I got to work on the prototype of the MEMS 217 Multiple Touch Controlled Voltage Source. This description is from Fluxmonkey:

A combination of four separate touch sensitive controllers, each with a variety of pressure and position activated outputs.

The Model 217 includes a 17 note electronic keyboard with both tunable and equal interval outputs. All pulse and pressure outputs are activated by any key: separate pulse outputs are triggered by individual keys.

A similar 4 key section has the above features plus individual pressure outputs for each key. Two analogue controllers provide output voltages that are proportional to lateral position.; they also feature pressure and pulse outputs.

The Model 217 incorporates design features that assure logical and responsive operation. Output voltages are "remembered" when a finger is lifted, the output remains at the last value; keys do not "block" the sustaining of one key does not inhibit the action of another: functions are activated by body capacitance, providing a level of responsiveness and reliability unavailable with mechanical or resistance sensitive keyboards.

This module requires 24V and outputs 15V signals.

MEMS designed a panel PCB with a 15 to 24V DC-DC converter so this is a three board module. The linear touch plates are on a smaller board in the middle. There are some early DTL parts that are rare so you can see five of the MEMS designed replacer modules using CMOS logic.

All the remaining touchpads and controls are on a large rear PCB.

There are a lot of wires and calibration trimmers in this dense module.

Operation

The linear control surfaces are interesting. A 80 KHz clock drives a shift register that generates 8 different square waves that connect to the 8 segments. Each square wave is operating at a 10 KHz cycle but shifted by 12.5 uS, the period of the 80 KHz master clock. These are the waveforms for the first 4 touch pads.

Summed together, they form an 80 KHz "sine" wave that increases in amplitude when the pads are touched. As you slide from minimum to maximum, the phase of the sine wave shifts. The phase between this sine wave and the master clock determines the linear position. At minimum, there is minimal shift but as you slide towards maximum, the time/distance increases. This superposition of four different images shows the sine phase changing as your finger slides along the touch pads.

The pressure output is determined by the amplitude of the sine wave. The pulse output is generated by a threshold voltage of the pressure output. The voltage output is generated by the phase between the sine and the reference. The pulse drives a track and hold so that the voltage is held when the pad is released.

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