FrSky DHT and HiTec - adding Buddy Box Support

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rkrishnan
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FrSky DHT and HiTec - adding Buddy Box Support

Post by rkrishnan »

I am returning to the Hobby after a long gap, and having kept my old HiTec Flash 4X and Flash5X 72 MHz radios from way back when, I was gratified to find that I could modify them for 2.4 GHz. by using the strangely named DHT (toggle switch) from FrSky. Only three wires are required to be soldered in:, V+, GND., & PPM. All are available in the rear 6 PIN DIN trainer socket of HiTec Flash 4X & Flash 5X Tx's.
I updated both my Tx's - I shoehorned the device inside, there is enough space inside!
Everything works just great, For $48.00 + shipping I have two Tx's that work perfectly with 3 different FrSky Rx's.

Now I want to stop crashing my planes and learn from an instructor, and would like to add a Buddy Box system that works between the two Tx's.
One Tx (the slave) would be powered down, (battery removed).
Can I safely parallel the PWM outputs from the two DHT's (Which is what the Buddy Box cable does) ? Should I add series resistors to each DHT PWM terminal to prevent that magic smoke puff?
I am hoping I do not have to resort to a 3 way DPDT switch to toggle between Tx's.

Daedalus66
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FrSky DHT and HiTec - adding Buddy Box Support

Post by Daedalus66 »

The conventional trainer setup works as follows. The buddy box doesn't have an RF module (or if it does, the module is turned off). Only the master transmitter (in the instructor's hands) transmits to the model.
The trainer switch is a momentary one on the Master transmitter and transfers control to the student when the instructor holds it on. When the switch is released, control reverts to the instructor.
rkrishnan
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Re: FrSky DHT and HiTec - adding Buddy Box Support

Post by rkrishnan »

Daedalus66,

Thank you. That is how I understood it, but with the HiTec Flash 4X & 5X, the cable has internal wiring such that when the DIN plug labeled "STUDENT" is plugged into either Tx, it turns off the RF power on that Tx, and when the other end marked "MASTER" is plugged into the other Tx, that one becomes the master. From then on everything works as you have described.
However, the DHT mod connects to the very same PWM pin (and return ground) ton both Tx's. In http://www.mftech.de/buchsen_en.htm
pins 4 & 5 are the ones shorted in the slave (student) buddy box cable end. This will only turn off the 72MHz side of the Tx (already crippled by removing the Crystal). The DHT remains in-circuit, maybe it will not affect anything if I remove the battery in the slave TX before a training session.
Guess I may have answered my own question - To play it absolutely safe I am going to have to add a special switch that completely isolates the slave side DHT from the DIN socket, and remember to flip it before each training session. Or maybe just do all my flying with one Tx, and make the other one into a buddy box/spare (it's easy enough to provide a 3 pin quick disconnect cable internally)
Daedalus66
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Joined: Tue Dec 27, 2011 8:22 pm
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Re: FrSky DHT and HiTec - adding Buddy Box Support

Post by Daedalus66 »

Sounds like you have a good grasp on the issue. I have an old Flash4X converted to 2.4 with a FlyDream module, but I haven't studied the trainer setup.
I think your last idea is the better one, as long as the Flash has enough memory slots for a single transmitter to meet your needs.

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