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PTT wiring: a sanity check, please

N546RV

Well Known Member
OK, so I've done a ton of searching and read a bunch of posts here on this topic, and I'm 95% sure I have the right idea, but the other 5% of my brain simply won't shut up. The question regards how to wire the PTT switch; in this specific case, it'll be for a Dynon intercom.

From what I've read in many, many other threads, the PTT switch works by simply pulling an input on the intercom (and, by extension, the radio) to ground. Which of course means that the wiring need only consist of a single conductor starting at the PTT input, going to the PTT switch (in the stick grip in my case), and then to ground.

This setup would greatly simplify my wiring, as opposed to the schematic provided by Dynon, in which the PTT switch closes a circuit between the tip and sleeve on the pertinent mic jack (see attached photo). This diagram passes the sniff test, since closing the switch is connecting the PTT input to the mic ground.

I suppose my question is this: if the PTT input need only be grounded, why depict this comparatively complex setup in the manual? In particular, why use the tip on the mic jack for the PTT input? I presume the tip contact serves no other function in that jack?

Thank you all in advance for hopefully assuaging tonight's electrical anxiety.
 

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Because, back in the dark ages, most pilots listened over a speaker, no headphones, and transmitted over a hand-held mike. The PTT was on the mike, so you had to be able to make that connection thru the mike jack (as well as the audio). You can also buy portable (velcro) PTT switches which plug into the mike jack (these have another jack for the headset mike).
If you don’t need or use these things then, as you have correctly surmised, there is no need to wire the PTT wires to the jacks. I didn’t.
 
PTT Ground

Bob's explanation makes sense, now it is clear why there are 3 conductors (instead of only 2 needed) on the headset jack for the microphone!

One other thing to be aware of (I wasn't) is that the radio and/or intercom may have separate grounds for both audio and power. The PTT switch must be connected to the correct (likely the microphone audio) ground. In my case with an MGL V16 radio, there is a separate ground specifically for the PTT. However, the PTT was incorrectly connected to the main ground and this caused the PTT to fail intermittently.
 
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Because, back in the dark ages, most pilots listened over a speaker, no headphones, and transmitted over a hand-held mike. The PTT was on the mike, so you had to be able to make that connection thru the mike jack (as well as the audio). You can also buy portable (velcro) PTT switches which plug into the mike jack (these have another jack for the headset mike).
If you don’t need or use these things then, as you have correctly surmised, there is no need to wire the PTT wires to the jacks. I didn’t.

Of course it's that simple. I like it when the answer to a question is rather obvious in retrospect.

It's especially amusing that you mention the portable PTT, because the Citabria I've been flying recently is equipped with one of those. That was a new one for this guy, who's previously only ever pushed buttons on the stick or yoke. :)
 
This topic is of interest to me because I'm in the process of changing out my POS Icom A210 com radio (which has failed twice) for a Garmin GTR-200B. Not knowing any better, when I built the plane I installed 3-wire jacks and 3-conductor shielded wire from the jacks to the radio. I'll NEVER use a handheld mic, so there was no reason to do it that way. I should have used 2-wire mic jacks (Edit #2: This is wrong! See below.) and 2-conductor shielded cable plus a separate wire from the PTT switch on the stick to the radio. The Garmin connector has Mic LO, Mic IN, and PTT inputs for both the pilot and co-pilot. Garmin shows the shields of all the audio cables grounded to the case of the radio, and the jacks insulated from the panel with shoulder washers/flat washers. Live and learn.

Edit: My audio system is mono only, but before installing your system, you need to decide whether you want stereo or "3D audio".

Edit #2: A friend pointed out that this is wrong. Since the mic output is between the ring and sleeve, you have to use a three-wire mic jack. You can still use 2-conductor cable if you want, connected to the ring and sleeve. My apologies.
 
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Dynon's drawing shows the tip being wired to to the sleeve because the sleeve is the ground and it is common to the radio.
 
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