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.


3 Answers 3


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.

enter image description here

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

  • \$\begingroup\$ 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"? \$\endgroup\$
    – Tripola
    Jan 25, 2018 at 19:39
  • 2
    \$\begingroup\$ @Elia its not like a wire short.. its more a resistor. \$\endgroup\$
    – Trevor_G
    Jan 25, 2018 at 19:46
  • 8
    \$\begingroup\$ @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. \$\endgroup\$ Jan 25, 2018 at 20:09
  • \$\begingroup\$ @Elia ...and continuity mode tends to indicate at less than about 200 Ohms \$\endgroup\$
    – Chris H
    Jan 26, 2018 at 16:24
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    \$\begingroup\$ Also worth noting, when designing a PCB for a membrane switch array like this, it must be ENIG to prevent corrosion of the contacts. \$\endgroup\$
    – Matt Young
    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.


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