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Video Tx covert from internal audio (mic) to external audio

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Hi everyone,

I bought this Video_Tx for use in my ground station as a repeater up to my goggles.

At the time I didn't realize it doesn't accept external audio but instead has a little mic built in.

I want to know if I can remove the mic and wire up my external audio to the solder pads adjacent to where the mic was located?

Would I require a resistor (value?) between the audio source and solder pad?

The external audio source is just one of the audio outputs of an Eagle Eyes.

Any tips or advice greatly appreciated.










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An external audio source will work with it. You need to attenuate the signal with resistors configured as a L-Pad. http://www.electronics-tutorials.ws/attenuators/l-pad-attenuator.html

The audio output of the EagleEyes will be up to a couple volts. The output of the electret mic is a few millivolts. Choose your L-Pad resistors to accommodate this.

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Hi Thomas, thanks for your reply. Its very helpful as always.

A reading of the Eagle Eyes manual indicates the audio output level as "line Level".

Reading your link and this one about Line_Level I'm going to build a L-Pad attenuator as follows using 0805 smd reistors and fit it inside the transmitter case.

I/P voltage: 2v

O/P voltage: ~ 3mV

R1 = 100Kohms

R2 = 150ohms

Will report back.

Best Regards





Edited by garris2

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Don't be surprised if you need radically different resistor values. My gut feeling is that the 100K is too large (because of the interaction with the existing resistors in the mic's audio amp on the vTx). So if your calculated values don't work then be prepared to choose the resistors empirically (trial and error). Just a gut feeling, but I predict the final choices will be closer to these: R1 = ~20K, R2 = ~1K.

The electret mic's circuitry will have a low value resistor (1K to 3K ohms) connected to V+ that was used to power the FET inside the mic capsule. To isolate this unwanted DC bias voltage I suggest adding a small value cap (~10uF) to the output of the attenuator (side that connects to the vTx mic input). Or you can identify and remove the resistor from the vTx.

BTW, the bare RF module on the vTx should accept line level audio. So if  you run into trouble feeding attenuated audio into the hacked mic circuity then a backup plan would be to use the RF module's audio input pin instead.

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Hi Thomas,

Feeding the Rf module directly would be my preferred method, thus far I couldn't identify the audio input solder point but I will look at that again. I assume if I correctly identify it I can simply cut the track close to the module to isolate the other Mic circuitry. 

In case,  I will buy some other smd resistors and play around with values as suggested.


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I can't offer specific pinout advice because your photo didn't show the PCB side with the RF module. But many of the Chinese vTx designs simply use some variation of the Boscam TX5823.

Here's an application circuit diagram for the basic DIP switch version (yours will be SPI version). The basic information seen here will allow you to confirm the Power, GND, and A/V pins. Ignore the other pins.



Use the details to investigate your vTx. If VCC power & GND match-up then I suspect the Audio pin will also be as shown in the drawing.


And the bare module looks like this:


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He's some pics of the RF board.

I hope the quality is sufficient to get some useful info. If not let me know and I will take some more.

I will do some probing and report back.








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So I had a close look at the cct board again.

I removed the channel select button to reveal more clearly whats going on.

I can see from the mic the trace goes to some audio amp circuitry then through to a hole taking to the other side of the board.On the RF side a simple trace takes the audio over to a solder pad for audio input to the RF module.

I've cut the trace isolating the mic circuitry.

I also wanted to make a clean hack for the audio input.

So I found that I could remove a resistor between the 5v output connector pin and regulator circuitry isolating the pin.

Now the 5v output becomes the audio input and a wire run internally directly to the RF module audio input completes the hack.

I've verified the vtx accepts extremal audio now and transmits by rigging up camera and mic on the bench and receiving loud and clear without distortion at my goggles.


Thanks very much for your help on this Thomas, it's very greatly appreciated.







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