Phone Radio Interface

The Phone Radio Interface is a device designed to provide an audio interface between a smart phone and a radio for use in digital modes. This is done through:

  • Capacitive isolation: preventing ground loops
  • Trimmer potentiometers to control audio levels to both the phone microphone input and radio input
  • PTT control by grounding PTT during TX, based on the presence of audio out from the phone
  • Powered by the “Plug-in Power Supply System”, or with a lower R10 value, compatable with iDevice microphone power

The cost of this device is currently A$12 or US$12 for a printed circuit board, or A$25 or US$25 for a board plus components. Purchasers need to provide cables at both the radio and phone sides.

Most components are surface mount technology, but are larger sizes: 1206 and 0805. With good eyesight and reasonably steady hands, soldering these components can be done without the aid of a magnifying glass. There are thru-hole technology transistors, diodes and potentiometers.

FT-817 to phone interface

Discussion about this and other devices by Wayne VK3WAM can be made in Yahoo Groups at:

My blog posts on the development of this device can be found here:
FT-817 (& FT-897) Phone Audio interface part 3
FT-817 Phone Audio interface part 2
Designing a Phone/Radio interface

Component List
Name Type Layout
10uF-1206 Capacitor C3
100pF-0805 Capacitor C1
10nF-0805 Capacitor C7
100nF-0805 Capacitor C2
2n4401 Transistor Q1
1k-1206 Resistor R4
4.7k-1206 Resistor R10
22k-1206 Resistor R3
33k-1206 Resistor R7
47k-1206 Resistor R2
100k-1206 Resistor R1
10kP Poteniometer R8

Note that R10 requires about 4.7K to cause a 600 uA current draw for iDevices. Most other devices should be compatible with the “Plug-in Power Supply System” for mono microphones, and for these devices, R10 should be around 12K. Samsung Galaxy S2 devices appear to be compatible with the “Plug-in Power Supply System”.

3 comments on “Phone Radio Interface

  1. begahouse2 says:

    Hi Wayne, I have just discovered your wordpress blog and have to write to say it has motivated me to get going with SOTA and general field work.
    Currently own a ft857 but would like an ft817 for lighter portable work.
    I have been reading your other blogs on radio/phone interface for digital stuff and battery options, having trouble understanding the more techo stuff but if your willing to spread the knowledge more power to you.
    Thanks for sharing
    kind regards Phill Noble vk2hpn

    ps. your travel blogs through the vk1 ranges was excellent made me wish I was there.

  2. senorfreebie says:

    Can you explain what these do, as if I were 5 years old? I’m very curious why you installed antennas on some of the Alpine mountains, and how that relates to using a smartphone in this way.

    • waynemerry says:

      Amateur radio is a hobby that can use voice, morse code (also called CW), but also digital methods of communication. Voice, CW and sigital methods are also known as “modes”. Digital modes need some device to generate the audio to be transmitted. The audio that is received is either difficult or impossible for a human to understand, so a device is needed to decode it. This phone radio interface is designed to connect a radio to a smartphone. The smartphone would have an app installed that can generate and decode the digital mode.

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