Touch sensitive keypad

posted Sep 18th 2009 9:00am by Mike Szczys
filed under: classic hacks, peripherals hacks

capacitive_keypad

[Viacheslav] built a keypad that uses human capacitance to detect key presses. Unlike normal keys which close a physical connection, his project detects touch through the PCB substrate. He uses the analog comparator of an AVR ATmega8 to detect the moment of zero crossing and then measures the time it takes to discharge in order to detect key presses.

I use analog comparator to detect the moment of zero crossing and the charge is evaluated by measuring time. Builtin AVR ADC probably wouldn’t be very useful here because the charge that can be built up is very small.

Update: [Viacheslav] left a comment to let us know we had it wrong. Instead of using the ADC, the analog comparitor is used. This change has been made above.

Comments [12]

tagged: atmega8, AVR, human capacitance, qmatrix, touch sensitive

12 Responses to Touch sensitive keypad

Insipid Melon says:

September 18, 2009 at 9:09 am

But I *like* my tactile feedback. :(

Reply Report comment

Alex says:

September 18, 2009 at 9:10 am

Neat. Would be cool for a door lock.

Reply Report comment

Belenos says:

September 18, 2009 at 9:28 am

I’d be more interested in this for a netbook. I could shave almost a quarter inch of thickness off of mine.

Reply Report comment

sly says:

September 18, 2009 at 9:59 am

@Insipid Melon

yeah… I prefer to feel up my keyboard too.

Reply Report comment

ank0ku says:

September 18, 2009 at 12:33 pm

This would be cool for a casemod maybe?

Reply Report comment

Brandonman says:

September 18, 2009 at 1:07 pm

That would look *awesome* on some door entry keypad, which I would love to get to building some time… Hm… I have an atmega8 or two laying around, and a programmer… Just need to find me a PC with a parallel port, or a converter to parallel for programming. Harharhar!

Reply Report comment

svofski says:

September 18, 2009 at 1:40 pm

Correction: I use analog comparator to detect the moment of zero crossing and the charge is evaluated by measuring time. Builtin AVR ADC probably wouldn’t be very useful here because the charge that can be built up is very small.

Reply Report comment

sneakypoo says:

September 18, 2009 at 1:48 pm

I’ll probably have to read that about 20 times before I start to understand it but it looks like it would be useful for a future project where space is a consideration.

Tactile buttons are still king though ;)

Reply Report comment

Pinochet's 21st century digital négligé says:

September 18, 2009 at 2:18 pm

@Alex

That would be very interesting as it would add a whole new layer to the process of opening the door. Someone may find the combination but they can’t open it without knowing how you press the buttons (how much pressure is applied on each one).

Of course in reality combination locks are only used in places that lots of people need to come and go from :)

Reply Report comment

aztraph says:

September 18, 2009 at 2:31 pm

make a whole keyboard like that

Reply Report comment

trax says:

September 18, 2009 at 3:17 pm

Very, very good. I love it!

Reply Report comment

Agent420 says:

September 19, 2009 at 11:27 am

That is very clever, I’m impressed by the low level sensing design – good job! I also love the edge-lit cover template!

I believe this is how the mouse touchpads work. I frequently use surplus touchpads for this type of application because they easily interface with controllers. My example is here:

http://hackaday.com/2009/07/20/touchpad-and-vfd-hacking/

Reply Report comment