D
Duncan Domingue
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.
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.
