Nice looking project rmteo. Do you have a PBP example for it?
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Nice looking project rmteo. Do you have a PBP example for it?
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The project was done in SF BASIC for a client so I cannot go into too much detail. As for the capacitive touch part, the keyboard is implemented as PCB copper pads on the top side (see picture). Each of the 24 pads is routed through a pair of 16-to-1 multiplexers (74HCT4067) to act as the timing capacitor to a TS555 (CMOS version of 555) timer running at about 2MHz in astable mode.
A timer in the PIC is set up as a counter. It counts the number of pulses in a fixed time frame (I use 2 ms). When a pad is touched, its capacitance increases thereby lowering the frequency (lesser pulse count) of the TS555. You will typically see 10-15% change when a key is touched. Because the capacitance can (and will) drift due to temperature (and the fact that each key will have a slightly different capacitance due to trace length, etc.), you will need to employ some algorithms to ensure accurate readings. There is an app note on the MC website that describes this.
The nice thing about this method is that you can use any PIC - it does not need to have a capacitive sense module to work and uses fewer I/O pins. I implemented a similar scheme using the ARM Cortex M0/M3 but the same principle can be applied to work with just about any microcontroller.
Last edited by rmteo; - 5th May 2010 at 01:35.
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Good looking unit you made there. Nice work. I guess i'm going to have to start playing a bit with cap sensing on the 16F1827 here shortly...
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rmteo,
Nice project. I have a project using CSM (capacitive sensing module) on a 16F727. In that project I'm using only two buttons. Yes, you can do it with any PIC but you will need some extra external components.
How did you do the external plastic cover for the buttons? Was it custom made by some company?
Robert
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I used a clear acetate (0.012in.thick) overlay for the buttons. This way, the end user can change legends by printing their own labels on paper using an ink-jet printer. This is one of the nice features of a capacitive switch - you don't have to spend a lot of money for custom labels (and the attendant lead times). And you can change labels easily and cheaply at any time. There is a downside though - there is no feedback (like tactile with a mechanical switch). I implemented an audible click using a buzzer like this one, which is also used for alarms, etc.
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Nice! Now, that is one more item for my "To Buy List".Originally Posted by rmteo
Robert
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I got the material from a local office supply place - OfficeMax. It is the clear stuff used as covers for book bindings.
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