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wire a computer speaker jack to an LED?

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Hello All!
I'm a newbie and am working on a special project for my girlfriend. At work, she has four computers setup to run a program. This program searches for various things on the web and when it has found what its looking for, it sets off an alarm. Now, with four computers running, it gets difficult at times to distinguish between the four alarms when they all ring at the same time. I was wondering, is there a way to connect LEDs to each of the speakers so that an LED would light up whenever electricity is passed through the speaker wire? Can this be done? Any help would be appreciated. Thank you.

Connect a LED and a 330Ohm series resistor in parallel with the speaker. Hope this works.
In parallel? You mean have the LED connected to the speaker and have the LED connected to the resistor which is then connected to the speaker? I'm new to electronics, played around with it a little in jr high, but its been a while since then. Thanks for the advice though, I'll try it out after work today.

Yes the LED and Resistor will be in series and this circuit will be in parallel with speaker. Be sure of LED and Speaker polarity. Positive of speaker to Anode(+) of LED through resistor and negative of speaker t cathode(-) of LED.
It didn't work... the LED wouldn't light. There must be a way this can be done? Can anyone offer advice?

- Tom
Ok try this. This may work. The cap. will filter out all the ripples and produce a constant DC for LED.


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Okay, the circuit didn't work either. It seems as though not enough current is getting to the LED to light up. When I wire up the circuit to a 3V power supply, the LED lights up. But the LED will not light up when connected to the speaker. What should I do? Please help.

Try out only with LED and serial resistor.( maybe lower value as 100ohm)On speaker output is a rectangular TTL signal, when the polarity OK. ,it work.
Okay that didnt' work either... Its definitely not enough voltage coming through the speaker wires. Is there a way I can wire up the LED to its own power supply and use the current coming in from the speaker wires as a switch. That is, when sound is coming from the speakers, it causes the LED to switch on? Help, I'm new to this but I would like to learn.

Well I think your PC speaker is drawing large amount of current leaving almost zero current for LED. It may be because you have a magnetic speaker of low impedance instead of Piezo-electric speaker.
Instead of parallel connection why don't you try and see if LED and Speaker works in series or not? In this case same current will flow through LED and speaker and may light up your LED.
Do let us know the result.
Nope, putting the LED in series with the speaker didnt' work either. I tried it with just a 100ohm resistor and I also tried it with that other circuit you posted. Neither one caused the LED to light up when sound was coming out of the speaker. I wish I knew a little more about electronics.

Are you sure you are connecting the LED with right polarity. Make sure that it is correct. If in series the speaker works and LED does not then the polarity of the LED should be reversed. Since speaker is driven by TTL output it should have enough power to lit up LED and sound the speaker.

The other problem I could think of is that the speaker works on high frequency square wave from TTL port. If the duty cycle of this wave is less than 50% then the intensity of the LED will be very less to be visible.
why dont you try to use something simple. at the computer in accesibility options there is something that mekes your screen flicker when a sound produces. search, it mught be metter this way
Hi PammyBoy,

While the circuit kinjalgp supplied is good, there is one major oversight. On all computers nowadays, the sound card requires powered speakers to produce descent sound, therefore a tiny current is output from the card (in the region of 20mA @ 1V I believe).

Being an Electronic forum, the solution above (to use the Accessibility Features to flash the screen) is too simple... ;-)

To operate correctly, this circuit needs a power source to drive the LED, in this case 2x1.5v AA batteries and a transistor and other minor components. The transistor should be a high gain unit, such as my favourite BC108 (they are still making them???), and should be provided an input at it's Base from the PC's sound card.

Hope you don't mind the hand-drawing.
If you find that the sensitivity of the circuit is more or less than what is required, drop in a variable resistor instead of R2. I built and tested this only 10 mins ago, and it worked OK off a small radio I have. Best of luck...

i think that he was talking about the speaker inside the computer the one that is not driven by the sound card.
Don't you think the polarity of the LED should be reverse?
Yep. It should....
....just checking if you are all awake.....

That will work off the internal speaker also, just add and tweak VR1.
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