LED ans speakers.

[pljack]

I there a simple inexpensive way to attach LEDs to computer speakers.
What I'm after is if there is any sound coming out of the speaker
the LED would light. Not so much with the sound, as in a light organ, but
just light to indicate the that sound channel is working.

I believe non powered(external) speakers have 3 volts going to them.
So if we tapped the input before the speaker we could get a power source?
Assuming that is true, how to light the LED if sound in being sent to the speaker.

Anyone done this before?

Jack

 

[audioguru]

Hi Jack,
Speakers don't have 3V going to them. They have an AC signal with a peak voltage of next to nothing when playing quietly to many volts when playing loudly. The necessary amount of voltage to light an LED would be when the speaker is playing quite loudly.

An old-fashioned red LED needs about 1.8V to 2.2V to light brightly and any more voltage fed to it will burn it out because its current would be too high. If you add a resistor in series with the LED it would limit the current.
A new ultrabright blue, green or white LED needs a voltage of about 3.5V to light brightly. They also need a current-limiting resistor in series with them.

LEDs have a negative voltage limit of 5V that must not be exceeded. Since the signal feeding a speaker is AC, it is negative half the time. You can limit the negative voltage with a diode, or just by connecting a second LED backwards across the 1st one.

Try it. Connect two LEDs in parallel with one connected backwards across the other. Connect a 390 ohm resistor in series with them. Connect the free end of the resistor to one speaker wire and connect the LED wires not connected to the resistor to the other speaker wire. Slowly turn-up the volume. If the LEDs are too bright, increase the value of the resistor. If the LEDs are too dim than they need their own amplifier. :lol:

 

[pljack]

Hi audioguru.

Speakers don't have 3V going to them.

Now that I think about it, your are right.
Sounds like if I just connect an LED and resistor inline with
each speaker I should get some indication, dim or otherwise that
the speaker channel is working.

The speakers are just for diagnostics on a PC repair bench so any
brightness of the LED would be fine.

Seems a better idea would be to use a small speaker extension cable
for the LEDs. that way no modification of the speakers would be needed.

Thanks again.
Jack

 

[audioguru]

Hi Jack,
You must have deaf people working at a PC repair bench or it is too noisy there for them to hear if the PC speaker output is working, right? :lol:

Don't use only one LED with a series resistor, or else the LED's max reverse voltage rating of only 5V might be exceeded. Use two LEDs, connected back-to-back with one wired backwards to the other.

The LEDs and their series resistor shouldn't be connected "inline" with a speaker, they must be "tapped" into both wires of a speaker. Therefore an extension cable can be used is if it has one plug to the PC and 2 jacks, one for a speaker and the other for the LEDs and their resistor. Or if you aren't using a speaker then an extension cable can have the LEDs and their series resistor connected to its end instead of a jack. :lol:

 

[Oznog]

Sounds like if I just connect an LED and resistor inline with
each speaker I should get some indication, dim or otherwise that
the speaker channel is working.
Thanks again.
Jack

Well, read carefully that the LED will NOT light unless the signal is above a threshold voltage. The reds have the lowest threshold voltage, but it may or may not be a a pretty "loud" level to reach that threshold voltage, it depends on what the volume is set at and the expected impedance of the speakers. This does not sound useful to your purpose since it means the light will not light if the volume is simply configured at a low level.

Is there something wrong with just attaching a known good set of speakers here?

 

[Someone Electro]

cant you just plug in some headphones or an PC speker to test it?

 

[stevez]

It sounds like you need something that can react to varying voltage that would be typical of what you'd see at the speaker terminals. As mentioned already that amount of voltage could be quite variable and wide-ranging.

I've seen LED VU (volume unit) indicators that indicate relative voltage with multiple LEDs. I know Radio Shack's handbooks over the years showed do-it-yourself designs. Sometimes the input/output response is log type - not linear so that a wide range of inputs can still be within the range of the indicator (the old VU meters had a log response). While it's not one LED it is fairly simple and should give a good indication. I don't know if the input to the VU indicator is such that you could connect it across the speaker lines or not - but with resistors or a simple amp that ought to get you there.

 

[jrz126]

I've seen LED VU (volume unit) indicators that indicate relative voltage with multiple LEDs. I know Radio Shack's handbooks over the years showed do-it-yourself designs. Sometimes the input/output response is log type - not linear so that a wide range of inputs can still be within the range of the indicator (the old VU meters had a log response). While it's not one LED it is fairly simple and should give a good indication. I don't know if the input to the VU indicator is such that you could connect it across the speaker lines or not - but with resistors or a simple amp that ought to get you there.

This definitly gets more complicated than a single LED, but I have a schematic for it. **broken link removed** I used this circuit for the LEDs in my roof.
Also, I know the LM3915 IC is a log scale, and I think the LM3914 is a linear version.

 

[audioguru]

Years ago, long before surface-mounted parts, National Semiconductor made little pcb's with an LM3915 under a black little blob and a bar-graph unit with 10 LEDs. I can't change the fairly dim bar-graph LED module easily because they welded tiny LEDs under the diffuser. Some day I might drill the LEDs out and put in a modern ultra-bright LED bar-graph module. :lol:

 

[machavelli]

I don't know if this will help or not, but I use one of the bulbs from a old string of christmas lights, works good.

 

[audioguru]

Hi Machavelli,
An incandescent light bulb draws 10 times or more its rated current when it is cold. Therefore it would be like a dead short to an amplifier or transistor that is trying to drive it.

Measure a lightbulb's resistance. I couldn't find a 3V series Christmas lightbulb like you used, but I measured a bigger 5W/120V clear one that I use for my night-lights: 274 ohms when cold and the tiny current from my ohm-meter makes it increase slowly as it warms a little. When hot, it calculates to be 2880 ohms.
If your 3V Christmas bulb is rated for 100mA, then it is 30 ohms when hot and less than 3 ohms when cold. If it is a 2V series bulb, then its resistance is even less. :lol: