I've been taking apart a recent, standard touch tone phone with the aim for automated dialing via a separate MCU with transistors activating the contacts (Smirk. An assistive technologies project designed for my grandmother, not 'robo-calling').
Removing the solder mask it is quite clear how to accomplish this in this case–
However, I had another question: Namely, does anyone know what process/method is being used here to trigger the switches ?
Rather than any physical device, the button contacts seem to be overlaid on the board themselves with a type of 'black substrate'. No visible connections are already apparent between the buttons.
At first I thought perhaps captive touch, but then they are activated by basically a rubber pad on the matching side, plus a plastic 'number button' on top.
My second thought here was that somehow the button press might be creating 'arcing'– but could the voltage really be so high in this case?
I wondered if anyone had any thoughts as to what is going on, or what process is being used?
Best Answer
This is not any sort of touch technology. These are old fashioned silicon rubber buttons. The pads on the PCB are carbon coated to prevent oxidation of a tinned finish. Immersion gold and silver can also be used. The black pads on the buttons are conductive (put a DMM across them, you'll measure a low resistance), and physically pushed down to contact the PCB pads. It functions like any normal normally open push button. No magic here.