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The device I want to make is simple to describe: a big red lightbulb, connected to the AC 110 / 220 V, stays dark when it's quiet in the room, starts lighting up somewhat if there's a moderate level of noise, glows brightly when there's a lot of noise. When the room goes quiet again, the bulb turns dark.

Basically, a visual cue that you're making too much noise and you need to keep quiet.

The device doesn't need any hysteresis or a lot of inertia - it's fine if it tracks the instant level of noise quickly. Of course this is easy to control via a low pass filter or something.

Back when I was a wee lad in school, I knew how to make the classic circuit with a triac that could control the AC bulb with a small DC signal. But that works with incandescent bulbs. I am less sure about them newfangled LED bulbs - do they work well with the chopped up AC sine curve that comes out of the triac? So that's issue number one.

The other part of the circuit should capture the signal from a microphone, amplify it, and inject it via an optocoupler into the AC controller. I'm pretty sure I can figure this one myself (could be as simple as an opamp and a few passive components), but anyway if you have a better idea (like if there's a dedicated IC or something) then I'd be happy to learn from you. The thing here is to introduce a logarithmic scale, to mimic the sensitivity of the human ear - other than that it should be pretty basic. I remember there was a trick with a FET in the loop that would stay pretty close to a logarithmic curve.

Perhaps there's an IC that does all this already? (except the pure AC stuff)

Any suggestion is appreciated. Thanks!

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    \$\begingroup\$ As for using new fangled light bulbs for your lighting idea, you would need to find a bulb that is workable with a dimmer control. Many of the older fluorescents and some existing LED bulbs do not work with a dimmer circuit. You would need to find newer fangled bulbs that specifically list that they do work with a dimmer circuit. \$\endgroup\$ – Nedd Jan 18 '18 at 5:55
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    \$\begingroup\$ Unlike incandescent bulbs, LED bulbs have regulators inside so that the brightness remains "nearly" constant for 85-265Vac or 185-265Vac input range. So I think that the conventional triac-based choppers won't work. Maybe you should pick dimmable LED bulbs but probably they provide one-step dimming. Another option would be picking a non-regulated DC LED bulb (12V or so) or making your own since it will be used as an indicator. \$\endgroup\$ – Rohat Kılıç Jan 18 '18 at 5:55
  • \$\begingroup\$ One source of unregulated LED bulb modules may be those used for automotive use (or replacement). Some of these lower price automotive versions are just built up modules of several LEDs with a resistor in line. Of course you would need a separate 12v DC supply to run them. Amazon has a wide assortment of these available: amazon.com/dp/B06XVT9D1W/ref=psdc_15730031_t1_B071P1QZS3 \$\endgroup\$ – Nedd Jan 18 '18 at 6:10
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My answer is different by using stuff commonly available although low tech.

Take a microphone put it in or near the room. Run it to a cheap 30 watt stereo. Get a 30 watt bulb and hook it where the speakers go instead of speakers. When it gets loud the light lights up to the sound decimals with a smooth light transition. You can run another bulb inside the same bulb housing hiding the 2 bulbs one being consistently on. A $10 Fm transmitter would make it work on any 10 dollar stereo. You could go farther and daisy chain separate LEDs to equal 30watts and make a led decimal meter.

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  • \$\begingroup\$ That sounds like it might work, though it does require an incandescent bulb. There are plenty of cheap class D amplifiers online. Have you actually tried this? \$\endgroup\$ – Florin Andrei Jan 26 '18 at 21:53
  • \$\begingroup\$ @FlorinAndrei itll work with a dimable led \$\endgroup\$ – user103579 Jan 26 '18 at 21:56
  • \$\begingroup\$ Yes.............. \$\endgroup\$ – user103579 Jan 26 '18 at 22:00
  • \$\begingroup\$ @Muze, upvote for outside of the box thinking .... you caught may attention with your weird question about the thin wire power transmission ... lol \$\endgroup\$ – jsotola Mar 26 at 4:27
  • \$\begingroup\$ @jsotola thank you I am trying to make it better \$\endgroup\$ – user103579 Mar 26 at 4:30
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LED bulbs are so often retro-fitted into existing TRIAC dimmer situations, that it's been necessary for the market to produce 'dimmable' LED lights. They tend to be more expensive than non-dimmable ones, but not by much.

While the bulb itself may be dimmable (that is, detect the phase shift angle and use that to program the LED current) beware that most TRIAC dimmers have a minimum current below which they don't work well. You may need to put an incandescent bulb, or some other resistive load, in parallel with your LED bulb to get an existing dimmer to work.

If you need to buy a new dimmer, then you may be better off starting from scratch with a properly DC controlled LED.

As you want a red lightbulb, it will be better to use a red LED, rather than a filtered white LED.

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There's "VU meter" ICs out there – they drive the little vertical "loudness" multi-LED bar on your 80's boombox :)

With these, you could very easily design a loudness-controlled "bulb", if you used the outputs to drive a high-brightness LED driver, which is not that hard: These ICs come in different output shapes: one where the outputs are switched "high" if thresholds are passed, others where the output are pulled low. You can convert one type to the other with a simple NPN-based inverter!

I'd really recommend just feeding these as inputs to a cheap microcontroller, which, in turn, PWMs an output, with which you control a LED driver.

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