Introduction: Capacitive Switches Using MTCH108
Ever wondered what goes into making the capacitive touchscreens, buttons which dominate most of our day to day applications like smart phones, various electronic appliances?
This instructable deals with the designing of capacitive switch buttons. If you go Googling about capacitive switch designing, it will throw you many thousands of results. The same thing happened with me. So here is the instructable for the designing of a capacitive switch board, which was used in me and my team's summer project. The ic used here is Microchip's MTCH108
Step 1: COMPONENTS USED
The basic components used for this project were:-
1. Microchip MTCH108 capacitive sensing ic (Package:- SSOP20)
2. Pullup resistors - 4.7k ohms (3.3k ohm will also work)
3. Noise eliminating resistors - 4.7k ohms (Recommended in the typical circuit given in the datasheet)
4. Trimm Potentiometer:- For adjusting the sensitivity (I tried all pots from 1k ohms to 100k ohms, and the ic works just fine)
5. Copper clad board
Step 2: ABOUT MTCH108
Microchip are currently the best in the field of touch and capacitive switches. This video proves what I quoted earlier.
Before turning to MTCH108, I tried Atmel's AT42QT1070. However I guarantee you that MTCH108 is far more better.
Turning to the specs of the ic, it is a 8 channel ic. That is, a maximum of 8 capacitive switches can be implemented usng this ic.
The special feature of the ic is the sensitivity adjustment using a trimm potentiometer. At maximum sensitivity it can detect your finger even if it is 6 -7 cm up in the air. Materials like acrylic, with a thickness of thickness upto 6mm do not pose a problem in the detection.
The ic can operate in low power mode as well as normal power mode. A pin has been dedicated for the selection of the power mode. Applying either VCC or ground selects the power mode. (Here the VCC is 5V).
The output of the IC is open drain, i.e. you will have to pullup the output to 5V, using 4.7k ohms resistors.
Step 3: PCB Designing
Here comes the most important part. It is extremely important that the PCB should be designed following all the guidelines. Since all the capacitances involved are generally in the range of picofarads, care should be taken while placing the touchpads.
First lets take a look at the keypad board we designed. The whole board has been shown so that you get an idea of what the ideal distance between 2 pads should be. As you can see, we took into consideration the area of the pad, so that our finger can be ideally detected. Secondly comes the distance between two pads. Note that when you touch the pad, the electric field changes in the area around the pad as well. So make sure that no two pads are too close to each other.
The next image shows the close up of the tracks between MTCH108 and the touchpads. Note that the track lines are kept at some distance from the pads, so as to prevent the false signalling.
One of the boards shown is for the voting buttons. In that board, you can see that the ground plane does not pass anywhere near the MTCH108. This has been done after going through a lot many pdfs regarding pcb designing.
Another important instruction is that avoid using a voltage regulator on the board of the capacitive touchpad. The signal distortion/ripples occuring due to the voltage regulator are far too much for the small capacitances to handle. As a result the ic cannot detect the changes occuring in the capacitance.
Step 4: OPTIMIZATION
I have not used the ic to its full potential. I did not use the guard channel feature, which is used in almost all the high-end applications of this ic. The guard channel has the maximum sensitivity, and hence it can be used to negate conflicting types of touches, i.e 2 touches at the same time, or a touch between two pads.
If anyone of you guys tries that feature out, please let me know.
And if anyone has got any doubts regarding this designing, feel free to comment, and lt me know your ideas as well.
Till then, happy instructing and learning!