How does the telephone's keypad work?

I disassembled some old telephone and mobile phone. Every keypad is different, and yet, all of them have those "spirals" (see the image below) under the button or something similar.

What I don't understand is how those "spirals" can put in contact two ends of a wire. Are they some kind of sensor?

The two ends of a spiral are not connected. My guess is that when the button is pressed, they are short-circuited; however, I don't see how it should happen.

There is a conductive pad on the bottom side of the button that "shorts" out the spiral. Usually dark grey since it is carbon based.

The spiral just give you a higher probability that the pad will touch both sides at the same time.

• I thought that solution, but I used my tester in "continuity test mode" to test the bottom of the button and it didn't show any sign of electrical conductivity. In fact, I thought that that dark/gray matter was an insulator... So why my tester didn't "ring"? Commented Jan 25, 2018 at 19:39
• @Elia its not like a wire short.. its more a resistor. Commented Jan 25, 2018 at 19:46
• @Elia Even if the resistance is 100 kΩ, that could allow enough current to flow for the IC to detect that a connection had been made. And the low current would help with the battery life. Commented Jan 25, 2018 at 20:09
• @Elia ...and continuity mode tends to indicate at less than about 200 Ohms Commented Jan 26, 2018 at 16:24
• Also worth noting, when designing a PCB for a membrane switch array like this, it must be ENIG to prevent corrosion of the contacts. Commented Jan 26, 2018 at 17:18

The spirals are electrical contacts. Shorting the traces in a spiral causes the keypress to be detected.

The backs of the keys have a layer of conductive rubber or plastic. Pressing that layer against the spirals closes the circuit just like you had use a piece of wire to make the connection.

The black rubber on the back of the keys is actually somewhat conductive and provdes a current path between these swirls once a key is pressed.