# VERY inexpensive way to operate relay from speaker output .

#### gramparich

Joined Feb 8, 2012
7
Hey folks,
I'm one of those old folks that know a little about a lot of things, so be nice.
I have a project that requires what the title says.
"I need to turn on something when there is sound coming from a speaker."
One of the first considerations is cost, as I am hoping to manufacture a sizable quantity of devices utilizing this function.
My first thought was to utilize the speaker voltage directly, and then I turned to the sound itself being detected and amplified to the point of firing the relay.
Am I missing something ?
Is there a simpler solution that could cost less ?

Thanx,
Grampa Rich (not Rich Grampa as my kids think)

#### PaulEE

Joined Dec 23, 2011
423
Hey folks,
I'm one of those old folks that know a little about a lot of things, so be nice.
I have a project that requires what the title says.
"I need to turn on something when there is sound coming from a speaker."
One of the first considerations is cost, as I am hoping to manufacture a sizable quantity of devices utilizing this function.
My first thought was to utilize the speaker voltage directly, and then I turned to the sound itself being detected and amplified to the point of firing the relay.
Am I missing something ?
Is there a simpler solution that could cost less ?

Thanx,
Grampa Rich (not Rich Grampa as my kids think)
May I (we) ask what the overall goal of this project is? Or, is it top secret?

There are approximately 298459823498 ways to do this, but your project specifications are going to narrow it down to a handful...at which point, we can do the cost-cutting.

#### gramparich

Joined Feb 8, 2012
7
Hey there PaulEE,

Yup - top secret
However, basic voltage available is 12vdc
- lookin' to keep overall device as small as possible
- will need to drop 12v to 5vdc for another little sub device (will gut a $.99 usb to 12v doodad for the 5v) - said little sub device will have 10 second timer to turn speaker off (relay to turn off too) - very low volume of audio is needed HALP Thanx, Grampa - Thread Starter #### gramparich Joined Feb 8, 2012 7 Anybody that likes golden's can't be all bad. grampa Thread Starter #### gramparich Joined Feb 8, 2012 7 I just read over what I forwarded, and I need to change "I want to manufacture----------etc" to "I would like to patent the idea--------etc" #### joeyd999 Joined Jun 6, 2011 4,477 I just read over what I forwarded, and I need to change "I want to manufacture----------etc" to "I would like to patent the idea--------etc" Well, if your intention is to patent the idea of: "I need to turn on something when there is sound coming from a speaker." You've already given it up to the public domain by posting it here! Hopefully, there is more to your invention than that, but, in the future, *be careful*! #### Audioguru Joined Dec 20, 2007 11,249 There is an Instructable that blinks an LED when music plays. It was designed by a 10 years old kid so it is missing two current-limiting resistors and a diode. It is just one TIP31 transistor. It is missing a series base resistor so its forward-biased base-emitter junction blows up and might blow up the audio amplifier. Its emitter is missing a series diode so when its reverse-biased emitter-base junction has avalanche breakdown at about 5V then it blows up and might blow up the audio amplifier. Its LED is missing a series current-limiting resistor so the LED will probably blow up. Thread Starter #### gramparich Joined Feb 8, 2012 7 Well, if your intention is to patent the idea of: You've already given it up to the public domain by posting it here! Hopefully, there is more to your invention than that, but, in the future, *be careful*! Oh yeah, there is much more than this one function, and I appreciate the "be careful", but I have been down this road before and know the bumps. Thanx---------- #### MrChips Joined Oct 2, 2009 23,253 Unless you have$100 grand to spend on a patent application you might as well forget that idea. In any case, if someone knowledgeable in the field of electronics can come up with the same idea in the normal course of practice that kills your patent.

But you can still build a simple sound activated switch, just not patentable.

#### Audioguru

Joined Dec 20, 2007
11,249
Haven't you heard about "The Clap Switch" that is being sold today?

It turns something on when you clap your hands quickly two times (or if your dog barks quickly two times). Then it turns off when you clap your hands quickly two times again (or your dog ...).

How can you patent something that is already used all over the place? How can you sell yours when everybody already has a few?

#### c0de3

Joined May 1, 2009
50
Oh yeah, there is much more than this one function, and I appreciate the "be careful", but I have been down this road before and know the bumps.
Thanx----------
So you are saying the sub-circuit would get power from a separate power source, you just want to switch it on and off with the speak line voltage? Do you care if this device causes issues with said speaker line? ie. does the speaker still need to work with this device inserted?

#### gramparich

Joined Feb 8, 2012
7
OK,
Let me start over here-----------
I have a neat idea for a gadget that needs the aforementioned function.
It might be patentable, it might not, I really don't care.
I just want to make one for my own use, and maybe it might interest someone who had the wherewith all to take it and run.
So, does anyone have an idea how to operate a relay with speaker audio voltage, or the sound itself ? cheap ?
thanx

#### Audioguru

Joined Dec 20, 2007
11,249
A problem with using audio from the output of an amplifier to turn on a relay is that a relay needs a certain DC voltage and audio is AC at many voltages.

The audio must be at least 4.1W into 8 ohms for a duration a little longer than the relay's turn-on time plus the duration tthat you need. Rectify the audio then filter it. The resulting voltage will be at least 5VDC. Use a 5V voltage regulator to turn on a 5V relay.

#### gramparich

Joined Feb 8, 2012
7
WOW,
thanks for all your help !

#### Ctenom

Joined Nov 1, 2010
59
Hey there PaulEE,