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Old Aug 21, 2012, 08:16 AM
ancora imparo
jj604's Avatar
Melbourne, Australia
Joined Jul 2005
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Mini-HowTo
Adding FrSky to a Spektrum DX8

The FrSky 2.4 system is popular because of its extremely reliable radio link and growing range of telemetry options at a reasonable price. They currently make plug in modules for suitable modular transmitters plus two add-on DIY modules that can be installed internally (DTH) or externally (DTH-U) on any transmitter.

Adding an internal one to a Spektrum DX8 is attractive and is straightforward enough for anyone who can solder two wires together and makes no changes to the DX8 at all, except a few holes in the back cover.

I really like having the power of the DX8 software and programming (and SD card storage!) together with the link reliability of the FrSky and its versatile cheap telemetry.

Leaving the DX8 intact means I can have FrSky or DSM2/DSMX at the flick of a switch, which is nice for the Spektrum bricks and parkfliers with cheapie Orange DSM2 receivers. You get the best of both worlds. The only thing you don’t get in the add-on is the Model Match facility since this is a function of the Spektrum DSM2/DSMX connection but everything else in the DX8 applies to the FrSky link.

NOTE: Zeeland has converted the first 4 posts into a single pdf that you can print out for reference if you prefer. See Post #9. Thanks!

The FrSky unit is installed using just 3 connections: power and ground plus the PPM control signal from the transmitter encoder section. The DX8 should be a simple conversion since the PPM signal is available at the trainer jack on the back. The DX8 does not provide suitable power at the charger port but it is a simple job to add another connector to the battery pack. Unfortunately if you connect it up as expected (see Connections), it doesn’t work.

I found plenty of information on converting other transmitters but little on the DX8. There are two special problems with the DX8 that do not occur with most transmitters.

1) It uses a 4.8V battery and the FrSky module specifies a minimum of 6V
2) Despite being clever about recognizing PPM pulse polarity the FrSky module does not respond to the DX8 output.

Short answer for electronics types.

Add a 47k pulldown resistor between the PPM lead and ground and supply the FrSky module with a minimum of 6V. Mine will actually work down to 4.9V but less than 6V is at your own risk.

Long answer.

The voltage problem can be solved in a number of ways. See post #3 for a discussion of four options.

I recommend the simplest and most effective solution of replacing the normal DX8 4.8V NiMH with the alternative 7.2V LiPo pack which is sold as an accessory (or the much cheaper HobbyKing equivalent). The DX8 is designed for it, includes the correct charge circuitry and it will double the operating time as well. The DX8 low voltage alarm will go off long before the FrSky stops functioning.

The signal problem is a little more subtle. If you look at the signal coming out of the DX8 trainer jack it looks like Trace 1. The “0-“ marker is zero volts. The 8 control pulses are negative going from about 0.7V positive to 2.6V negative (3.3V logic). However when you connect the FrSky module the voltage levels shift to Trace 2. The pulses now go from 6.1v to 2.8V. The FrSky input discriminates logic 0 from a logic 1 at 1.65 V so never sees any pulse input. Adding a 47k pull down resistor to the output of the DX8 solves the problem and results in Trace 3, a series of 3.3V to zero pulses.

The next post shows how I added an FrSky DTH module internally but connected to the external trainer port and an FLD-02 LCD display to give a full FrSky telemetry system that can be switched at will back to a completely unmodified (except for the case holes) DX8.
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Last edited by jj604; Aug 21, 2012 at 10:15 PM.
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Old Aug 21, 2012, 08:16 AM
ancora imparo
jj604's Avatar
Melbourne, Australia
Joined Jul 2005
6,148 Posts
Build

Providing power.

1) Change the Tx pack from a 4.8V NiMH to the alternate 7.2V LiPo one – SPMB4000LPTX. Either the Spektrum original or the HobbyKing equivalent will work fine. I used the latter.
2) Add a second battery connector to the new battery for the FrSky module. The simplest way is to use a good quality servo extension lead cut to length. Solder the normal connector (the smaller end) to the battery lead as shown in NewBatteryWire. You can remove and discard the third signal wire. Make sure you do one wire at a time and properly insulate each one. If the battery wires ever touch you will have a massive short circuit! This has a bonus advantage in that you can now charge the Tx LiPo using your normal charger and a Rx pack charge lead simply by removing the battery cover. Remember it will be an unbalanced charge however, so avoid high rates.
3) Run the other part of the lead through the hole in the battery compartment where it will be joined to the red (+V) and black (power ground) wires of the DHT via the new power switch.

Providing signal.

1) You will need either a stereo or mono 3.5mm jack and a 47k resistor. Also a small on/off switch for the power supply to the DHT. I chose a good quality slide switch as this is the main On/Off switch for your transmitter when in FrSky mode but a toggle switch is simpler to mount and operate and is probably a better alternative. It is shielded by the jack when in use so is well protected from accidental movement. Choice is yours. See StereoJack and SwitchOptions.
2) You can get everything at any electronics shop, even Radio Shack, but it may be more convenient to cannibalize an existing stereo earphone lead from a Walkman, CD player, iGadget or similar. You want a right angle jack so it doesn’t stick out the back of the transmitter and the thin round black stereo wire is neat. The DX8 only needs a mono jack but stereo ones are far more common and work fine. The wire that goes to the tip of the jack is the PPM signal (Yellow wire on the DHT) and the body is the ground (Black wire – also the power ground). On a stereo jack the center band and its third wire are not used and must not be connected to anything. Many stereo leads use a red wire for the tip, white for the center and the outer copper braid for the body but you should check this with a multimeter to be sure.
3) You need to connect the 47k resistor between the PPM signal wire and ground. Either inside the jack like I have done or between the yellow and black wire at the DHT end. See ResistorAdded and JackConnection.

Putting it together

1) The first step is to trim a bit off the end of the DHT indicator board as otherwise it will just touch the works inside the transmitter and stop it closing properly. Cut off up to the mounting hole on the switch end. See BoardTrimmed.
2) Open up the DX8 with the battery unplugged and removed. First peel off the two contoured rubber back grips. They can be pulled up and away from the thick end. The double sided tape does not have to be replaced as they will stick back on fine and there are rubber tags to hold it as well. Then peel the side panels from the black back half only. Leave them attached to the silver front. You can now take out the 6 screws that hold the back on and separate the two halves. See RubberGripRemoval.
3) Fit the DHT indicator board inside the back next to the Aux knob pot and as far forward as possible. Mark the position of the LED, measure the distance and drill a 3mm hole for it from the outside. Check it is in the right place then drill a 3.5mm hole for the bind button at the correct spacing. Drill a 6.5mm hole for the aerial connector next to the Trainer switch. See ThreeHoles. The DHT bind board should be held solidly in the case by the LED through the hole and some hot glue. Check that the bind button just protrudes enough and that it switches without catching on the case.
4) Make a hole or cutout for the power switch next to the Trainer Jack. See PowerSwitchCutout. A drilled hole for a toggle switch is a lot easier. Fasten in the power switch (epoxy is good for a slide switch) and cut the red wire of the new power wire and solder to the switch. I wound the excess black wire round the unused third terminal just to help anchor the wire. See PowerSwitchWiring. It ends up looking like PowerSwitch.
5) Attach the DHT with some double sided tape, route the wires neatly, and thread the DHT Mode wire through the battery compartment hole. The switch itself sits in the battery compartment since I only use telemetry receivers and will only need to access it for firmware updates. If you want to use both telemetry (Mode2 – Two Way series) and non-telemetry (Mode 1) receivers you will want to drill another hole in the case and mount the mode switch externally for easy access. See OpenedView.
6) Reassemble the transmitter making sure that none of your new wiring can touch the control pots or their wires when they move. I had a puzzling interaction between the two controls on one stick until I figured this out.
7) Fit the battery and DHT Mode Switch and the new connector into the battery compartment. BackView and BatteryCompartment show how it all fits.
8) Attach the FLD-02 display. I used a wedge of rubber cut from the packing a brushless motor came in and 3M Outdoor double sided tape. See DisplayMounting. You could also use the FrSky custom handle mount.

Using it.

1) For the original DX8 DSM2/DSMX just use it as normal leaving the new power switch off and the jack unplugged. A bit of Velcro on the jack makes a handy storage.
2) For the FrSky, leave the DX8 off, plug in the jack, wait till the DX8 starts up in Slave mode then turn on the new power switch. Bind the FrSky receiver as usual using the new power switch and the FrSky bind button. Never turn on the DX8 main switch if you are in FrSky mode.
3) Note that the model memories are part of the transmitter, not which mode it is, in so you can have the same model with identically stored settings use either a DSM2/DSMX or an FrSky receiver. The only difference will be that Model Match only works with the Spektrum system.

Alert: I came across one gotcha by accident. You can set the failsafe for the Rx as usual and it will work as expected if you turn off the FrSky Tx module. However on one of my ESCs at least if you unplug the SIGNAL (ie pull out the trainer plug) with the Tx and Rx still powered you may find the throttle will go to full speed - and will not throttle back when you plug the signal back in. Very dangerous! Don't know if it is just a feature of this one particular ESC but worth knowing about. It isn't something you would normally do - but be warned.
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Last edited by jj604; May 27, 2013 at 02:24 AM. Reason: New info
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Old Aug 21, 2012, 08:17 AM
ancora imparo
jj604's Avatar
Melbourne, Australia
Joined Jul 2005
6,148 Posts
The voltage problem

The FrSky DHT is specified to operate from 6 to 13 Volts. The normal battery in the Spektrum DX8 is a 4cell NiMH pack with a nominal voltage of 4.8V (unlike many transmitters which have a 7.2V or 9.6V battery and which work fine with the DHT). Not high enough. So what is the solution?

1) The cheap and nasty approach.
Both voltage ratings are a bit conservative. My DHT operates properly down to 4.89Volts at which point it begins cutting in and out and stops transmitting. The Spektrum battery actually stays above 5V for much of its discharge. The DX8 uses just over 70mA current in trainer mode while the DHT plus FLD-01 uses just under 130mA. Total current drain is thus 200mA. The graph shows the actual voltage curve of my fully charged Spektrum 2000mAh NiMH pack when discharged at a constant 200mA.

If you set the DX8 low voltage alarm to 5.1V (green line) you will get about 100 minutes flying time and still stay at least 0.2 above the FrSky required voltage. A 5 volt alarm (red line) will give you 200 minutes but only 0.1V leeway. Be sure to check that your DX8 Tx pack voltage reading is really accurate with a good quality voltmeter before you rely on this method though. You would also need to be sure your FrSky system will work at 4.9 volts. It’s risky and not for me.

2) Boost the voltage.
All we need is a method to boost the NiMH pack to a steady 6V. Unfortunately the 6V BECs we use for models won’t work as they are all voltage reducers and will not output a voltage higher than the input. A similar circuit can be used as a voltage booster though and they are sold as USB->6V converters which have adequate current capacity of 300mA or more.

http://usb.brando.com/usb-to-6v-volt...2c042d015.html

Even more useful is the so-called Solar Boost Power Converter module which will produce up to 9V from 0.9-6V in at several 100mA. Perfect. At $3.61 delivered I have ordered a couple to try out. However they will require modification to remove the USB connector and may well have noise issues. I am not keen on putting a cheap switch-mode regulator inside my transmitter. See update in Post #14.

http://www.goodluckbuy.com/dc-dc-sol...-module-1.html

3) Add an additional battery to run the FrSky module.
This is the approach adopted by the very successful FlyZone Tactic Tx-Ready system to make it simple to attach their module to a DX8. Works fine for their low drain system using a small LiFe 200mAh battery but in my opinion becomes a clumsy solution with the bigger battery needed for the much higher drain FrSky system. You also have no monitoring of the state of charge of the additional battery, which is now running everything.

4) Change the battery
The DX8 has actually been designed to use either a 4cell NiMH or a 2cell LiPo. If you replace the original battery with the SPMB4000LPTX LiPo 4000mAh alternative the problem is solved. The transmitter already contains the LiPo charger and you alter the voltage cut off alarm in the system setup. Minimum voltage is then 6.3V (or whatever you choose). The official Spektrum replacement is, as usual, expensive but HobbyKing make an exact equivalent at 1/3 the price.

http://www.hobbyking.com/hobbyking/s...ter_Pack_.html

This is the best way to go in my opinion and what I show in the how-to.
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Old Aug 21, 2012, 08:17 AM
ancora imparo
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Melbourne, Australia
Joined Jul 2005
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Options

Simple options:
1) Omit the FLD-02. If you are not interested in telemetry, omit the FLD-02 and its connecting lead. You can either extend the 4 pin DHT connector into the battery compartment for future firmware updates or decide they you will take the case apart any time you need to do one.
2) Omit the DHT Mode toggle switch and its lead. If you only use 2-way (D series) receivers then you can disconnect the toggle switch and its lead, as the default with no switch connected is 2-way. You will have to take the case apart if you ever need to do a firmware update and re-attach the toggle switch.
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Last edited by jj604; Aug 21, 2012 at 08:45 AM.
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Old Aug 21, 2012, 08:46 AM
ancora imparo
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Melbourne, Australia
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Happy to answer any questions people have. Just remember the time difference. If you are in NA I'm asleep when you are awake.

That's enough for one night.
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Old Aug 21, 2012, 09:05 AM
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Quote:
Originally Posted by jj604 View Post
... at will back to a completely unmodified DX8.
Other than the holes in the case, right? Nice detail posts on your mod.
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Old Aug 21, 2012, 09:08 AM
ancora imparo
jj604's Avatar
Melbourne, Australia
Joined Jul 2005
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OK, OK.

What I meant was that there are no electrical mods. Many guys are happy enough hacking holes in plastic but get freaked by soldering things to their transmitter. It was just shorthand but you are technically correct.

John
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Originally Posted by freechip View Post
Other than the holes in the case, right? Nice detail posts on your mod.
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Old Aug 21, 2012, 10:50 AM
59 years of RC flying
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Canada, ON, Ottawa
Joined Feb 2006
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Thanks very much for the detailed, interesting and very well presented information. I don't happen to own a DX8, but I still found the material offered many good ideas about FrSky conversions and related matters.
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Old Aug 21, 2012, 09:51 PM
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Little help

For share and print xD
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Last edited by Zeeland; Aug 30, 2012 at 07:59 PM.
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Old Aug 21, 2012, 10:12 PM
ancora imparo
jj604's Avatar
Melbourne, Australia
Joined Jul 2005
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Hey, thanks for that Zeeland. That's a neat conversion.

Would be interested in using the same tool on some other web postings I have.

What is it called?

Regards, John
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Originally Posted by Zeeland View Post
For share and print xD
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Old Aug 21, 2012, 11:43 PM
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Hi John,

I just copied your words and photos (Ctrl+C & Ctrl+V ) into word 2010 and "print" into PDF.

Nothing complicated but a little bit time.

Regards,

Kai
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Old Aug 22, 2012, 12:16 AM
ancora imparo
jj604's Avatar
Melbourne, Australia
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Well, thanks for the effort. Very much appreciated.

John
Quote:
Originally Posted by Zeeland View Post
Hi John,

I just copied your words and photos (Ctrl+C & Ctrl+V ) into word 2010 and "print" into PDF.

Nothing complicated but a little bit time.

Regards,

Kai
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Old Aug 22, 2012, 03:48 AM
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Added to main Frsky thread links index, Conversions section.
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Old Aug 30, 2012, 05:00 PM
ancora imparo
jj604's Avatar
Melbourne, Australia
Joined Jul 2005
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Voltage Booster

This is a follow up to my comment no3 in post 3. I also posted it on the DX8 and FrSky thread in case anyone else is doing a conversion with aTx that has a 4.8V pack.

If you want to add an additional RF module to the DX8 (like an FrSky or similar) the 4.8V standard battery is a problem since many of them need 6V or more. You can change to the 7.2V LiPo battery which is what I did but there's another option.

This little voltage booster is $3.61 (that includes delivery from China!) and mine arrived in 13 days from placing the order.

http://www.goodluckbuy.com/dc-dc-sol...-module-1.html

It produces a steady 9.31Volts at maximum adjustment from my Spektrum standard DX8 NiMH.

I haven't measured the ripple but they claim 50mV max and 94% efficiency with a minimum current capacity of 800mA. It seems to work my DX8/FrSky conversion just fine. I removed the USB connector which is the OUTPUT and added a standard Futaba/JR IN and OUT for testing. You would need to put it downstream of the battery switch of course, otherwise it would drain the battery. They say it only needs 130uA but I bet they didn't include the LED in that.

If you removed the LED (and it still works) then you could leave it in circuit permanently. If my maths is correct it would drain a fully charged 2000mAh pack in 42 years.

Even with the tiny SMD LED it isn't much more than the NiMH self discharge rate.
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Old Aug 30, 2012, 08:00 PM
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Added into PDF.
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