Can an AC signal light an LED?

Cliff_J

KingNothing - if you search around the internet you'll find all types of sources for working with LEDs and the approach to it. Here's a quick summary.

Each LED will have a working voltage and current, you want to avoid exceeding either one but most importantly the current.

Lets say the LED is 3V and 10mA and you have a 12V car battery. Well the battery is more like 12.8V when charged and something like 14.4V when the car is running. It should be obvious that 14.4V is a lot bigger than 3V and would burn this out is no time.

In a series circuit the current would flow into the resistor and then out and into the LED and back. Since we know ohm's law means that if we know the voltage and the current we want we can calculate the resistor that will allow that amount of current to flow (see link below for a calculator for the easy answer). Also we can use ohm's law to double check to make sure that if the correct amount of current is flowing the resistor will drop enough voltage to bring it down to a safe level for our LED.

http://home.cogeco.ca/~rpaisley4/LEDcalc.html

You could wire up many of them in series too, this page has an example of that where the author puts 3 in series and then calculates the resistor.

http://unclean.org/howto/led_circuit.html

To complete my example, the easy solution with a 14.4V source and 3V diodes would be to wire 5 in series and that would drop 15V - pretty close and requires no extra parts at the expense of a tiny amount of brightness replaced by a tiny amount of longer lifetime.

Just make sure when wiring in series that the diodes are all facing the correct direction, the anode (longer one) facing toward the positive.

And I still think a christmas tree light bulb is much cheaper and simpler since they will be much more forgiving if you overvolt them.

KingNothing

Okay...thanks, that does help. What about lighting an LED with an audio signal? I bought some red LEDs from RS a few hours ago. I plugged the prongs directly into an old stereo (the speaker terminals)...it lit, although very dimly. Although with the volume on full blast, it followed the music very well (although it was way too loud)...but that's also one speaker terminal down that I can't use for the actual speaker.

I won't have any problems running an LED off the 12V battery in the car. Why is the LED so dim when connected directly to speaker terminals? Does it have too much current and not enough voltage?

I have heard of a 'voltage multiplier' circuit, but only seen them used at much higher voltages. Could that help me at all? How do I make a simple voltage doubler or tripler, even if that won't help here? Im not familiar with a lot of schematic symbols.

Can someone move this to electrical engineering forum? I think the topic better applies there.

Cliff_J

An LED is very fast, it can blink many times faster than your eyes can see. Like others, a series of still pictures running at 24 frames a second appears as a moving image in a movie theater. A sound at 24 times a second is outside the range of most musical instruments this side of a pipe organ. A guitar or piano are going to make sounds that vibrate 10x faster than your eye can see - it blurs this fast repetition into a dimmer light.

You don't need a multiplier, you need something like an audio compressor and a VU (volume unit) meter to do what you want to do. An analog VU meter circuit might have enough output to drive a single LED to varying levels of brightness by itself and could drive a single transistor that could drive any number of LEDs.

http://www.shure.com/support/technotes/app-meter.html

KingNothing

Sounds a bit too expensive. I figured my poor music quality (128-bit mp3) and crappy old stereo woulda taken care of that! Nevertheless, how would I make a voltage amplifier (for use outside of this).

Also, what kind of light bulbs would operate at around 3V and 2mA? (these are the values I measured of the sound my stereo puts out)

Danger

My graphic equalizer has LED bar graphs that follow the frequencies of the music. If your's does too, it might be simplest to just relocate them to the speakers. Maybe not what you're looking for, but easy to do and no messing about with intermediate circuitry.

KingNothing

Oh there's probably many easier ways, but I like to mess around and build stuff, then I can feel more proud. I'll have pics if I ever get this project done. Also, I see what a Volume Unit does...where can I buy one?

I can't seem to measure the electrical properties of the audio signal with my multimeter - I can measure the AC voltage at around 1.8V, the AC current around 2mA, but the LED limits are given in DC - will this be a problem?

Rogue Physicist

You are best isolating a part of the speaker signal, and having it control another circuit which powers the LEDs. Here are the reasons:

1) "Adding any circuitry will distort the audio signal." This is probably not that important if the impedance of the LED circuit is 100 or more times the impedance of the speaker system. However, you should do two things to minimize this anyway: Connect the circuit to the bass or midrange circuit (post crossover) so that you don't affect the high end, which is more sensitive to distortion. Don't load the circuit down with a lot of LEDs and voltage-dropping resistors which waste power and voltage.

2) The varying voltage of the audio is difficult to control. Peaks are very likely to overload the LED reverse bias and fry them. If you have to add more sofisticated circuits, you might as well have just done it right.

3) It is a pain in the *** to make diodes work from high voltages. Putting them in series means if one burns out they all go out, for minimal reliability. Voltage-dropping resistors waste tons of power.

So the right way to do it is have a fixed low voltage power supply that can be controlled by a simple circuit that doesn't present a varying or low impedance load to the speaker signal. This can then control any number of lights to flash to the music. The circuit design will be the same as that in any Radio Shack Organ Light from the 70's.

Danger

You are one busy little dude, aren't you? I've been following you all over the place because your posts are so nice to read. It's a pleasure to see so much information presented without losing clarity.
I cheated on the colour organ thing, since I couldn't afford a real one. I just glued a bunch of mirror chips to a piece of Saran wrap, stretched it across the speaker, and shone coloured lights on it.

Rogue Physicist

LOL I fell off my chair reading this! I had forgotten how much fun can be had on zero dollars.

As I recall, one of my first experiments as a 9 year old was to make an electro-magnet by wrapping about eighty turns of lamp-cord around some spikes and plugging it in the wall! I was just lucky it created enough inductance not to explode and blind me with molten copper shards. I happily stuck it to dad's car after attaching it to the outdoor socket. Eventually we took it to class and stuck it to the metal frame of the fish-tank. (The fish mysteriously died later.) No one, child or adult seemed aware of any danger. Years later I did a quick double take going over it in my mind.

KingNothing

Rogue, thanks man! That helps a lot. I am thinking about posting in the Electrical Engineering forum asking how to do this, but I have a feeling I would just end up hearing it from you. Do you have an e-mail I can contact you at?

I think I will need 3 LEDs per small speaker, and 4 LEDs per large speaker. I plan to use these LEDs: http://www.lc-led.com/products/500tb4df.html