Sensitive Audio Detector - would it work in reverse?

Thread Starter

Hugh Mongoose

Joined Nov 1, 2016
6
I'm wondering if the Sensitive Audio Detector - http://www.allaboutcircuits.com/textbook/experiments/chpt-4/sensitive-audio-detector/ - would work in reverse? Here's the situation I'm working with.

I want to replace a sound impact sensor (a little microphone that completes a circuit at a sufficiently high volume, and looks like this: http://elcodis.com/photos/32/57/325729/ma300dp_sml.jpg) with a 1/8" mono audio jack, so I can more finely control the response of an audio-triggered set of LED lights. The lights change whenever the circuit is completed - I can just short across the two solder points on the board to make it change. I can't just do a direct swap of the audio jack ( http://www.hiviz.com/kits/images/components/stereo_jack_123_med.jpg ) onto the board, because there isn't something built in to complete the circuit at a specific volume threshold. As soon as both ends are plugged in, the circuit's complete and the lights start going crazy.

The Sensitive Audio Detector lesson looks a lot like what I'm hoping to do, but instead of the signal coming from the test leads, it would come from the 1/8" plug. Including a potentiometer is a plus, to adjust the sensitivity of the lights.

If someone can let me know if I'm on the right track here, or if I'm not, to point me down the right path, it would be greatly appreciated. If this is the wrong forum to ask this question, please point me to the right one.

Cheers!
 

Colin55

Joined Aug 27, 2015
519
The 120v to 6v transformer is around the wrong way!!!

Suppose the input signal is 120mV. You are only going to listen to a signal that is 6mV in amplitude and this will not be very loud.
You want a circuit that detects 10mV signal and produces a signal of 1,000mV in the headset.
 

Colin55

Joined Aug 27, 2015
519
"No, the connections are correct as shown. The headphones are very low impedance and the transformer is there to raise the impedance of the input."

I am talking about an entirely different problem.

I am talking about the statement: Very sensitive audio detector" when you are reducing the signal by a factor of 100.
 

AlbertHall

Joined Jun 4, 2014
12,338
I am talking about the statement: Very sensitive audio detector" when you are reducing the signal by a factor of 100.
If the source of the signal has a high impedance, using the step down transformer will result in more voltage across the headphones than without the transformer.

This is why valve audio amplifiers use a step down transformer between the output valve(s) and the speaker.
See http://www.electronicecircuits.com/...5-class-a-vacuum-tube-valve-amplifier-circuit
 

Colin55

Joined Aug 27, 2015
519
"This is why valve audio amplifiers use a step down transformer between the output valve(s) and the speaker."
You are talking about an entirely different situation.
Valve amplifiers have a 385v signal. We are talking about 700mV signal.
 

AlbertHall

Joined Jun 4, 2014
12,338
So do you think that connecting the loudspeaker direct to the output valve would make the sound much, much louder, getting the full 385V across the 8 ohm loudspeaker?
 

Colin55

Joined Aug 27, 2015
519
"So do you think that connecting the loudspeaker direct to the output valve would make the sound much, much louder, getting the full 385V across the 8 ohm loudspeaker?"

I really don't think you understand the difference between the two arrangements.
The value amplifier has 385v and possibly 50 to 150mA.
The speaker needs about 20v and 3 amps.
We are converting the 40 watts from the amplifier to 40 watts into the speaker.
This arrangement is totally different to creating a sensitive audio listening device.
I agree that the transformer impedance matches the audio to the headphones but you are certainly not going to be able to hear low amplitude waveforms without some type of amplifying stage.
A simple transistor stage could provide the gain you need and match the two.
This transformer idea is a complete waste of effort.
 

AlbertHall

Joined Jun 4, 2014
12,338
I agree that a transistor could also do the job of impedance transformation but so will a transformer.
Indeed, transistor amplifiers used in such as radios and tape players used an output step down transformer.
 

Colin55

Joined Aug 27, 2015
519
You are missing the point completely.
In the process of impedance matching with a transformer, you are losing 95% of the amplitude of the signal.
This means the circuit will not be able to detect small signals.
 

AlbertHall

Joined Jun 4, 2014
12,338
It will be able to detect signals of low current/high impedance and transform them to a larger, and therefore more audible, current in the headphones and that is the whole point.
 

Colin55

Joined Aug 27, 2015
519
"It will be able to detect signals of low current/high impedance and transform them to a larger, and therefore more audible, current in the headphones and that is the whole point."

This is entirely untrue.
That's why you have to learn about impedance matching.
 

Thread Starter

Hugh Mongoose

Joined Nov 1, 2016
6
Colin55, AlbertHall-

I may have missed it in the back-and-forth, but I'm not sure if there's been an answer to my question here. Could a circuit like the one for the Sensitive Audio Detector work in reverse, to close a circuit when the signal reaches a certain volume? So to clarify, the signal is coming in through the 1/8" plug - there are no headphones involved here, just a headphone-level signal.

Thanks!
 

AlbertHall

Joined Jun 4, 2014
12,338
I may have missed it in the back-and-forth, but I'm not sure if there's been an answer to my question here. Could a circuit like the one for the Sensitive Audio Detector work in reverse, to close a circuit when the signal reaches a certain volume? So to clarify, the signal is coming in through the 1/8" plug - there are no headphones involved here, just a headphone-level signal.
Used that way you increase the voltage of the signal (but it will also reduce the available current).
The original circuit does not have any means of closing a circuit.

You need something more like the circuit discussed in this thread:
http://forum.allaboutcircuits.com/threads/audio-detector-switch.76451/
 
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