Adapting a real KT76A transponder for AM

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ChuckK
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Joined: Sun Nov 06, 2016 2:33 pm

Adapting a real KT76A transponder for AM

#1 Post by ChuckK »

I purchased a KT76A transponder off eBay for $23 with the intent of adapting it to AM for my full hardware Piper Malibu cockpit. As an aside I am currently using a homemade transponder with a 16x2 character display...works but cheesy. Much prefer the real thing. FYI, I have also adapted an S-tec 55 autopilot head (including the LCD display) to AM, as well as a KMA-24 audio panel. I'll likely post on these in the near future. Back to the KT76A,

After initial disassembly it's easily doable using the actual switches, front panel and lighting. The switches actually work out to your advantage in that each digit is actually a 3-bit selector, meaning there are only 12 wires (3x4) plus a ground required for reading the code set by the dials. An additional 5-6 inputs would be required for the mode selector and the ident button. One output line is required for the reply light. I have the 3-bit digit decoder script working and will share the full instrument code when complete.

In summary, the KT76A can be easily interfaced to into AM with only 19 pins, pretty sweet.

Chuck


ChuckK
Posts: 95
Joined: Sun Nov 06, 2016 2:33 pm

Re: Adapting a real KT76A transponder for AM

#3 Post by ChuckK »

Thanks Tony, I have read that thread.
I was a bit concerned with non continuous rotation on the purchased switches. I've never seen one like that, so it would have to be modified.

Also, discussed in that thread, I was reading the KT76 manual, it does mandate only selecting the code in Standby, then going to On or Alt. It's old technology so I would assume its replying at all times in on or alt mode regardless of the movement of the code dials. All very easy to program in AM, including the 45 second warmup delay.


Chuck

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Sling
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Re: Adapting a real KT76A transponder for AM

#4 Post by Sling »

Great. I didn’t want you to miss out on that discussion. Like you say this should be one of the simpler units to get working. Don’t forget to show us your finished project.

Tony

jasong
Posts: 101
Joined: Sun Oct 20, 2019 8:18 am

Re: Adapting a real KT76A transponder for AM

#5 Post by jasong »

Hi Chuck good luck with your project hope mine helps with any code process. Also those switch’s are continuous they will rotate either direction from 0 to 0 and back around to 0 for as long as you want. :D
Glad you got one with the switches intact would like to see a photo of what they actually look like or if they have part numbers.

I also have the kma 24, I was going to start on that after finishing these.
viewtopic.php?f=14&t=2541

Keep me updated on your progress I’m happy to work together on it. I’ll add some photos tomorrow to compare units and how to maybe how to wire them up.

ChuckK
Posts: 95
Joined: Sun Nov 06, 2016 2:33 pm

Re: Adapting a real KT76A transponder for AM

#6 Post by ChuckK »

Hi Jason,

The switches are continuous rotation. There are 3 terminals and a ground. As the switch rotates, the terminals make contact to define the 8 positions. Attached is the truth table for the switches along with an up close of one of the switches.
image.png
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The KMA-24 was very easy to adapt. On both of these units I also converted the indicator light to LEDS, works really well.

Chuck

jasong
Posts: 101
Joined: Sun Oct 20, 2019 8:18 am

Re: Adapting a real KT76A transponder for AM

#7 Post by jasong »

Hi Chuck,

Below are some photos of the KMA 24 I have, I think I'm going to have to make a pcb for it. :|

Interested if yours is the same and if you have any ideas for it? As I said I probably won't start this till I finish with the displays for the radio stacks.
KMA24 1.JPG
KMA24 2.JPG

ChuckK
Posts: 95
Joined: Sun Nov 06, 2016 2:33 pm

Re: Adapting a real KT76A transponder for AM

#8 Post by ChuckK »

HI Jason,

My KMA24 is a different model but very similar. There is no need for a circuit board. The push buttons are accessed via the terminals on the top of the switches bodies, see pic. These switches are not normal switches in that they do not go to ground (~zero resistance) when pushed in. Instead they vary between 2 resistance values. Because of this you need to use the AnalogIn function and an analog pin on the Arduino. For the rotary switches, some probing with a DVM on the other side of the board will reveal the terminals, again solder right at the switch. For the back lighting, I used the existing incandescent lamps running on a separate dimming circuit. I put some notes on your picture. The switch terminals are from memory so they may be incorrect, but close.

Also, I actually cut the circuit board and housing about 3" back from the front so it didn't take up as much room behind my panel.
image.png

ChuckK
Posts: 95
Joined: Sun Nov 06, 2016 2:33 pm

Re: Adapting a real KT76A transponder for AM

#9 Post by ChuckK »

I have finished adapting the KT76A to my simpit. It works just perfect. I've attached some pics. Because of the ADS-B mandate, the KT76 transponders are getting to be very inexpensive on ebay and make a great option for adding real hardware to your rig. Here are some notes..

As mentioned before the code switches are actually 3-bit selectors. With some simple scripting logic they are easily adaptable, and only require 3 wires to the 4 code selectors. The mode switch is a bit more difficult on 2 fronts:

1) STBY, ON and ALT are straight forward, but OFF and TST are electrically the same. Given the modes are cycled sequentially the prior mode can be used to determine if OFF or TST is being selected, but....see item #2

2) When going from STBY to OFF and from ALT to TST unit electrically goes thru the ON position. This requires a look back of two prior modes to determine if TST or OFF is being selected. Not difficult but a bit more code.

As can be seen in one of the pics, I replaced the incandescent IDENT light with a white LED, looks perfect. One feature I added in Lua is a hidden refuel function. I have a fully to scale Piper Malibu Cockpit, and it's a squeeze to get in and out of just like the real thing. So when it's time to do fuel planning, getting out to go to the control PC to add fuel or use the Control App is a buzz kill. So I programmed a refuel function. It works like this...

When in OFF mode,press and hold the IDENT button for 2 secs. The IDENT LIGHT will flash rapidly. Release the IDENT button. Dial the code switches to select the amount of fuel to add to each tank, e.g to add 12 gallons to the left and 20 to the right dial up 1 2 2 0. Press the IDENT button to add the fuel. Works great.

X-Plane's transponder logic is not very correct. The reply light and ident should only work in ON or ALT mode, in XP they are always working. I added some code to make it work more correctly.

So in total is took 17 digital signals for the entire project, (12 (4x3) for the code, 3 for the mode switch, 1 for the IDENT switch and 1 for the IDENT light).

I depopulated the PCB behind the switches, and will eventually cut the circuit board down so its easier to fit in my panel. Once interesting thing is that wiring into the full PCB, AM cannot recognize the high impedance "open" between the switch points and ground due to some 10k pull up resistor packs near the rear of the PCB. Took a little while to troubleshoot that ;) Once removed all worked well.

A great project, that will be a nice upgrade to my cockpit. My final step will be to wire up the backlights to my panel dimmer bus.

If anyone is interested I will post the code as needed, and provide a wiring diagram of the switch connection points.

Chuck

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jasong
Posts: 101
Joined: Sun Oct 20, 2019 8:18 am

Re: Adapting a real KT76A transponder for AM

#10 Post by jasong »

Hi Chuck that’s looking great.

Also you mention putting back lights in, where abouts do they go? mine didn’t have any pcb, are they behind the squawk digits?

Also thanks for providing the info on the audio selector that’s going to help when I get to that part.

Are you using this PA-46 in xplane?
https://store.x-plane.org/PA46-Malibu-M ... p_292.html

I noticed the weather radar in the Malibu, I’m wondering if you know of any weather radar plugin that can pop out for X-Plane?
I have the weather radar tv instrument and would like to adapt it as it won’t take long to do.

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