Some time ago I came across the Wolphilink Interface. It's a small box which connects an smartphone to a Yaesu FT-817/857/897 transceiver. I was about to buy one, but the interface was quite expensive, especially if you include the optional cables and the shipping to my QTH in Germany. I googled a bit and found an interface built by Christian Petersen, DD7LP. I made the decision to take a closer look on his schematic diagram and built my own interface to use APRSdroid and PSKdroid on my FT-817.
Luckily Christian also provided a buying list besides the schematic diagram. As I don't have any knowledge and also no tools to etch a circuit board, I first planned to build the circuit on a simple stripboard. When I compared the required parts from the list with my stock, I saw a small hint „board by DH4YM". I simple googled and found out that Dirk, DH4YM is an Germany-based electronics engineer and he also supports the ham radio scene with great professional etched board in a superior quality at affordable prices. I ordered a few interface-boards and started building one circuit the same evening.
After 1,5 hours the whole interface was assembled. My transmitted packets went straight into the APRS network. Some minutes later I found out, that my smartphone (Samsung Galaxy S3) did not receive any packets. I checked everything twice and measured almost the whole night but could not find the problem. I used the YLs smartphone (Motorola Moto G 3rd Generation) the next day and it worked just perfect.
I emailed Christian, DD7LP and also gave him a call later. This guy seems to be a real electronics professional, he explained way faster than I could thought. The problem are slightly different impedances in the microphone inputs of every different smartphone model, so I had to vary the resistor R7 in the DD7LP circuit.
I downloaded a very intersting app (Sound Analyzer) from the Google Play Store. On the provided spectrum analyzer I could see how the graph was changing while I added different resisors with a jumper wire. I used the 33k and simple put a 10k parallel, so with 7,67k for R7 I received the first packets with my Samsung Galaxy S3. Other smartphones might need different values.
Other changes away from DD7LPs original schematics were made with the hints from Dirk's, DH4YMs website. For some smartphones there are slightly differnt values recommended: R1 10k, C5 & C6 4,7µF.
I am so happy that this now works, especially after my quick and dirty Baofeng APRS interface broke some time ago.
Big "thank you" to the founder of this project Christian Petersen, DD7LP who spent a lot of time for developing everything and also to Dirk Ruffing, DH4YM for etching the boards and the great support. This project is a very good example what this hobby makes so great.
The interface also works with the Yaesu FT-857 and also FT-897.
If you like to work portable on APRS or PSK-modes and don't want to build you own interface, you can simple purchase one online in the USA direct from Wolphilink. The manual and schematics are online and worth a look. They also make great software for digimodes.