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Pull-up circuit for altitude encoder tester needed.

I am building a altitude encoder tester. Altitude encoders are installed in aircraft and work in conjunction with the aircraft's ATC transponder to report an aircraft's pressure altitude via ATC's radar. (ATC = Air Traffic control)The basis of my tester is this:

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I came across one for free! My problem is this. The encoder is normally connected to the transponder in the aircraft, which provides a hi voltage to the encoder's relevant pins. At specific altitudes, the encoder provides a ground to selected pins at the transponder to trigger a corresponding altitude code in the transponder's reply signal. (known as greyscale or Gillham code)

My tester must operate without the transponder providing the +12 volts to the selected pins. the display on the AMS-2000 jumps around because of float. (nothing hooked up to the pins). Can a simply connect a +12V supply trhough a resistor to each pin for this? will the encoder still pull it to ground? If so, what value of resistor? Thanks for any help with this.

Earl

Reply to
Earls61
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and then to the bibliography for the altitude encoder tester article I wrote for Kitplanes about five or six years ago.

Jim

Reply to
RST Engineering (jw)

I have a difficult time believing that Shadin didn't put pullups in their product, but perhaps they did not.

Jim

Reply to
RST Engineering (jw) 

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Probably.
Reply to
John Fields

Perhaps. I didn't call them or anything. I lost confidence in Shadin because they coudn't repair this unit after I removed it from the airplane and sent it to them. I got it back and fixed it myself. (bad solder joint at the display connector)

When I power up the unit, with open pins, the display reads random numbers. Grounding them quiets things down. (and changes the altitude) I haven't checked it while connected to a xpdr. I think I will try the

10k resister idea next.

Thanks for the link to your website. Very interesting reading. I thought I'd make one using the shadin unit since it was free and it also reads out serial altitude from an encoder so equipped.

Earl

Reply to
Earls61

Transponder mfrs use various, including the voltage "to which pulled up." 10Kohm pullup should be just fine.

Fred F.

Reply to
TaxSrv

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