Rogers postbag - FM radio band transmitter

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RogerClark
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Rogers postbag - FM radio band transmitter

Postby RogerClark » Fri Feb 17, 2017 5:56 am

If anyone read my thread on the Bluetooth audio receiver dongles, they'll know I ordered a low power FM radio transmitter module so that I can get Bluetooth audio into my car radio via FM radio (as it doesn't have Bluetooth or even a Aux line input)

So this module arrive today

http://www.ebay.com.au/itm/291722004450

fm_tx.jpg
fm_tx.jpg (91.51 KiB) Viewed 346 times




And ... It works really well.

With a suitable antenna, I'm sure the range would be hundreds of meters, but its not legal to operate it like that, so I'm just going to put a resistor for an antenna, so that it only broadcasts enough power to be received by the car radio when the unit is in the car.

Interestingly, on the back of the board I noticed it had 3 pads labelled TX, RX and GND

Image

So I hooked up a USB serial adaptor, and I could see it output "FM TX mode" at 38400 baud

Looking on the web, I found this post, which implies that you can send it commands

http://falconchristmas.com/forum/index. ... l#msg57368

However I couldn't get it to respond to anything, with or without newline or carriage return after the command.

If you plug the USB socket into a PC, Windows initially give an error about an unknown device, but then seems to load a USB audio driver, and if you select that as the audio output on the PC, the transmitter broadcasts that audio.

Strangle when the board is plugged into the PC the baud rate of the Serial TX and RX changes from 38400 to 115200.
But I still cant get it to respond to commands.

BTW. The markings on the main IC are.

JL AC1618CGCL -12E

However I can't find any details on this device.

Looking at the 8 pin chip, in the bottom left of the photo, its a 24C02 (I2C Serial EPROM), which is quite interesting, so if i had time I could probably read that and see what data is inside.


There are 2 pads in the top left of the board, labelled M and G. G is connected to Ground, but I'm not sure what M is for.

I measure the voltage on M and its 3.3V. Initially I thought perhaps these were terminals for a battery, as I know some versions of this board are supposed to support battery operation and change the battery via USB, but I expected that voltage to be 4.3 V is the board was trying to charge a battery (i.e max charge voltage for a LiPo)

Some of the documentation on this module seem to suggest you can control the RF TX power but by pressing the Pause button then pressing some other buttons (the instructions were vague), but I can't seem to enter the power control menu at all. The only things I can change are the audio input sensitivity and the transmission frequency.

Anyway. The next step is quite easy, I just connect this to a Bluetooth audio receiver module, and put it in the car !



PS. Shame the Serial commands don't work

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ahull
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Re: Rogers postbag - FM radio band transmitter

Postby ahull » Fri Feb 17, 2017 10:03 am

There seem to be a number of variants on that module, including this

http://www.ebay.co.uk/itm/FM-Transmitte ... Sw5cNYNrz9

Image

Which is based on an 8 bit STM - > http://www.st.com/en/microcontrollers/stm8l052c6.html
- Andy Hull -

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RogerClark
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Re: Rogers postbag - FM radio band transmitter

Postby RogerClark » Sat Feb 18, 2017 1:04 am

Thanks Andy

I just bought the board which @vassilis found, (in the other thread)

I mistakenly thought that the big chip in the middle of the board was some sort of all in one processor / RF PLL generator etc etc

But it looks like that chip is just some sort of microprocessor to run the display etc, and the other 8 pin chip in the photo you linked to (which is the same chip as on my board) is probably the FM radio transmitter

The markings on that chip are 8027 0428 6TCC

Which seems to be a QN8027

http://down.cosou.com/xintechsz.com/QN8027.pdf

Which has an I2C interface and direct audio line inputs (for left and right)

So, the MCU on my board must be providing the USB Audio interface, and also the hardware UART interface (though the UART seems to be one way - output only - unless I'm doing something wrong when I sent the commands), and controlling the radio chip via I2C

I don't know if the STM8 has USB, as far as I can tell it doesnt have hardware USB, but I could be wrong. In which case... some of these boards would have the USB audio capability and some would not.

The spec for the eBay listing for the STM8 based version doesn't seem to have anything about USB audio (But I don't know if the one I bought had that listed either)

Anyway. its an interesting / cheap way to get a FM transmitter that can be microprocessor.

Probably snooping on the I2C traffic would be the best place to start, to find what setup commands are being used, if anyone wanted to make their own board.

But for me, I just need to connect up the Bluerooth audio receiver and sort out a 5V regulator so I can power it in the car, and put it in a box.

raphik
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Re: Rogers postbag - FM radio band transmitter

Postby raphik » Sat Feb 18, 2017 11:10 am

I also bought one of those about one year ago. AT commands can be sent through UART (RX-TX-GND pads) with SSCOM32E.
Image
It can be downloaded from https://storage.googleapis.com/google-c ... com32E.exe

AT commands to be sent are (I first wrote it in Spanish, sorry)
Image

One interesting tip: Campus Mode can be activated/deactivated by pressing VOL- and VOL+ simultaneously when turning the module on. Campus ON extends the frequency range from 76 - 108 MHz.
Image

Regards.

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ahull
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Re: Rogers postbag - FM radio band transmitter

Postby ahull » Sat Feb 18, 2017 4:33 pm

You could also send data using these devices... but that might be illegal in some jurisdictions. Two methods spring to mind... audio encoded data (think modems and cassette interfaces).. and RDS data (which the transmitter chip appears to support).
- Andy Hull -

raphik
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Re: Rogers postbag - FM radio band transmitter

Postby raphik » Sat Feb 18, 2017 8:25 pm

As you said, ahull, QN8027 seems to support RDS/RBDS. So I tried to handle it via I2C but I got no success.
Image

It's hard to make welds so small.
Image

I attempted it without 4K7 pull-up resistors, what could have caused the failure.
Last edited by raphik on Sat Feb 18, 2017 8:30 pm, edited 1 time in total.

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Pito
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Re: Rogers postbag - FM radio band transmitter

Postby Pito » Sat Feb 18, 2017 8:30 pm

You need a better wire and solder iron. The soldering job above could introduce shorts..
There should be the i2c pull-ups on the board already assuming the chip needs to be programmed to work..

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ahull
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Re: Rogers postbag - FM radio band transmitter

Postby ahull » Sat Feb 18, 2017 9:22 pm

I think you should be able to talk to the chip, but you may need to 'cope the pins to see what is going on. I suspect that the micro-controller on the board may be talking at the same time as you are trying, so you may need to hold the micro-controller in reset while you speak to the QN8027
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RogerClark
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Re: Rogers postbag - FM radio band transmitter

Postby RogerClark » Sat Feb 18, 2017 9:26 pm

@raphic

I will try the Serial commamds again usng that terminal.

What MCU does your board use ? is it the same one that I have.

Re:I2C control of the TX

You may need to cut the tracks from the MCU on the board, as it may be doing something non standard with those lines and holdimg one of them when it should be releasing.

Re:out of band.

Thanks for the tip, but my car radio only goes down to 87.5, so I dont really need it.

If you could get the I2C control working, I susoect you could move it well out of band by changing the multiplier on the crystal, but you would probabky need to change the external inductor if you wanted to use this for the 6 m ham radio band.

Re:RDS

I noticed that as well.

It looks like the QN8027 is capable of generating all sorts of data in the transmissions, including traffic alerts.

Unfortunately for me, my Subaru car radio, doesn't even have RDS display. Subaru really skimped on the radio, for in what is / was a top of the range model.

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RogerClark
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Re: Rogers postbag - FM radio band transmitter

Postby RogerClark » Sat Feb 18, 2017 9:27 pm

Andy

Our posts crossed.

I had the same idea about the I2C

If I get time, I will attach my logic analyser to the pins and see if I can capture the data commands that are being sent to it, as I just need to solder on 2 wires.


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