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replace LED with relay that activates large LED panel

Hi there,

I have a circuit that, under some circumstances, lights a LED. The problem is that the LED is too small and can only be seen while in clos e proximity of the circuit.

What I'd like to do, is keep the LED in place, but add some additional circ uitry after the LED so the original LED keeps functioning, but when it dete cts the led has voltage applied to it, drive a relay that in turn turns on/ off the 12v that powers a large LED panel like this one:

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What would be the preferred/lower cost design to achieve this?

Thanks, FC

Reply to
Fernando Cassia
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rcuitry after the LED so the original LED keeps functioning, but when it de tects the led has voltage applied to it, drive a relay that in turn turns o n/off the 12v that powers a large LED panel like this one:

You could turn on a scr from the original LED. And have the scr ground one side of the large LED panel. You would want to have a resistor in the li ne from the original LED to the gate of the scr, or else the scr will drop the woltage enough that the original LED will no longer be lit. This will not turn the large panel off when the original LED goes off. But you could put a switch in series with the supply to the large panel and reset the sc r manually.

Dan

Reply to
dcaster

Do you have something against using transistors?

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Regards, 
Martin Brown
Reply to
Martin Brown

If the voltage drop above the LEDs series resistor is greater than 1.5V you could replace the original resistor with the diode of an optocoupler and a smaller resistor. The transistor of the optocoupler then drives a relay or a transistor with sufficient power.

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Dipl.-Inform(FH) Peter Heitzer, peter.heitzer@rz.uni-regensburg.de 
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Reply to
Peter Heitzer

First thing to find the GND of the supply that powers the LED as you will need that to be connected to your external 12V supply. Then you need to know the voltage of both sides of the LED in on as well in off state. Depending on the outcome you will likely need a transistor, a resistor, a diode and the relay.

I do not advise an SCR as it can easily be turned on but is difficult to turn off.

Off course, you need to know the current required by the LED panel to choose the relay. In turn the relay will decide what transistor to be used. This and the voltage(s) measured from the LED define the resistor. The diode can be about everery general purpose SI diode.

petrus bitbyter

Reply to
petrus bitbyter

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Power MosFET.

What's the current draw of the LED panel? What drives the existing LED? Preferred connection would be upstream of its current limiting resistor (if any). What sort of 12 volt supply do you have available and how is it related to the circuit's supply (grounds, etc). If you have to add a supply and its a sensitive circuit, you might want to throw in an opto-isolator as well.

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Paul Hovnanian     mailto:Paul@Hovnanian.com 
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Reply to
Paul Hovnanian P.E.

I second that.

Another option that's not as good would be to replace the LED with an optoisolator, then string an LED back from whatever that optoisolator controls to give you your one twinky-light.

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

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