Phone/Tablet to X1M Audio Interface Box

I built a little (2″ x 3″ x .7″) box that has an audio interface so I can hook the X1M to my Android devices running apps like DroidPSK and Pocket RxTx for portable field operation. The interface routes all the audio signals to the right place, adjusts their levels and also develops a PTT signal to key the X1M when the Android tablet starts sending PSK31. Here’s the schematic:

Audio Interface Schematic

Tablet computers and phones have a powered microphone input that supplies a couple of volts to the electret mic element through a resistor of a few thousand ohms. We ‘steal’ a little bit of the DC current available through that interface and filter out any audio in RC circuit R2 & C2. The resultant DC voltage is applied to the emitter of Q1. The audio signal from the tablet which carries the TX PSK31 audio signal is applied to the base of Q1. D1 prevents the signal on Q1’s base from going more than about 0.7V above Q1’s emitter. so most of the available waveform excursion drives Q1’s base lower than the emitter, turning Q1 on and ff at the audio rate. RC network R1 & C1 filter this signal into a DC voltage at M1’s gate, turning M1 on when Tx audio is present and off when it ceases. M1’s drain switches on to near-ground level when Tx audio is detected, activating the X1M’s PTT.

The tablet’s headphone output is coupled to the X1M’s mic input through level adjusting pots and the X1M’s headset output id coupled to the tablet’s mic input through a level adjusting pot.

 

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