I'm planning on making some sort of synthesizer. Are there any buttons with a low activation force, and whose resistance varies based on how far down they're pressed?
Well, of course there are! But here in the 21st Century you might have to custom manufacture your own. There was a (now expired)
Kickstarter campaign to produce such buttons, but I can find no evidence that manufacturing actually occurred, beyond the delivery of a few prototypes to supporters of the campaign.
However, "planning on making some sort of synthesizer" leaves a lot a room for hobby-type development and experimentation with different concepts. You should probably initially concentrate on producing a suitable actuator, a button with a limited travel that self-restores to its initial position when released. Think of a rod in a cylinder with a light-weight restoring spring in the bottom.
Travel distance of the rod, and the force required to move it, will be determined by factors that you set. Once you have those details worked out, just add a sensor/transducer mechanism that translates rod position into an analog signal output. There are dozens (if not thousands) of ways to do this.
@VenomBallistics suggested using Hall sensors.
@BobK suggested using pressure sensors. I suggest that you investigate using a Texas Instruments invented
chip that measures displacement inductively, by sensing the position of a small stationary coil of wire relative to a moving conductive surface, which could be the rod part of your push-button.
The LDC1000 inductance-to-digital output is digital, but this digital signal can be easily converted to an analog signal with a digital-to-analog converter, available in low-end (inexpensive) PIC microprocessors from Micrologic. You would need one PIC per push-button, but they are quite small and could be integrated with the button and the LDC1000 chip. However, you might want to consider staying in the digital domain with your synthesizer... analog synths or sooo 20th Century. Also, consider using a COTS (Commercial Off-The-Shelf) MIDI (Musical Instrument Device Interface) keyboard to control the notes produced by your synth.
You will need a computer and appropriate software to translate the MIDI keyboard signals into synth commands, but you can also purchase a COTS synth that will interface directly to the MIDI keyboard and NOT require a computer, making a kind of electric piano or organ or whatever instrument the synth is capable of reproducing. I did this late in the previous century, but I am no musician. The setup worked, but I was incapable to producing anything that sounded like music, at least to wife's ears (and maybe my children's ears too, but its hard to tell with teenagers). Your mileage (or kilometers) may vary.
