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I'm looking for a type of device I can hook my arduino (micro-controller) to that will control a set of lights that require 120VAC. Basically I will be making a light organ, where the arduino will be processing an mp3 file, splitting up three frequencies, then turning on the set of lights which is set for a specific frequency. Basically making the lights flash to the music.

The problem is I can't just use a relay because they have a delay time, and if the music is to fast the lights won't turn off in time. So I need another solution.

Does anyone know a device that is low dc voltage controlling 120 VAC, and has a very minimal delay time?

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The canonical answer to this is a solid state relay. If you aren't switching much power you can use counterfeit Foteks found all over the place on eBay for $3. This is not a good idea because they are counterfeit, but it is a cheap idea.

As for the "lights wont turn off in time", I think that will be primarily dominated by the speed at which the lamp filaments cool down. Mechanical relays can disengage in 30-50 milliseconds.

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  • \$\begingroup\$ Interesting. So you are saying the SSR should be fast enough for this? I would be using regular incandescent light bulbs. In addition wouldn't the Relays be loud (I know they make clicking sounds), technically they would be clicking to the beat)? Also by counterfeit, what makes it bad? Is it not as safe as the regular ones? Or is it because it just won't perform as great as the regular one? In terms of Power, each relay will have 4-5 light bulbs (in parallel) turn on and off (but not stay on for to long). \$\endgroup\$ – ProspectiveEng Feb 10 '14 at 15:44
  • \$\begingroup\$ An SSR is a great choice for this. They will switch on or off at the next zero-point in the AC cycle, so the max delay would be 1 second / 120 or about 8 milliseconds. \$\endgroup\$ – Eric Gunnerson Feb 11 '14 at 4:40
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These days I'd consider operating the bulbs on a "safe and isolated" DC supply - then you can just use MOSFETs. Obviously, the power supply voltage needs to match the lamps ratings but you'll be safe and they are a lot easier to drive and develop circuits for.

Here are other ideas: -

From the 70s onwards sound-to-light units used triacs or SCRs for switching the lights on and off: -

enter image description here

This is a simple circuit that pulses a lamp when the microphone detects sound. Be aware that this circuit is not isolated from the lethal AC voltages.

Here's one that connects directly to a speaker AND uses an isolation transformer: -

enter image description here

Solid state relays are OK but many of them do not have fast activation times (just like mechanical relays) - if you are to use SSRs then check the data sheet.

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As safety is the most important thing in mains switching, and given the level of capability involved in design your own mains circuits, this previous question could be relevant.

This is the latest model suggested in that question, utilising TRIAC control. Should be fast and not suffer the failure rate (~100,000 operations) of a mechanical relay based device.

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  • \$\begingroup\$ That's a nifty little widget. \$\endgroup\$ – Bryan Boettcher Feb 11 '14 at 14:46

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