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Topic: Multiple ISD1820 sound modules with one speaker (Read 1 time) previous topic - next topic

st3f0

Hello arduino community,
I have some isd1820 sound modules (datasheet) and they're working well so far. Now i want to use several different messages in my project and therefore use multiple sound modules. My question is, whether i can connect multiple isd1820 to one speaker (in parallel) without damaging the modules.

If nobody can tell me, i'll probably just try and report, but i assume this is an easy question for many people.

DVDdoug

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My question is, whether i can connect multiple isd1820 to one speaker (in parallel) without damaging the modules.
No, you can't do that...   There's a rule-of-thumb that you can connect multiple inputs together but you can't connect multiple outputs together.  

To mix audio signals you need a mixer so you'd need a mixer followed by an amplifier that's going to drive the speaker (since the existing amplifiers are going to the mixer instead of to the speaker.   You can make a simple passive mixer with resistors but you still need to follow it with an amplifier.

The problem is that the effective source resistance of the audio amplifier is a lot lower than the speaker it's driving.   The amplifier might have a source resistance of less than 1 Ohm and that means the other amplifier is seeing a 1 Ohm load instead of an 8 Ohm speaker, etc.   Too much current flows and stuff overheats, etc.

It might be easier to think about what would happen if you connect two digital outputs together...  If one is trying to go high and the other is trying to go low, they are fighting each other, you'll get too much current, and you'll get an unknown output.

st3f0

#2
Oct 12, 2017, 09:23 pm Last Edit: Oct 12, 2017, 11:17 pm by st3f0
Thank you very much for clarifying this, DVDdoug! What if i ensure with diodes that current flows only towards the speaker? Would that be an option?

Grumpy_Mike

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What if i ensure with diodes that current flows only towards the speaker?
Then that would screw up the negitave part of your audio waveform and it would not sound good.

st3f0

Thanks a lot for these clear explanations.

To mix audio signals you need a mixer so you'd need a mixer followed by an amplifier that's going to drive the speaker (since the existing amplifiers are going to the mixer instead of to the speaker.   You can make a simple passive mixer with resistors but you still need to follow it with an amplifier.
This sounds pretty simple to me. So all I'd have to do is connect the outputs using a 5k (right value?) resistor each to an amplifier (like lm386) and then I'm good?

Grumpy_Mike

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So all I'd have to do is connect the outputs using a 5k (right value?) resistor each to an amplifier (like lm386) and then I'm good?
That sounds wrong as stated. Can you draw a diagram of what you want to do.

st3f0

#6
Nov 04, 2017, 07:24 pm Last Edit: Nov 05, 2017, 12:05 pm by st3f0
Out1 ---------R1----------|
Out2 ---------R1----------|-------amp-----C1-----speaker-----------gnd
Out3 ---------R1----------|                           |---R2---C2---gnd

Thats how I meant it. The vertical lines are supposed to be connected. I hope it's understandable. If not I'll make a proper drawing.


Edit: I attached a "proper" drawing.

Edit2: corrected attached drawing according to Grumpy_Mike's suggestions.
R1 = 5k Ohm; R2=10 Ohm;
C1 = 250 uF; C2 = 0.05 uF;
amp = LM386;

Grumpy_Mike

Yes that is better your phrase:-
connect the outputs using a 5k (right value?) resistor each to an amplifier
implied to me that each Arduino output had its own amplifier.
I would put a capacitor between each output of the Arduino and the resistor.

Why are you connecting R2 and C2 across the speaker though?

st3f0

First of all I'm sorry that my formulation was misleading/incorrect. I do my best, but I'm obviously not a native speaker.

To answer your question about the RC part in the diagram: Honestly, I don't really know what it's good for. I found it on several sites like here. I assumed that it is something that is necessary when using the lm386 audio amplifier.

I'll play around with it as soon as I find the time and report back.

Thanks again for your helpful expertise, Grumpy_Mike.

Grumpy_Mike

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I do my best, but I'm obviously not a native speaker.
That is fine, it is why a schematic removes all doubt. Many native English speakers don't have the skills to describe a circuit correctly or follow a correct description. Again a schematic removes all doubt.

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Honestly, I don't really know what it's good for. I found it on several sites
If it is on the data sheet it is fine. It increases the stability, they are very small values. That amplifier is a bit unstable and is likely to oscillate.

Your schematic should have:-
1) shown the values of the components
2) not drawn so that it looks like C1 has 3 connections to it, that wire to R2 should be to the left of the symbol C1

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