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

Thread Starter

kaotica

Joined Oct 1, 2020
2
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
2,538
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
9,408
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
2,538
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
2
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
2,538
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
5,646
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
2,534
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
11,628
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
6,985
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
5,646
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
6,985
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.
 
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