A weekend project which was completed back in 2011. I wanted to convert a green FeTAp 791 rotary phone which was quite common in Germany in my youth into a mobile phone. The intention was a phone to make and receive calls and not much else. Forthemore it should cost next to nothing. The mobile phone is a Simvalley RX-180 which costs 10 Euros. I ripped the case open and connected opto-couplers to the keypad pads on the mobile's PCB. The mobile's keys are organized as a matrix.
The microphone and loudspeaker were de-soldered from the PCB and moved into the handset of the old phone, using the original cabels. The removal of the case of the mobile phone left the phone's battery and SIM-card without support. So I picked up a new SIM card holder (the original was basically an array of spring loaded pins which pressed the card against the case). The battery was gaffa-taped into the old phone. I use the charging circuit of the mobile phone to charge the battery.