Electronic – Silk-screened ‘buttons’ on a touch-tone phone


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?
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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.