help- looking to trigger a physical switch with a speaker audio signal

Thread Starter

kaotica

Joined Oct 1, 2020
7
Hi Everyone,
I am a maker and tinkerer and I am putting together a project where:

I am looking to use the electric signal in a wire pair that leads to a speaker
to turn on a switch / closing a circuit
for the duration of the signal coming through to the speaker.

that's the technical part- the real world version:

basically for halloween i want to hack a door chime which plays for ~15 seconds and use the signal that the chime's pcb sends to the speaker to instead turn "on" a small MP3 player which in turn plays music through it's own speaker instead. The mp3 player currently has a push button switch to activate its audio playback.

Let me know if you need any other info to help me out w this.
thanks
fabio
 

SamR

Joined Mar 19, 2019
3,179
Let's start with the MP3. You say a pushbutton start. Do you have the schematic or wiring diagram? Is it a start/stop PB?
 

Dodgydave

Joined Jun 22, 2012
9,914
Need to measure the AC voltage on the speaker to what level it is , the use a precision rectifier to operate a relay.
OR
Tap into the bell push with an optocoupler.
 
Last edited:

SamR

Joined Mar 19, 2019
3,179
I was thinking an op-amp or FET on the bell wire to trigger a logic level SCR possibly, but just what does the MP3 require to start/stop? Lots of possibilities. We do need to know what the signal level is on the bell wire.
 

Thread Starter

kaotica

Joined Oct 1, 2020
7
i will check to see what's coming to the speaker using a multimeter, what reading should i be looking out for?

as for the mp3 player, yes it's a play/stop button, two wires when they touch it plays the file and when you touch again, if the file is still playing, it stops
 

SamR

Joined Mar 19, 2019
3,179
Then you need to add a timer to the list. Start timer when ON and it turns the MP3 off when timed out.
 

Bernard

Joined Aug 7, 2008
5,772
Assuming that a transistor can act as a switch for the MP3, might use one or two comparators to produce a square pulse. We will turn on MP3 on leading edge, off with trailing edge. I wonder if one speaker lead is common to chime power supply?
 

BobTPH

Joined Jun 5, 2013
2,989
So the button requires a push and release to start and a second push and release to end.

What happens if you push and hold?

This could make a difference in how it is done.

This could make a difference in how ot is done.

If it really works the way I stated at the top, your circuit needs to produce one short pulse to start it and one short pulse to stop it. This Is is not trivial, it will require multiple chips or a microcontroller, which is what I would use.

Bob
 

Alec_t

Joined Sep 17, 2013
11,972
i will check to see what's coming to the speaker using a multimeter, what reading should i be looking out for?
We don't know the chime supply voltage, or whether the speaker is driven with AC or pulsed DC. I suggest you take readings of the voltage between the two speaker wires on both the 20VAC and 20VDC meter ranges and report back.
 

MisterBill2

Joined Jan 23, 2018
8,199
It seems that it would be much simpler to connect a small relay across the wires to the input of the electronic chime device, presuming that it is a standard doorbell circuit with an external power source. Then you could have isolated contacts to trigger the player and no other added complications.
 

Bernard

Joined Aug 7, 2008
5,772
Is your MP3- FN-BC04 ? ; yes then use the Level hold loop playback function. Chime signal ( when & if we find what it is ) rectified , low pass filter, & neg. pulse driver. Do you have a 12V supply or what is chime supply V. The clock is running.
 

MisterBill2

Joined Jan 23, 2018
8,199
OK, if the trigger wires for the present sounder are not accessible then you do need to use the speaker signal for the trigger. Insread of the speaker load, use a transistor as the driver for a small relay, and couple the speaker signal to the base through a reasonable capacitor, 0.47mFd or so. Use a 10K ohm or greater potentiomenter as a voltage divider to bias the base just short of turning the transistor on, so that any sound signal voltage will turn the transistor on and operate the relay.
 

Thread Starter

kaotica

Joined Oct 1, 2020
7
hi everyone, sorry for the delay, so here are my findings:

mp3 player side: i replaced the speaker with a larger one and it works perfectly so dont need to touch anything there
i removed the switch and shorting the wires plays the soundfile as i imagined

on the chime side, i used a multimeter and the signal coming over the speaker wires is .012V AC
if i stop triggering the sound it goes to 0V

so i need to find a way to monitor the wire pair for a change in voltage from 0 to .012V and on "the other side" short the two wires from the mp3 player for it to start playing the sound
 

MisterBill2

Joined Jan 23, 2018
8,199
For a signal that small one option could be a "sound operated switch" module connected across the chime connection. That may not be the least expensive but it should work.
 

Thread Starter

kaotica

Joined Oct 1, 2020
7
For a signal that small one option could be a "sound operated switch" module connected across the chime connection. That may not be the least expensive but it should work.
that sounds quite simple , could i just wire the speaker out from the chime to the microphone "in" wires ?
 

Thread Starter

kaotica

Joined Oct 1, 2020
7
OK, if the trigger wires for the present sounder are not accessible then you do need to use the speaker signal for the trigger. Insread of the speaker load, use a transistor as the driver for a small relay, and couple the speaker signal to the base through a reasonable capacitor, 0.47mFd or so. Use a 10K ohm or greater potentiomenter as a voltage divider to bias the base just short of turning the transistor on, so that any sound signal voltage will turn the transistor on and operate the relay.
could you help me with a diagram of the circuit? i get the gist of what you're proposing but i've never worked w/ transistors or relays before so a diagram w/ parts would help a lot
 

MisterBill2

Joined Jan 23, 2018
8,199
that sounds quite simple , could i just wire the speaker out from the chime to the microphone "in" wires ?
Probably very close to that, wiring the chime speaker wires to the sound operated switch input wires. You might need to reverse them to make it work, maybe not. You might even need to put a capacitor in series, depending on the sound operated switch module. You would want to verify that the module could trigger your MP3 player before going any farther. But that should not be so very complicated.
 
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