How to record a speaker?

Question

Complete noob here. I don't need a ready to implement solution, pointers on what to read upon and ideas for even simpler projects to gain experience first are perfectly fine.

I'm trying to feed both microphone and the right speaker of a motorcycle helmet intercom (Freedconn TCOM-SC) into the 3.5mm external microphone adapter of an action camera (Ghost Drift X). There's no ready to use solution for output from the intercom (it uses weird plug) so I need to tinker one out.

I got a "Y" headphones splitter that I can use to connect two microphones to the camera and record both simultaneously so I thought I could take that further, cut the splitter's input terminals off and tap into + and - on the intercom's speaker and microphone cables instead. However, after reading on how microphones and speakers work I realised there can be a huge difference in both voltage and current there. How do I match those? What are the terms I'm looking for and how do I Google for this specific subject (all I found in Google and here is how to match speakers to amplifiers)?

Answer 1

AC couple the connection by using a series capacitor.
A 10 uF electrolytic (or non polarised if you had one to hand) would be an OK start but higher and lower may work as well (0.1 uF - 100 uF range may be OK).

If, as is likely, the level is far too high for the mic input use a resistive divider .
A 10k or maybe 1k pot will make life easier.
A "pot" has 2 end of track terminals and a wiper terminal.
Connect as per diagram below.

Start with the wiper at the common or "ground" end.
It actually matter too much (usually) which end you consider ground in each case BUT on the camera side the ground connection will usually be hard connected to the camera metalwork. Not always.

Adjust pot slowly while noticing results.

IF IT PROVES necessary we may then start in on impedance matching :-).
If. ______________________________________

I don't understand how I'm going to prevent the signal from the speaker breaking the microphone.

Update: I now see what you mean by "... breaking the microphone."
The altered circuit may work as is. Failing that a very simple opamp buffer will allow one way signal feed.

The capacitor I suggested removes any possible DC component.
This may not be present but the cap does no harm.
The 1K (say) potentiometer reduces the level from about zero at one extreme to full signal at the other.
The combination will work well enough in many (most) cases.

^{simulate this circuit – Schematic created using CircuitLab}

^{simulate this circuit}

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Here is a 4am, must go back to sleep rather rough basic mixer.
There are far far better on web and a search under "mixer" in images will produce examples to ask us about.
The LM324 is a quad op-amp that sells for maybe $1 (or 3 cents in volume in China as an indication of its ubiquity).

IC1a and OA2 (whoops) provide solidish buffering against speaker to mic feedback - which OA3 also provides as OA3- is a "virtual ground". I may improve or delete this circuit later.

A few zillion examples here

Even something as simple as this, with a few pots thrown in, may well be good enough: