What is the purpose of a three-terminal momentary contact in a matrix keyboard?

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The motivating example is that I have a damaged membrane keyboard where some of the keys do not work.

At first I thought it should be easy to rig up a matrix of keys to replace the damaged board, but as I was trying to determine the wiring, I realized that there are three terminals in each button pad. If you look carefully at the scan below, you can see the two distinct layers and, on the top layer, either interleaving traces on the top layer, or a fully connected grid on the top layer. Where the top layer isn't interleaved, the bottom layer is interleaved. All the circuit examples of matrix keyboards I've seen have a simple row/column grid, and does not have the extra contact in the button.

With the three terminals per pad, hooking up a standard 4x4 matrix doesn't seem reasonable -- it would require pushing adjacent pairs of buttons simultaneously. I eventually rigged double-pole momentary buttons to bypass the keyboard but...

My question is why would they design three-terminal momentary contact keys like this and what are they used for?

Actual Membrane Keyboard with 3 Terminals/Key:

This is the left keypad from a ProForm Sport 1200 Treadmill, Model PFTL59506. The membrane has some identifying numbers: "LUXE ETPF51105,LEFT#222227.

I think that traces 1, 2, and maybe 7 are damaged as in Fixing/Replacing Conductor Ribbon Cable .

From close visual inspection of the membrane and some continuity checking, I found this:

From the bottom-right and proceeding clockwise, ending on the small center pad, the terminals shorted together are:

3-6-8 - No Label/hidden -- ??? / Rings
3-7-8 - Trainer Program -- dead
2-1-4 - Incline Up    -- dead
2-1-5 - Incline Down  -- dead
2-1-6 - Fan           -- dead
2-1-7 - Calorie Program --dead
3-5-8 - Weight Down   -- Works / Rings
3-4-8 - Weight Up     -- Works / Rings

Proform Sport 1200 Keypad Left

  • \$\begingroup\$ Improve reliability of connection possibly... \$\endgroup\$
    – Solar Mike
    Apr 10, 2018 at 6:33
  • \$\begingroup\$ Power-on if any key is pressed. \$\endgroup\$
    – Oldfart
    Apr 10, 2018 at 8:00
  • \$\begingroup\$ @oldfart -- Power-on is handled separately. There is an enabling rocker switch on the base of the treadmill, and the console has some magnetic reed switch enabled by a magnet clipped to the user. If the master switch is on and the magnet is in place, the system is on. \$\endgroup\$
    – Dave X
    Apr 10, 2018 at 19:15
  • \$\begingroup\$ @SolarMike -- Reliability for safety makes some sense -- you might not want a frizty keyboard accidently ramping up the speed or incline unexpectedly. \$\endgroup\$
    – Dave X
    Apr 10, 2018 at 19:18


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