Hey all,



After searching a long time I found this circuit for my little project.



I fly a paramotor and was looking for a solution to hook up my radio + phone to my BT earbuds.



I want the radio to have priority always. So if I get a transmission over the radio, it will mute my phone (change the input to the radio).

I can also get traffic alerts through my phone so it’s a good safety feature to be able to listen to it too.



The circuit will detect the radio reception ( radio has squelch so it should work in theory) and it will drive a small relay that will switch the phone line off and switch it to the radio.





I know driving a relay from the 1/3N battery is far from optimal. I’d need two relays and the drain from them is about 200mw. I’m guessing the battery will last for maybe 1-2hours of reception. Which is not that bad, but still.



I already have plenty of batteries “on board” when flying. Phone, the radio, portable Bluetooth transmitter. Would be best to draw the power from them, but then I’d need somekind of voltage regulation for the circuit that I’m not sure how to do. Searched for buck converter but gave up since I didn’t find a suitable one where I ordered all the rest of the parts. Any recommendations?



Also, I’d like to confirm that this circuit poses no threat to my radios electronics. As I understand the circuit needs to be hooked in parallel with the radios line out (speaker line). So just want to make sure it can’t push voltage into my radio. Not sure a 3v battery would be a problem even if it did.



Bare with be since I have very basic understanding of electronics. I’ve built some circuits before but actually understanding and modifying them is another thing.

So would be nice to get some advice and tips on this.

 

Is the circuit working OK to your specifications and why do you need two relays?
What are the voltages available from the other devices you can use to power this circuit?

 

Is the circuit working OK to your specifications and why do you need two relays?
What are the voltages available from the other devices you can use to power this circuit?

I haven't got the parts yet, ordered them a few days ago so haven't had the chance to test it out yet.

I need two relays because I need to switch between a stereo line and a mono. -> 3 channels. I'm sure there are relays that have 3 channels but I could find one. Anyway, I've been looking at different options and I think I'll just change the power supply to 2 x AAA batteries so this way it wont be an issue. Though I might need to add a boost converter since the Eneloops batteries I have drop off quite quick below 3V. Nominal voltage for those is 1.2V.

Other option (but risker) is to crack open the Bluetooth TX and try to get power from there. it has a 200mah battery so should be more than enough. Has the added benefit of one less battery to remember to charge before a flight.

 

What is the voltage of the Bluetooth battery?

 

Quite sure it's a typical Li-ion battery. 3.7V. I haven't tried to open it yet.

 

The question still is not clear. Please post a wiring diagram showing how the devices and signals are interconnected. Be sure to put a unique reference designator on each component so we can discuss them.

ak

 

The wiring diagram is attached to my first post. The only thing I'll add to the circuit are the two relays. The MOSFET will drive them. heres the relay specs https://www.tme.eu/Document/3d9fb13bc216892f8c7ae2000a20fad3/ftr-b3.pdf

One relay will control the stereo that will come from my phone. It will go straight to the relay.
The radio line out will go to the input of the circuits and also to the other relay. Stereo will be connected to NC and radio to NO. Both of the relays outputs will be connected to a 3.5mm jack and from there to a Bluetooth TX that will transmit it to BT earbuds.

Sorry don't have time to draw a diagram now, getting ready for bed. If it's still unclear just tell me, I'll add the missing components.

 

I'm asking about a diagram that shows all of the other components, not the audio circuit schematic. Something that shows the signal sources, destinations, relays, relay coil drivers, etc. Boxes for the components, such as a box for the audio detector circuit, are fine as long as they convey how everything is intended to work together.

ak

 

So you need a diagram for the radio, phone and the BT TX?

Not sure I can find them anywhere...

 

Again, not for the parts themselves, but for how you are connecting them. Where are the stereo signals, where are the mono signals, selector switches, the audio detector input(s) and output, headphone (or whatever) connectors, etc. Describing signal interconnections clearly and unambiguously in text takes a *lot* of text. Or a sketch.

ak

 

I think the relay contact arrangement would look like this:

 

Behold my photoshop skills

 

I think the relay contact arrangement would look like this:
View attachment 304088

Yes, thank you.

 

If the common connections are all connected together as you show then you only need one DPDT relay.

 

The ultimate solution would be to just to do it in the phones software. Would only require a BT TX to the radio.
Receive radio

If the common connections are all connected together as you show then you only need one DPDT relay.
View attachment 304092

Ah yes, didn't see that. Thanks.
I was already thinking of adding a bigger power supply because 2 of these relays would draw quite a bit for a small battery.

 

Two alkaline AAA batteries or one CR123 should last many hours depending on the duty cycle.

 

If the common connections are all connected together as you show then you only need one DPDT relay.

Agree.

Are you locked into a 3 V source for the circuit? If the signals being switched are line level and not actual speaker drive, then with 6 V (such as two of the 1/3 N cells making +/-3 V) you could replace the relay with CMOS analog switches.

ak

 

Agree.

Are you locked into a 3 V source for the circuit? If the signals being switched are line level and not actual speaker drive, then with 6 V (such as two of the 1/3 N cells making +/-3 V) you could replace the relay with CMOS analog switches.

ak

These are speaker level lines. Since I can ditch the the other relay it should be fine if I'll go with 2 x AAA or maybe a small power bank and a step down converter 3.3 V

Just got my BT TX in the mail and of course it doesn't want to connect to my Sony Earbuds.
Don't really want to randomly buy these and just hope that it works....