FM and BT wireless microphone (car ear) : comments

Sunday,18-June-2023

FM and BT wireless microphone (car ear)

Posted by longgo at 14:28:10 in Tech

EN
As you probably in some point repair something on your car, you should know how nice is if you can “hear” from where exactly the mechanical noise is coming from. Sure, you can buy cheap stethoscope which look like doctor`s one , or similar with metal small rod connected at the bottom. That is great tool, but unfortunately sometimes you have to investigate the issue when driving.
Sure, there are kits of four wireless units that you can buy, but unless you`re working at some service center or you can spare some 400-600$/£/€ for it, make your decision wisely..
So what was my approach?

FM radio transmitter with option to set the frequency

that is one interesting device, that you can connect to serial to USB connector and program the frequency it will transmit.
It look like that one:

It have one great option - programmable frequency! And second it cost about 2-3$..
Of course not “any” frequency, but something in the range of regular FM transmission. For my purpose I choose 88 and 88.5Mhz.

- How you can set it up?
- With any serial terminal and FTD-RS232 unit. I use that one:

You need some small solder wires in that order:

Select either Speed: 115200,8, N, 1 for the serial terminal, or if you use the arduino environment select speed 115200 , no line ending.
Then the commands are:

AT+RET - that will bring the software version, and the current set frequency. Keep in mind that in decimal. For example 950 is same as 90.5Mhz.. Then the second one is:

AT+FRE=885 <- in my case I set that one to 88.5Mhz.

That done, you need metal average strong clamp (about 3-5$) , some battery holding box with switch (about 1-3$) , elastic band (2-5$) that will hold the box under the car for when you need , and some wire....

I selected some clean and quite close to the clamp part:

After the wire been soldered on place, added antenna wire, and pace selected, the cover “black filter” in front of the mic was removed and with moment glue the metal surface been glued for the clamp. Then with hot silicone stuck on spot:

Bluetooth module

it look like that one:

and you can buy them for about 3-5$/pc.
That one is quite smart small “bug”, that you can adjust once gain via serial port. Connected as that:

Then on the serial port you see the devices around, to which one is connected, volume level and etc..
List of datasheets, what commands are supported and so on you can download here
It also can be set to receiving mode by the small jumper on top of the board and then via serial port you can set the receiver`s mac address so they bond all the time together. In my case I left is as is, and by pressing the button you can force the unit to “find” any speakers or headphones to connect to.
In the data sheet it is stated that you need to run it anywhere from 3-5V, preferably at 5V.
The idea was to use the very same box, battery cell and clamp, so in case I decided to move to 5V for sure similar DC-DC step up convertors can be used:

To my surprise, it worked fairly well on 3.7v so I didn`t bother to add one. Or either battery regulating chip, as I do not plan to charge them on spot or to use it until battery is flat.

Microphone was like that:

And I connected it as that:

For some odd reason that was the only way that I get it to work without any huge hum or noise. In any other way I try it was horrible.. Still, small hum is present, but doesn`t bother the purpose that is made for.
The external antenna bring extra range so it can be placed at any under body-shell places.

And finally I get something like this:

I try also some 100nF capacitor between power and ground but still didn`t make the things much better so I left it as is..
Two FM and one BT “ears” cost me less than 30$. Which almost make similar things as the “pro” ones. Sure, you can make any extend version of that project with more complicated design. For me was enough that I can connect both modes to parasitically any car stereo (the FM ones) so you can use it just without extra costly equipment.
The issue can be if you have some powerful radio broadcasting on the used range. In that case you program right “gap” in the frequency range to be used, or use BT version , or.. you can buy FM programmable receiver and set both to be just on the edge or above the regular FM band.

Hope that was helpful and good luck!