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Audio activated LEDs

Melonsprout

New Member
I want to have an LED light up every time there is an audio signal sent from a contact mic into a potamp and out to a jack. Theres a contact mic soldered to a guitar potamp which is soldered to the output jack. Is there any way I can (without the usage of a battery) solder an LED into this to light up when the potamp sends the amplified signal of the contact mic? Thanks!
 
Yes, you can connect LEDs directly to an audio amplifier output signal without any external supply and they will work superb. If you do it right.
The audio fed to a bridge rectifier
1711147974927.png

and its DC voltage turns on a led trough limiting resistor and the next led is fed by a diode in series. Done it many times and gave away to friends that enjoyed them. Somehow unable to attach the photo of the gadget. :(

1711202349357.jpeg

Edited, added image above. By my thumbnail is the rectifier in whitish.
 
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Yes, you can connect LEDs directly to an audio amplifier output signal without any external supply and they will work superb. If you do it right.
The audio fed to a bridge rectifier (oscured at the bottom)
View attachment 145079
and its DC voltage turns on a led trough limiting resistor and the next led is fed by a diode in series. Done it many times and gave away to friends that enjoyed them. Somehow unable to attach the photo of the gadget. :(
Thanks so much!
 
What do you mean by "potamp"? I thought to start with it was supposed to be preamp?
You do say its an amplified signal??
I mean the potentiometer and the amp. It is an amplified signal. Contact mics are passive, they can be soldered to this and then an output jack as pictured below.
 

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You showed a piezo vibration sensor connected to a potentiometer (level control) and to a jack.
The amplifier output is not shown. The signal at the jack is probably too low to drive an LED.
 
as pictured below.

There is no amplifier in that image. There is a piezo element that can generate a voltage without any other electronics or power source, a potentiometer (volume control) than can *only* attenuate the signal, not amplify it, and an output connector.

To your original question - almost certainly no.

While the output signal from a contact mic can be relatively high in terms of a normal electronic instrument voltage level, two things about LED power:

1. Voltage. It would have to be greater than the minimum voltage an LED needs to light (called its forward voltage, Vf), usually around 2 V peak for red and green, and higher for blue and white. A white LED with a Vf of 3.5 V would need a signal that is 7 Vp-p (volts peak-to-peak). That's a lot.

2. Current. The typical current as listed on a datasheet for many small, garden-variety LEDs is 20 mA. While many LEDs will make usable light with much less than this, the current out of a piezo element is way less.

ak
 
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Yes, you can connect LEDs directly to an audio amplifier output signal without any external supply and they will work superb. If you do it right.

To clarify for the OP:
That method applies to audio POWER amplifiers that drive loudspeakers - not low level guitar signals.
 
I don't know what components your circuit actually consists of. Anyway, let's talk about a general case. Without an amplifier, I do not think the signal strength will be enough to drive an LED. You will need an OP-AMP or transistor to amplify the signal strength. For audio activated LED projects, I find LM3916 the most convenient. LM3916is a Dot/Bar Display Driver. It's known as the VU-meter IC.

 
The LM 3915 and LM3916 LED drivers are obsolete and not made anymore. Fake or defective ones might be available.
An audio power amplifier can light an LED if the signal voltage is high enough, if a series rectifier is used to block reverse voltages and if a series resistor is used to limit the current.

A peak detector circuit night be needed to see short duration audio peaks.
 

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