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Whatchamacallit.. The Box(tm) (a sort of source selector and bluetoothRX/TX thingie)?
Whatchamacallit.. The Box(tm) (a sort of source selector and bluetoothRX/TX thingie)?
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Old 9th April 2019, 07:38 PM   #1
JSL is offline JSL  Finland
diyAudio Member
Join Date: Sep 2008
Default Whatchamacallit.. The Box(tm) (a sort of source selector and bluetoothRX/TX thingie)?

Hello gentlemen, ladies!

I'm a total beginner when it comes to circuit design so I'm posting here in case someone might be able and willing to help.

What I'm trying to build is not any high-end audio system, but a.. well, I don't know what to call it. An audio gateway and (automatic) source selector? Maybe better to explain the use case and what I'm trying to achieve.

The use case:

When riding a motorcycle, allow:

1. listening to music and navigation instructions from a mobile phone / gps / some other device
2. communicating with other riders

Existing solutions:

There are commercial motorcycle intercoms (rider to pillion, bike to bike(s)) that allow communication between riders and listening to music etc. from a mobile device. They are using bluetooth for connecting to the mobile device and also for the wireless communication, which under good conditions works reasonably well and having full duplex audio when talking with others is nice. However, the longer distance communication i.e. bike-to-bike is not that reliable and as the distance between the bikes increases the reliabilty decreases (it more or less requires a unobstructed line-of-sight between the riders). Not to mention the better ones are ridiculously expensive for what they are.

What I'm trying to achieve:

I would like to build a Box(tm) that would allow me to listen to music and navigation instructions from my mobile phone and to use PMR446 radio for bike-to-bike communication! They would be connected to The Box(tm) with wires. The Box(tm) would then connect to a(ny) commercial Bluetooth headset.

1. Music / navigation instructions

Mobile phones have wired headsets so you can get analog audio out (and mic-level audio in, but let's forget that for now). No issues here.

This is the audio source that would be selected by default.

2. Bike-to-bike communication

I've been using PMR446 radios in various outdoor activies for years and they work very well. The communication is reliable, range is reasonably good and they work surprisingly well even when there are obstacles in the transmission path. The PMR446 radios typically have connectors for wired headset so you get analog audio out and mic-level analog audio in.

When there's an incoming transmission from another PMR radio The Box(tm) should automagically switch to this source. After the transmission ends, The Box(tm) should switch back to the default audio source (mobile phone).

Also, when the tangent of the connected PMR radio (or actually the wired headset's tangent) is pressed, The Box(tm) should receive the audio from the BT headset and route it to the PMR radio's mic input. Not sure if we should do so, but if we also switch the analog source when the tangent is pressed, once the tangent is released The Box(tm) should switch back to the default audio source, of course.

What I have so far:


This is the easy part For now, I'll be using my RHA MA750 Wireless headset. The placement of the microphone is a bit of an issue when wearing a helmet so either I'll find another, more suitable headset or I'll have to hack the wires to allow better positioning of the microphone. But it'll be OK for now.


MA750 Wireless | High Quality In-Ear Headphones with Ear Hooks | RHA

Bluetooth RX/TX:

The bluetooth part of The Box(tm) will be handled by a CSR87xx module which are available from Aliexpress etc. The CSR SoC is highly configurable already with the standard firmware so in case the module is not good-to-go as it is, it can likely be configured to do what is needed. However, based on my research it should be OK as it is (I managed to download the necessary SW tools from a shady chinese web forum in case it's not). The module takes in analog audio and transmits it to the headset. It also receives the transmission from headset and outputs analog audio.


BTM875 B / CSR8675 Stereo Bluetooth 5.0 Audio Module Module SPDIF / I2S / Differential analog/ support aptx aptx ll aptx hd-in Replacement Parts & Accessories from Consumer Electronics on Aliexpress.com | Alibaba Group
CSR Bluetooth burner USB to SPI downloader production tools chip Bluetooth module with development software-in Integrated Circuits from Electronic Components & Supplies on Aliexpress.com | Alibaba Group

Logic control:

While the CSR SoC includes a user programmable MCU, they are apparently notoriously difficult to program and the documentation is severely lacking. Thus, for handling the control logic I'm planning to use something like Arduino Pro Micro, just to make my life easier. Arduinos are well documented, the SW tools are publicly available and the world is full of examples of how to use them.


Micro USB ATmega32U4 Pro Micro 3.3V 8MHz Board Module For Arduino ATMega 32U4 Controller Pro Micro Replace ATmega328-in Integrated Circuits from Electronic Components & Supplies on Aliexpress.com | Alibaba Group

What is still missing:

Everything else! And this is where I would need "some" help. I believe at least the following will be needed:

1. Detecting if there's audio signal coming from PMR

Feed the signal to Arduino and write the code to detect if there's an audio signal or not. I have no idea how to design the circuit for this.

2. Switching audio sources mobile phone <--> PMR

Arduino can handle the logic and triggering, but how to actually switch to the relevant source? SSRs, (latching) relays?

I have no idea how to design the circuit for this.

3. When audio from BT headset's microphone is coming, I believe it will be line level when it comes out of the bluetooth module so I'm guessing it will have to be dropped to mic level before feeding it into the PMR's mic input?

I have no idea how to design the circuit for this. (And not really sure how to trigger the RX either, yet)

4. The Box(tm) will be powered from the motorcycle's electrical system i.e. from 12V. The bluetooth module and Arduino are expecting 3.3V so voltage regulation is needed.

I have some idea how to design the circuit for this, a linear regulator and.. something...

(It would also be nice if The Box(tm) could be a standalone unit i.e. have its' own battery so that one could jump of the bike and take the whole setup (phone, PMR and The Box(tm)) with you to be able to still talk to others over PMR etc. However, this is not a critical feature at this point.)

So there you have it. I hope I was able to explain it clearly enough. I can handle the software part, I can solder the whole thing together and with some (more) practice maybe even design the PCB, but I have no idea how to design the circuit to make this all work. I would be very thankful for any help on that part.

Last edited by JSL; 9th April 2019 at 08:59 PM.
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