages, then one of them will not operate correctly.
Try connecting the opto in series with the status LED, or simply replace the status LED with the opto.

Also, if I were doing this, I would not run the +5v supply from the "source equipment", out on 100 feet of wire.
I would have a separate supply for the remote LED.
That way there is no possibility of the long leads conducting "noise" back into the source equipment.

JimB
 
B1-1-small.jpg

Very good reply jimB.

We could use the source led and a phototransistor. This way we would be dealing with a known voltage all the time of 5 VDC. The circuit would be very reliable and most likely work in all cases.

In addition, your point about the noise is something I did not think of.

I have done a prelim circuit picture of this above. I picked the R1 value out of the hat. I chose Q1 because I believe it is sensitive to visible light. I added Q2 to boost the output. Did I do good?

In the meantime, does anyone know of any phototransistor examples which would apply?
 
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