this device is a passive dual unit standalone force sensing resistor box with (3.5mm, 1/4" or banana) jack connectors. it requires no power of any kind. it makes no sound or signal whatsoever unto itself. it has been tested to pass voltages between -5 to +10 volts but will likely pass wider ranging signals with no problem. the pads (which are the actual resistors themselves), when untouched, inherently stop almost all signals (that we will deal with). there is a very small amount of signal bleed which can be heard/witnessed if the incoming signal is hot and the output monitored at a high volume level. this bleed is natural. these devices are not meant for deeply precise processing. they are meant for fun interactive play-time. passive etc.
the way the circuit works is that the resistance of the pad is decreased as pressure is applied. the more pressure applied, the more the input signal is passed. so if we were to compare this to an attenuator pot: when the pot is closed (CCW) that is the same state as the FSR with no force applied. when the pot is fully open (CW) that would be when the FSR has maximum pressure applied. the sensing of force is not dependent upon surface area, just overall force. further, the force-related resistance is not dependent on the X or Y axis location on the FSR itself. the FSR response is immediate and continuous while force is applied. the amount of resistance is proportional to the total pressure so it does not matter if you use one finger, a palm, etc. in addition, the resistance is also the sum of all forces so if you place a second finger on the FSR it simply adds any pressure to total pressure sensed.
the touch/feel/response by itself is quick. this is ideal for many applications. this can be changed by placing different materials on top. my favorite way to give a nice slow subtle feel is by placing an eggshell foam material on top of the box or even better by by adding slew as well. it is also fun to try different materials (rubber/foam) in order to best suit your own personal touch preference. being a touch interface makes it pretty personal so what works for some may not be ideal for others. experiment! i would also note that while i have not found a pressure amount which could damage the device there are likely limits to how much the pads can take. the device itself responds to light touch so there is no need to squeeze excessively.
a fun part of this design is that you can run anything through the pads. audio or control voltage. there is no designated in or out hence no labeling. each pad has two jacks. an input signal can be put into either of the jacks. whichever of the jacks sits unused is subsequently the output. i initially made the controller as a tool to simply add variable vibrato in the style of the ARP PPC pads. that is just the tip of the iceberg as to patching possibilities. i have designed the device to be comfortable both sitting on a surface as well as in your hand. when used as a dynamic hand-percussion tool, i like to hold it and when it is doing the controller/modulation dance it feels best (to me) to sit it on top of my modular synthesizer CV controller keyboard which in my case is an Roland SH101. again, experimenting is key. the box itself is constructed of a very sturdy aluminum. it has a heft that i find perfectly enjoyable to hold (like an old console game controller).
in terms of modular patching, the bare necessities in order to get the widest use with the box in the modular synthesizer context is a VCA, slew limiter and fixed voltage source. as this box is passive, all scaling of voltages must happen either before or after the box. but again, being passive means that there are MANY ways to patch. for instance, patching a touch sensitive vibrato could easily be done a number of ways. i will list two and discuss potential differences:
- patch a continuous modulation source (LFO) directly into one side of an FSR (input jack). patch the other jack (in this case the output) to a VCO exponential fm input. now as you press the FSR, the modulation will be routed to the VCO therefore modulating it variably.
- patch a continuous modulation source (LFO) directly into the signal input of a VCA. patch the signal output of said VCA to the VCO exponential fm input. now patch a fixed voltage source into one side of the FSR. take that adjacent FSR output to the VCA control input. turn the VCA offset (if available) down (CCW). now as you press the pad the VCA will open and allow the modulation. VCA should be DC coupled.
you may see different results depending on voltage sources, module settings and attenuation at various stages. in the first example there is more likelihood of getting some signal bleed as well as a generally more drastic result. the second example allows for more dynamic control which i find to be more musically useful.
in either case, adding slew can REALLY smooth things out. experiment. i approach this thing as a musical instrument/interface so like any other instrument, practice time spent learning the behaviors will greatly increase the likelihood of feeling pleasure when you touch the box.
my current favorite use is to pass stereo audio through one channel per side and use the pads as percussion-ish triggers to allow the audio through only when pressure is applied. the response of the pads feels ideal to me for this. as i said before, there can be some bleed. if precise closing off of signal is desired or necessary, it would have to be patched in (noise gate type patch) which is another reason why this is perfect for the modular context. anything is possible. patching a number of these boxes next to each other in audio pass applications could yield some really fun results. i am currently scheming/looking for a nice housing for a bigger/longer box to get more pads on it.
special thanks go to JLR, decaying.sine and James Husted (ersatzplanet) for design inspiration and direct help + kindness with this project (and others)!