Taranis QX7 bluetooth

Hardware help and support for the FrSky Taranis
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jhsa
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Re: Taranis QX7 bluetooth

Post by jhsa » Sun Sep 16, 2018 1:05 pm

I have a few P Channel Mosfets which are logic level. Will try them when I have some time.. The problem is most of what I have are.SMD :(

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MikeB
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Re: Taranis QX7 bluetooth

Post by MikeB » Sun Sep 16, 2018 9:46 pm

This is a version using a MOSFET to switch the power:
BTpower1.png
BTpower1.png (4.25 KiB) Viewed 56 times
I had this built (without the capacitor) on a piece of stripboard using surface mount devices so I tried it.
The capacitor could be a lower value than 1uF, I think 0.2uF would be OK. As shown, the circuit holds the power on for over 200mS after the TxD signal goes low, 40mS would be fine.

I'm running from 3.3V and using a 120 ohm resistor as the load, so around 27mS, which is, I think about the current needed by a HC-05.

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jhsa
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Re: Taranis QX7 bluetooth

Post by jhsa » Mon Sep 17, 2018 6:33 am

Thanks Mike, I see you omitted the resistor going from the base of the BJT to ground on your first schematic. Won't that prevent the transistor to turn OFF? Or is it enough when you pull the processor pin low?
Also is the pin floating at any occasion, for example, at power ON? That could activate the power to the Bluetooth, right? Or am I completely wrong here.
I am asking because I had trouble when buding my safe power switch for the 9x radios. The radio didn't want to turn OFF without the resistor between the base.of the transistor and ground. he circuit is very.similar to yours. My circuit prevent the radio and RF module from turning OFF if any part of the power switch fails. You might remember that it happened to my 9XR-PRO. :)

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Kilrah
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Re: Taranis QX7 bluetooth

Post by Kilrah » Mon Sep 17, 2018 6:45 am

A FET is controlled by voltage, so leaving the pin floating is bad since any stray signal can turn it on. A BJT is controlled by current, so won't turn on unless something actively pushes current through it.

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Re: Taranis QX7 bluetooth

Post by MikeB » Mon Sep 17, 2018 8:18 am

In addition, the TxD signal, in operation, is driven both high and low so the resistor in the signal path doubles up as pulling the base low.

In the second circuit, I had it already built so the resistor to ground was already present.

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Re: Taranis QX7 bluetooth

Post by jhsa » Mon Sep 17, 2018 12:30 pm

Kilrah wrote:
Mon Sep 17, 2018 6:45 am
A BJT is controlled by current, so won't turn on unless something actively pushes current through it.
Well, as I said above, I have seen the opposite. The circuit wouldn't turn off if I didn't connect a resistor between base and ground.

Looking at the circuit Mike posted (without the resistor), that is why I also said that it might not be necessary as the processor pulls it low. But then I thought that the pin might be floating at power ON for example..

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Re: Taranis QX7 bluetooth

Post by jhsa » Mon Sep 17, 2018 3:17 pm

Mike, Through hole N channel Mosfets are more readily available than P channel. this would allow people to build the circuit on a bit of stripboard. Could something like in the picture be used? I tested the circuit in a circuit testing software and it seems to work.
Although the BS170 datsheet states that its maximum current is 500mA, I am not sure it would be enough for the BT module?? Must try it :)

João

EDIT: Hmmm, the BS170 Vgs might not be low enough at 200mA.. It would be good if powering the module with 5V, I think. Need to test this :)

EDIT_1: I have just checked the HC-05 datasheet, and apparently only takes around 35mA. I think that makes the famous BS170 suitable for the job.
According to the datasheet, the graphic shows that for a current of about 150mA the Vgs has to be around 3.3V for the Mosfet to be fully ON. Please check attached picture. That should be ok for the HC-05??
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Mosfet Bs170 VGS at 3.3V.jpg
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Re: Taranis QX7 bluetooth

Post by MikeB » Tue Sep 18, 2018 11:30 pm

You are switching the ground rather than the power. With the positive supply still connected, I think it likely that driving the TxD signal low will cause the protection diodes on the TxD input to conduct and try to power the BT module.
I think we need to switch the power not the ground.
Even though I used surface mount devices, I have the second circuit built on stripboard. I'll see if I can post a picture of it tomorrow.

I have some firmware changes in place for the SKY, AR9X, 9XR-PRO, 9Xtreme and QX7 that drive the TxD signal low if the BT module function is set to OFF. There is a new function "Telem", if you want to enable the BT module for sending telemetry out rather than any of the other functions. While this new option appears immediately after "OFF", the internal codes used for the BT function are unchanged. Currently, you will need to also set the "BT Telemetry" option to "ON", although I think that will become unnecessary.

While not obvious, the BT function setting is a model setting not a general setting, so the BT module enable may change as you change models.
These changes will be in the next test version (R222c2), and I may get that posted tomorrow. I'd like to test the above power switch circuit with the firmware changes before I post them.

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Re: Taranis QX7 bluetooth

Post by jhsa » Wed Sep 19, 2018 6:10 am

Thanks Mike, I have been quite busy lately, but I have a radio with skyboard open on my computer desk, so I think I could test on that one.. Did you find a through hole low Vgs P channel Mosfet? Or did I misunderstand your post as usual? ;)

I do have a few logic level P channel Mosfets, but they are all SMD power Mosfets. I can live with it, but I was thinking about the people that can only handle through hole components, as they don't need a real PCB, and are easy to solder.

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Re: Taranis QX7 bluetooth

Post by MikeB » Wed Sep 19, 2018 11:58 am

This is my stripboard (without the capacitor), but it is all surface mount.
BTpower.JPG
BTpower.JPG (24.89 KiB) Viewed 64 times
I've just tested this on an AR9X board and a HC-06. It all worked fine, setting the BT Function to off, turned the power to the module off, setting the BT Function to something else and it powered on and functioned correctly. I ran BT trainer between this and my SKY board radio that has a HC-05 in it.

I changed the 1uF capacitor to a 0.2uF capacitor (two 0.1uF in parallel) and with the BT trainer running at 115200 baud, the capacitor only discharged by 0.2V at most.

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Re: Taranis QX7 bluetooth

Post by jhsa » Wed Sep 19, 2018 12:52 pm

Thanks Mike, what MOSFET is that one? Can't really understand the markings

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Re: Taranis QX7 bluetooth

Post by MikeB » Wed Sep 19, 2018 3:49 pm

I just updated the resistor and capacitor values on the circuit I posted above. The two transistors are as shown on that circuit, BC817 for Q3 and SI2301 for the MOSFET.

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Re: Taranis QX7 bluetooth

Post by jhsa » Wed Sep 19, 2018 4:21 pm

Thanks Mike..
I think I found a suitable Mosfet in my components box :)
AO3401, which was used on the first Ar9x board (I think), and then replaced with the AO3407A. Both are identical, but the 07A has a higher maximum Vgs, 20V against 12V from the AO3401. The 3401 has a lower Gate threshold voltage, so perhaps more suitable for our application. Please see the attached picture. I believe it is low enough for the 3.3V supply. What do you think?

Thanks

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Re: Taranis QX7 bluetooth

Post by MikeB » Wed Sep 19, 2018 4:49 pm

That looks OK.
I've just posted a new test version of ersky9x that sets the TxD signal low if the BT Function is set to off.

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Re: Taranis QX7 bluetooth

Post by jhsa » Wed Sep 19, 2018 11:23 pm

Thanks Mike.. Will try to test it on Friday. Too busy with work tomorrow.. Not complaining though :)

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