I've always seen a button either built into the yoke or attached to the yoke with velcro on older aircraft. However, the other day my instructor used an older-looking headset that had a push to talk button built into the side of the headphones. He had no cable plugged in except the mic/headphone jacks and never touched the yoke's PTT button to transmit. My question is: how is the PTT function activated? Is this done with a load resistor or something that loads down the mic line, triggering radio transmission? I've done some research and looked at various manuals of audio panels and navcoms and haven't found an answer. The audio panel in use in this case is the Garmin GMA-430 audio panel, pictured here: 
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$\begingroup$ This page has a pretty good explanation of the basics. In essence, the PTT switch operates to circuits or relays. One to transmit and one to mute the receiver. $\endgroup$– SimonAug 8, 2015 at 12:34
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$\begingroup$ I'm not sure what you are asking, it seems you wonder how the commutation between transmission and reception is done when the PTT is not used. If this is the case, then the answer is a vox circuit is used to detect audio from the mike and perform the PTT-equivalent function when the sound from the mike exceeds some adjustable level. Here is a headset with a vox included. The sensitivity button is visible. $\endgroup$– minsAug 8, 2015 at 13:27
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2 Answers
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The standard microphone plug on GA planes is the PJ-068 plug, which has three electrical contacts (tip, ring, and sleeve). The tip of this connector is a push-to-talk signal (the ring carries the actual audio signal, the sleeve is ground).
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PTT on aircraft radios and audio panels is activated by taking the PTT contact (tip of the microphone plug) to ground or microphone low. All you're doing is literally closing a switch that grounds PTT and that switch can be located just about anywhere. Putting it on the headset makes it nearly impervious to damage from cable strain. It also makes it so he ALWAYS knows where the PTT button is, regardless of which aircraft he's in.
