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I have a Plasma Ball with a built in microphone and would like to add another switch S1 and an audio-in port to the circuit J1. How do I calculate the line attenuation R1 and R2 resistors?

My idea is to add another switch S1 so I can switch between using the internal Microphone MIC1 or use the audio-in from an external source J1. Is this the best way to do this?

Some questions I have are:

1) Do I need to use a shielded wire for the microphone MIC1 when I move it or when I add the audio-in input J1? I would like the plasma ball not to fry these components or the devices they are connected to.

2) Also how would I calculate the (resistors) R1 and R2 to limit the voltage that comes in from the audio-in jack? J1 my audio input would be around 343mV

See image of circuit board with built-in microphone circled in RED along with schematic of what I want to change / add to the circuit.

Plasma Ball Circuit

New Circuit

Link to circuit

I tried using the built in circuit builder to electronics stackexchange below but it was missing some objects

schematic

simulate this circuit – Schematic created using CircuitLab

Adaptor Model YHGD-1200500 Input: 100-240v 50/60hz, 0.18A max Output: 12V 500mA

Mic1 is a CZN-15E Omnidirectional Electret Condenser Microphone Mic1 is a CZN-15E Omnidirectional Electret Condenser Microphone

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    \$\begingroup\$ Given the presence of the high voltage flyback transformer on this board, making electrical connection to it without full understanding of the workings sounds unsafe. You should probably not try to reassemble and re-use this one, rather get a new one and never open it, but instead put a small high-impedance speaker on the outside near the microphone. \$\endgroup\$ – Chris Stratton Jun 30 '17 at 19:20
  • \$\begingroup\$ It seems to be powered by a DC jack in the top right corner so the risk is probably very low. If, however, the high voltage got grounded somehow then it could destroy the audio device plugged in as Chris cautions. \$\endgroup\$ – Transistor Jun 30 '17 at 19:39
  • \$\begingroup\$ Yes it's powered by a 12vdc 500ma adapter...please note I wasn't going to use this as is I was going to place it back into it's glass housing. The audio voltage input I was going to use max'd out at around 343mV. I do see how touching the glass portion of the plasma globe when put back together then touching the wire leading to the audio device could zap it. I just wasn't sure if shielded wire would prevent this. \$\endgroup\$ – Rick T Jun 30 '17 at 21:49
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My guess is the microphone is acting as a resistor divider for a PWM modulation controlling the voltage... It may not be straight forward to replace a passive microphone with an active audio input.

Your best bet is to diagram the circuit, and understand how the microphone is being used. If it is just varying the voltage to a pin on a PWM driver, you could scale the audio signal, and do the same thing. Preferably through an audio transformer (they are small) so you have some level of safety and can avoid ground loops etc.

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What you are doing, is replacing the microphone with some audio generator. It should be obvious that you need to know the characteristics of the mic1. Without them, you can't design something to replace it. For example, depending on the characteristics, you might end up needing amplification, rather than attenuation!
Using shielded wire to a mic connection is always a good thing. At the very least, it keeps electrical noise out. However, in your case, the random noise might add some "character" to your display, as long as the induced noise is not large enough to zap components.
For more detailed help, I need the mic1 and Plasma Ball input specifications.

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  • \$\begingroup\$ The mic1 is a CZN-15E and the power supply used is a Adaptor Model YHGD-1200500 Input: 100-240v 50/60hz, 0.18A max Output: 12V 500mA I'll attach the specification sheet to the original question. \$\endgroup\$ – Rick T Jul 9 '17 at 13:07

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