Comments on: Speaker Power Detector

By: Lyle

Lyle — Tue, 08 Dec 2015 03:29:42 +0000

Hi, Keith — after much Googling to solve my problem of inadvertently bothering the hell out of my neighbors with my outdoor speakers, I found your blog! The last time I built a circuit was in college introductory physics (pretty sure that was just a battery and a lightbulb) — so needless to say, this is the first real circuit I’ve ever built. But the good news is, I’ve done it and it works…kinda. The problem I’m having is that I can’t get the LED to stay solidly lit at my desired low volumes. It flickers, but I want it to be ON. I’ve adjusted my trimpot to be JUST under the reference voltage and I have my speaker pot turned all the way up — still no dice.

I used my voltometer to measure the voltage across my speaker terminals, and at the volumes I’m interested in, it fluctuates between 15-25mV. That seems shockingly low. Turning it up to the top of the range at which I’d normally listen, I got 200-200mV; and at the absolute max volume my receiver is capable of, I got 1-3V. My A/V receiver is a Yamaha HTR-5860 (110W per channel) — is it odd that the voltage is that low? (And if the voltage is that low, is there any reason for a speaker pot at all?)

So, big question: Any thoughts on how to get that LED to stay solidly lit at these low voltages? My goal is to have that LED on solid if ANY voltage is in the line of my outdoor speakers.

By: Phil

Phil — Tue, 23 Mar 2010 15:46:46 +0000

Good to hear that! I’ll give it a try and let you know how it works out.

Thanks Keith,

Phil

By: Keith Neufeld

Keith Neufeld — Tue, 23 Mar 2010 14:19:42 +0000

Phil, yes, it would work great with only one speaker input — in fact, splitting it in half would eliminate all the cautions I was issuing that stemmed from using it with two speakers.

Just remember that you’ll need to provide a DC power source to run the IC.

You might prefer to substitute a dual comparator like the LM393 — I only used the LM339 because that’s what I had on hand.

By: Phil

Phil — Mon, 22 Mar 2010 22:21:52 +0000

Hi Keith,

After much searching (and confusion) I’ve come across your 2007 Speaker Power Detector design. I’ve been looking to add pulsating led pilot lights to a pair of old AR-4x speakers that I’ve just done some crossover work on. Your circuit looks like it may work for my purposes if it can be modified to work with only one speaker input. Can I just eliminate the bottom part of the circuit going to the second speaker?

I’m a photographer that has some electronics background (a long time ago) Thanks for your help and cool blog!

Phil

By: yeni

yeni — Tue, 09 Sep 2008 10:32:39 +0000

This circuit is simple , but very good
I ever built it and work very weel..
Thanks

By: Keith Neufeld

Keith Neufeld — Fri, 28 Mar 2008 00:10:54 +0000

Tim, you already have the remote start, right? (I can’t tell you how to wire an ignition switch if you don’t already have that done.) And it’s smart enough not to try to start the car if it’s already running?

Power this circuit from your cigarette lighter, or some other fused 12V source in the car, and I think everything else will pretty well follow from there — I think most of this circuit will work exactly as it is. And you don’t need to worry about the caution I posted to people with stereos — that only applies when you’re connecting two inputs, and you’ll only be using one.

Let me know how it goes! And of course be careful; I can’t take any responsibility for what you build when I’m not there to watch. But the fact that you’re connecting a cell phone (low voltage) to a wireless remote (low voltage) and only connecting to the car’s 12V system to get power for this circuit makes it a lot safer to do.

By: Tim G

Tim G — Wed, 26 Mar 2008 16:11:27 +0000

How many time have you wanted to start your car using your remote start and it didn’t reach? You still had to either go up to the window or actually walk outside to get close to the car? Well, The idea that had was to put a remote inside the car and get a prepaid phone. Connect the speaker output a circuit that will short the switch just like pressing it. Set all my calls to blank ringer, or no ringer except the one from my phone so when I call it will send a ringer to the circuit to activate the relay. There are services that can do this but they cost about 140 bucks a year. With a prepaid phone it would be free because you you would technically never answerer the phone. In theory it sounds like it would work. Thanks.

By: Keith Neufeld

Keith Neufeld — Wed, 26 Mar 2008 14:19:04 +0000

Tim, I don’t think you’ll be able to power a relay with the audio signal, and I also suspect that this particular circuit is more than you need for what you want to do.

Tell me more about your project and the functionality you’re after and I’ll see if I can suggest something.

By: Tim G

Tim G — Tue, 25 Mar 2008 20:47:26 +0000

Im trying to do something similar here except the audio input will be coming from the signal that goes to a cellphone speaker. The relay will be need to be able to replace a push switch on a wireless remote powered by a 1.5v battery. Is that even possible to use a audio signal to power a relay, (maybe charge up a capacitor and then let that power the relay). I only know basics about electronics. Any help would be appreciated.

By: Steve

Steve — Thu, 20 Mar 2008 15:15:00 +0000

Hi Keith,

I would like to build one as soon as possible, but think I need to wait for the “revision” because my surround sound speaker connections are not tied directly together, ~18K resistance between R- and L-.

What are your thoughts regarding the input redesign, maybe a transformer or optocoupler?

Best Regards, Steve Haran