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Hacking the I2C interface of Spektrum DX and AR ?

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It looks reasonable, but I don't have time to carefully review it. It might be convenient to add another pad on IC pin-8 for the gnd jumper you need. Also, the body size for C1 and C2 seems a bit large. If you shrink them to their actual size, your board could be smaller.

Lastly, it would be a good idea to check with Arthur P. to verify that the final schematic accurately reflects what he used.

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  • 2 weeks later...
i´ve routed it.... take a look on the photos.

I nee only one wire to plug pin 8 to gnd

there is any mistake here?

Hi osvaldoljunior,

Any luck with your converter? I'm very curious...I'm nearing the completion of my MK and would like to use my existing DX7/AR7000. Please update us with your results. Thanks :D


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I should hopefully be able to update the results of the interface within about a week now. I-ve finally managed to get a number of sufficiently powerful ESCs for my motors up and running as TWI/I2C controled ESCs. I actually posted a thread on converting those TowerPro 25A type W ESCs from PWM to TWI/I2C in both RC Groups under DIY electronics, and in the Mikrokopter Wiki. So I-m now integrating all the parts into two frames and then up to flighttest. Do't have much time this weekend and week, but definitely expect to reach that point next weekend.

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Arthur P., I have seen some of the rcgroups posts regarding your efforts. The build details definitely look fascinating.

If the HC221 interface does not work out then there are other solutions to try. The next attempt would probably be best solved with a dedicated microcontroller at the helm.

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Hi. My controlers and motors dont arrive yet...

but i have a little problem with the componentes.

the caps C1 and C2 that are drawing as they are polarized caps (.022uF and .01uF)

I lost a lot of time to find it with out any suces. them loking to the pfotos posted i dicover that this caps are not polarizad caps !?!

Is that correct... is the only missing part for my conversor.

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Arthur P., I have seen some of the rcgroups posts regarding your efforts. The build details definitely look fascinating.

If the HC221 interface does not work out then there are other solutions to try. The next attempt would probably be best solved with a dedicated microcontroller at the helm.

I may well try a dedicated controler. I have a few Holger ESCs which refuse to work properly as ESC but which have perfectly fine ATMega8's sitting around. I may well remove the FETs and parts related to driving them. That leaves me with a nice small ATMega8 PCB with ISP, serial and I2C ports. And with the info here http://www.rcgroups.com/forums/showthread.php?t=714299 on the digital signals coming off of the two Spektrum AR7000 satelite receivers it should be possible to just feed them the serial datastreams and put out a real PPM signal which would not be at risk of reordering of the channels as each digital datavalue for a channel includes the channel number.

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The layout below represents Arthur's final design. I'd like to move forward in having it etched and completed. The 1st layout includes a 3.3v regulator @ 400ma..not sure if this will be a good solution. The 2nd is without the regulator.

BTW, Terry over at RC Groups produced both layouts.

Any feedback is greatly appreciated.



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Hi Mr. RC CAM,

Good point on the Caps on the VReg... I'd like to say I checked this before bolting the component onto my design... but I didn't, having said that... it appears from the Datasheet that Caps aren't required on that VReg (but can be added for good practice/better stability).

Looks like I got lucky in not adding them!!!

What I wasn't sure of, is whether a 400ma VReg would be sufficient to run the circuit...

Anyway, if anyone spots any other issues let me know,



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  • 3 weeks later...
  • 1 month later...

I think I need a little bit of help here. I made the circuit and eventually (after 2 mistakes) got it working. It was not reliable though. Some of the PPM channels would drop out (on the scope they would flicker on and off) if I just let it sit. I started checking things and found that the PWN signal was only 2.2 V max. If I unplugged it from the converter it was back to 3V. The HC221 is running at 3.1V and the PWM signals after the diodes were now down to sub 2V and I was thinking that that might start creating problems for the HC221 to reliably trigger. I suspected (I am not a EE) that the 2.2K resistor at R3 was too much of a drain on the AR7000. ( As an interesting point, the channel seven pulse from the HC221 would show up at ~3V after the diod). I decided to change the resistor at R3 to a 10K. Voom, the PWM out of the AR7000 was now at ~3V and appeared very stable. Even the MK started loving the PPM. (with the 2.2k at R3 the channels would bounce around and sometimes the MK would reports signal loss, that’s when I hooked up the scope and could see the channels randomly dropping out on the PPM side)

The only problem is that now I lost a channel. (only 7 pulses in the PPM signal) The channel seven signal out of the second vibrator was not generating a PPM pulse (from what I can tell). How do I get this back to work and stable. Should I increase the R2 to generate a longer pulse?

Please help.



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Using a 10K at R3 is reasonable.

(1) Since you have a scope, can you monitor all the pins on U1B and report what you see?

(2) With the circuit connected, use a Y-cable and add a standard servo to the Rx's CH-8 port. Does the servo work too?

(3) Grab the magnifying lens. What is the HC221's exact part number and brand.

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(3) I used CD74HC221

When powered with 3V, that is a compatible part. When using 5V, the CD74HCT221 seems to be the better pick in this application.

(2): servo works with Y in 7th channel

Ok, that is good info.

Seems like it is time to get out the scope and snoop around all the pins related to U1B.

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(part 1) Here are some pictures (the scale is one volt per division) for the PPM-out picture the 0V is at the bottom, for the rest it is about one division up from the bottom. I only have one good probe so the bottom trace is accurate. For the pictures with 2 traces, the top trace is from my bad probe for reference and is set on the PPM-Out (U1A-Q-pin13) so that you can at least see the signal timing in relation to the output. Except for the first picture (ppm-out) the time scale is 1ms/devision.







Edited by brashley
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