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I have a circuit in which I want a pre-built stereo amplifier kit (with + and - outputs for each speaker) to drive a speaker and an LED in parallel. The circuit works, however is there any way to stop the LED flashing with the audio output? In the circumstances the LED drive has to come from the amp, and therefore has to have an audio signal input. Will some kind of transistor do it? Any help is much appreciated. EDIT: I want the LED to be constantly on, instead of flashing. Sorry for confusion.

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    \$\begingroup\$ Run it from the power supply to the amplifier (with a suitable current limiting resistor). \$\endgroup\$ – user_1818839 Jun 22 '16 at 20:11
  • \$\begingroup\$ So, when do you actually want the LED to be off? \$\endgroup\$ – Dave Tweed Jun 22 '16 at 22:22
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Yes, you can simply use a rectifier circuit and a smoothing/filter capacitor. You can adjust the value of the resistor and capacitor to suit your particular conditions.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Not to be picky Richard, but it usually not wise to put an electrolytic capacitor directly across an LED. If the LED becomes very warm its avalanche behaviour changes and schott noise increases, suddenly dumping the full charge of the capacitor and destroying the LED, even if the root cause was a cluster of atoms that formed a leakage path for excess current flow, starting at a tiny amount but having a non-linear rise. I have seen this problem several times. Insert even a 1K resistor in series with the LED and the potential problem goes away. \$\endgroup\$ – user105652 Jun 22 '16 at 20:36
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    \$\begingroup\$ The resistor would be better in series with the LED. The cap is then charged to the full voltage, with the resistor dropping the voltage after that. The advantage is that the LED output will change more smoothly. There will be a larger range of capacitor voltages over which the LED will light reasonably. The circuit shown relies on the LED resistance for the cap to have any effect. Think of the limiting case where the LED has a fixed voltage across it. In that case the cap does nothing except delay the initial brightening. \$\endgroup\$ – Olin Lathrop Jun 22 '16 at 20:44
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    \$\begingroup\$ @OlinLathrop: That's a valid point, but then the rectifier + capacitor would present a hugely nonlinear load to the amplifier, which will likely have an adverse effect on the sound quality. Two resistors would probably be a good idea, one in each place. \$\endgroup\$ – Dave Tweed Jun 22 '16 at 22:25
  • \$\begingroup\$ @Dave: Agreed. Most power amp outputs are very low impedance, but of course not zero, and the impedance of the speaker cable might be significant. Something like 10 Ohms before the diodes, and a larger resistance in series with the LED is probably a reasonable compromise. It all depends on how good of a voltage source the amp is. Oh, and the diodes should be Schottky for better low-volume operation. \$\endgroup\$ – Olin Lathrop Jun 22 '16 at 23:31
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    \$\begingroup\$ Note that I showed a 1000-ohm isolation/dropping/limiting resistor in what is essentially a 10-ohm (or lower) circuit. That is 10x what is normally considered a "bridging load" in the audio world. If we were worried about distortion, we wouldn't be implementing a horrible kludge like driving an LED from an audio amplifier!! \$\endgroup\$ – Richard Crowley Jun 23 '16 at 1:18
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Ultimately, no, you can't keep the LED on steady. Think about it. When there is no sound at all, there is no power available to run the LED from.

You can harvest power from the audio signal and store any excess that isn't needed to immediately run the LED. When the audio level gets too low to provide enough power to run the LED, you then use some of the stored energy. However, any stored energy will be finite, so this can't go on indefinitely.

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As Olin mentioned in his answer, the LED will be off if there is no audio output. The best compromise I can think of involves inserting a 1N4007 diode in series with a speaker (+) output. The end of the diode with a white line goes to a 1K ohm 5 watt resistor, then to the (+) wire of a 470uF 150vdc capacitor.

From this connection a 10K 5W resistor connects to the LED (+) terminal. The LED (-) lead goes to the capacitor (-) lead and ties to the speaker (-) lead. This circuit simples stores energy long enough to keep the LED from flickering, however, low volume levels will dim the LED, high volume levels will make the LED bright.

If the sound cuts off suddenly, the LED will slowly become dark. It is not a perfect solution but it does remove the flickering effect.

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The output of an audio amplifier is a voltage that varies with the sound content, so the LED will flash with the sound.

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