Pulsing LED to Audio using a 2N2222 transistor.

[skinind95]

Hello,

I am a beginner to electronics and came up with this circuit on my own, so there's a good chance i've done it completely wrong. The circuit is supposed to pulse an LED to music, and it does this just fine for the first few seconds. After letting the circuit run for a short time, the LED will slowly start to dim, as if the volume of the input were decreasing. At first i thought that i was frying my LEDs, but they are just fine. Removing either the power, or the audio source and then replacing them will bring the LED back to full brightness.

Could someone explain to me why this would be, or a better way to design this circuit. (I have a very limited supply of components)

Here is the schematic of the circuit:
**broken link removed**
[This was my first time drawing a schematic, so things may be backwards. The transistor, Q1, is a 2N2222 NPN.]

Thank you in advance,
Travis

 

[erosennin]

Look for colo(u)r organ circuits, they also do this job. The first thing i'm guessing is the input signal amplitude if it's just from an audio player or computer is going to be quite low... The other thing is the frequency, if you are playing music through it, it is going to be completely random and be flashing at rates too quick for your eyes to see...
These colour organs have filters to filter out certain frequencies and then activate the outputs...

Just a few suggestions...

 

[Torben]

Hello,

<snip>

[This was my first time drawing a schematic, so things may be backwards. The transistor, Q1, is a 2N2222 NPN.]

Thank you in advance,
Travis

Hi Travis! Welcome to the board.

I don't know whether it's the same on your actual circuit, but the transistor is upside down on the schematic.

Other than that, I don't know. I'd recommend taking erosennin's advice and looking up a simple colour organ schematic. I think he's also right about it probably flashing too fast to see as it responds to the actual waveform, when what you probably want is for it to respond to the perceived loudness of the music.

How is the performance of the thing before it gets too dim (um. . .besides the fact that it keeps getting dimmer)?


Torben

 

[skinind95]

erosennin - That was the term that i was trying to find while researching this! They appear to be a more complex version of what i am building. Thank you.

Torben - Yes, the transistor is upside down in the schematic by accident (and inexperience), the real circuit is not like that.

The high tones coming from the input are not visible on the LED, but the low tones pick up nicely making the LED pulse to the bass, which is what i was after.

Performance is great for the first 3-4 seconds, the LED lights brightly to the beat.

After a little bit of messing with it, i've added a 10k resistor between the positive and negative audio input lines. I'm not sure why, but this fixed it and now the circuit works exactly how it should. ..one day i hope to understand why. =)

Here is the current schematic for the working circuit:
**broken link removed**

Thank you all for your help!

 

[raresh_r]

sorry to bump on this topic but i want to know if that works fine on a pc subwoofer (i have a creative 5+1 system) i'm not that good on building this thing but i wanna know if it is good so i can buy the parts. what parts do i need and what is the "Gnd" ?

 

[Hero999]

Gnd is the 0V rail or negative terminal of the battery.

The problem is that if the amplifier is bridged and shares its 0V terminal with the circuit supplying the 5V it will overheat the amplifier. I think this might be happening here, the IC in the amplifier is probably getting hot and shutting down. Adding another resistor in series with the speaker's - terminal and the circuit will solve this problem.

 

[skinind95]

raresh_r, Gnd is the "Ground", or the negative side of the circuit. (As Hero999 stated). There are a few things that i will note now that i've learned a bit more.

I believe the cause of the fading lights were due to a capacitor somewhere inside the audio source charging up to prevent damage to the player. The previous circuit must not have been drawing enough power and it was being treated as a short. The added 10k resistor fixed this.

Also my audio source (An iRiver H320) has a headphone jack and a line-out jack. The headphone jack goes to an amp then to speakers, and the line-out jack goes to the circuit. I've found that the line-out jack will "Clip" the audio above a certain volume (headphone does not), which may be important to actually seeing the lights pulse. I know this is not good design at all as it relies on something like that, but as i said i am only a beginner and have few components.

I have never hooked this up to a subwoofer, but that may work as well as the clipped audio. It's worth a try

For parts:

R1 = 2.1khm: resistor
R2 = 470hm: resistor
R3 = 10khm: resistor

Q1 = 2N2222 transistor (NPN)

D1 = Just an LED i had laying around.

And you'll need a breadboard or a project board or something, and some kind of headphone jack would be useful.

 

[Hero999]

The output of the MP3 player clips because the volume is turned up way too much and the peak output amplitude is limited by the power supply.

 

[skinind95]

Yes, line-out is meant to output to an external amp and under normal use the volume would be kept at ~75%, but the clipping is desirable in this circuit, i think, because the high flat spots in the waveform allow the led to light long enough to be visible. Adjusting the equalizer settings effects how the audio clips thus altering the lighting effect, which is kind of cool. Like i said, bad design to rely on this, but it works.

Headphone output doesn't clip at all.

I will hook this up to a subwoofer sometime and let you know what i find out.

 

[audioguru]

The signal is AC. The transistor responds to only the positive half of the waveform. So The LED is lighted for only half the time and is dim.

The 470 ohm resistor limits the max LED current to only 6mA if it is red and to only 3mA if it is white or blue. So the LED is really dim.

 

[mojoman0]

Transistor characteristics change greatly during use. They have an internal capacitance as well, but it usually isn't too high. You may want to set up a path for the capacitance to discharge from the collector input, as the diode is keeping the current from flowing back where it came from.

 

[audioguru]

Audio is not at many MegaHertz so the capacitance of a transistor won't make any difference.
The very high stray capacitance of a breadboard might reduce 20kHz audio a little.

 

[philm400]

This is exactly what I've been looking to do with a pair of old AR-4x speakers that I've just done some crossover work on. I'd like to put a circuit across the woofer that would give me a pulsating led (pilot light). Does anyone know if this circuit will work for this purpose? This circuit appeals to me as it is very simple. I don't need a color organ with multiple channels / just want to drive a single led. I tried running it across the input of the speaker. This achieves the effect I'm looking for but without the circuitry the led eventually burns out. Any help on this will be much appreciated!

 

[philm400]

Excuse me, a new member, for not being familiar with older posts in the forum. After looking back a few pages I've come across the post Audio and LED's in which Hero999 posts a circuit that looks like it may work for me. https://www.electro-tech-online.com/threads/audio-and-leds.102803/ Will this work with one led?

 

[audioguru]

This is exactly what I've been looking to do with a pair of old AR-4x speakers

I also have a pair of AR-4x speakers that are 46 years old. One had the flexi wires that feed the woofer break.

 

[philm400]

Mine sound great after updating the caps! Woofer and other spare parts are available on ebay, plus lots of info on plenty of forums. Refinishing and reworking the crossovers was a project. My original pair were bought by my father in 66. I bought two additional pair recently that needed a lot of work. Was thinking of using them in a 5.1 system but scrapped the idea preferring them in a stereo setup. I cut the grille overhang flush to the speaker plane in the two new sets. They're the ones I've added the led pilot light to (upper left corner). Now I just need to get it working properly. Here's a link if you are interested in viewing some photos. **broken link removed**

 

[audioguru]

My AR-4x woofers make a nasty BRAAP sound when they hit something at the limit of excursion at their resonant frequency. It occurs at only about 40W.

 

[philm400]

Maybe the voice coil?

 

[harps]

hello (im a new member here) i hope some people are still interested in this subject .

i bought a little vox circuit last week that takes a line input and switches on/off a 12volt relay, to which you can attach a 240v light or anything you wish. however the release time of the relay was terribly slow, just under 1 second!!! i thought this slow release was due to a capacitor, but could not figure out how to bypass this stored energy in the circuit? so that circuit was no good for the purpose of quickly flashing a bulb to a line level pulse.

incidentally the circuit input range of that circuit was 10mv - 300mv , wiki state consumer line level is .3 volts.

i was just googling AC to DC conversion. To convert ac into dc a diode bridge rectifier can be used Diode bridge - Wikipedia link it seems quite simple to do this conversion, just solder some diodes in a particular way!! But i am unsure which rating diode to use. so tonight after another google search I found this forum. : )

i shall try your circuit out and post back with any more info.

peace: )

 

[mneary]

I suppose if you told us which "little vox circuit" you bought, it would ruin the mystery.

But it would result in better answers.