Receive only interface cable for Android
To improve the decoding quality with receive only apps like Morse Decoder, HF Weather Fax and DroidNavtex when transmitting is not required.
To detect an external audio source Android phones require a transistor or FET connected to the audio in pin (pin 4) and ground (pin3). A simple resistor seems to work on some devices as well but the value of the resistors seems to vary between devices. A transistor as seen in the schematic below seems to work always as long as the voltage on pin 4 is at least 1.4V.
Important: The interface uses the voltage of the microphone pin of the phone. The voltage on the microphone pin is usually used to supply an electred microphone in headsets with a voltage. The voltage varies from phone to phone. We measured voltages between 1.8V – 2.7V on various phones but it might be possible that certain phones have a lower voltages. If the voltage is lower than 1.4V this interface will not work but so far we are not aware of any phones or tablets which have a voltage lower than 1.8V.
This interface cable will not work on a Nexus 7 tablet (Version 2012). It works fine on the Nexus 7 (Version 2013).