Variable capacitance buttons!

J

Jeremy Leach

Senior Member
Just when I'm warming to this idea you bow out Alband ;) I think the pads might not be too difficult, especially if etching on PCB. But the whole thing has lots of potential problems and all a bit experimental. I might try some experimnets of my own if I get time.
 
A

alband

Senior Member
alband tepmoreraly bowing in:

A combination Iknow to work (from t'internet) is circular pad size of your finger this is surrounded by a ground plane with a gap 0.5mm thick (this is crittical) the pad mus have a via going into the second layer so that you can get power to it (don't know if you have the facility to do that - I don't - hence bowing) then, a piece of pure glass 10mm thick (no clever special window stuff), this is glued onto the touch area using a glue that doesn't conduct and isn't a dialectric (can hold charge). Finding a glue like this can be tricky as most glues hold a certain amount of charge.

This is all taken from boriz's "Working link". (Don't know what went wrong with mine but thanks anyway).

alband bowing out...

oh an good luck!
 
B

boriz

Senior Member
@Jeremy
"Must use as high frequencies as possile".
The COUNT command can only count upto 25kHz. Schmitt inverters can oscillate in the MHz range.
 
H

hax

New Member
As promised a picture of my glass face plate.

Capacitive sensor chips are QT240. They work very well if the design guidelines are met. The capacitors must have a X7R rating for stability.

They control a 3KV power supply which we use here for physics experiments.

The backlight LEDs also have to have a bypass capacitor because otherwise they would cause false on states every time they turned on/off which would self trigger the Qprox chip.

But the best thing about this technique is that the board can be re-used for other designs very easily. Only a new glass front panel needs to be designed, and the firmware changed for the new purpose.

The front panel was cut by a glazier and the sides were polished.

The graphics were printed onto the glass with a special toner transfer sublimation machine but similar home grown results would be had with a panel printed from photo paper and an inkjet printer.
 

Attachments

You must log in or register to reply here.
Top