Hello group. I'm currently working on a little device that will have an interface similar to a piano keyboard, but each "key" will be activated by a human finger. I was wondering if anyone had any good tips on touch activated switches.
Here are some of the things I've already thought about/tried/designed:
- The device will require *lots* of these touch switches, and be able to handle simultaneous keypresses. I'm using a PIC microcontroller so I need to multiplex the devices. I can go up to 2048 (8 * 256) individual switches using a simple multiplexing scheme where I read the outputs of each bank of switches.
- I haven't found many of what I consider to be good circuits for the touch switches online. I have found a couple that use simple AND and NOR, but those still take up quite a bit of space on a PCB (DIL parts, I can't do SMD.)
- I have looked at simple transistor types but they're a bit too finicky with choice of resistors to accomodate skin resistance. FETs are nice because they switch on voltage and not current, so the skin's resistance is no big deal. SCRs look really nice even though they use current to switch since they act as solid state switches. I think I may have some solid state switches in a DIL package from Texas Instruments, but I haven't yet checked.
- I plan to make the actual interface to the switches on PCBs by having two long, wide traces run next to each other, kind of like whale teeth/barcodes. Should I worry about capacitance between the two stips of copper? If so, how should I avoid them?
So, if y'all have any ideas on what type of circuit to use (transistors, FETs, SCRs, TTL gates, etc.) and how best to multiplex them, I'd be glad to hear them.**Note, I've already thought of using capacitance type touch switches, but those will come later. Right now I'm working on a proof of concept for myself.