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7 February 2011
Author: Giorgos Lazaridis
PIC Capacitance Sensor with 4 buttons and Multitouch Function





The circuit on a breadboard for test

Some time ago i wrote a theory explaining how the touch sensors work, covering the resistance, the AC Hum and the capacitance touch sensors. Microchip has developed the mTouch(tm) sensing solutions. Part of the mTouch(tm) sensing solutions, is a series of PIC microcontrollers with embedded capacitance module. A list of these PICs can be found in this link.

In this article, i will use the PIC 16F1937, a powerful microcontroller with a 16-channel capacitance module. I will only use the 4 of them, but the idea to expand it to all 16 channels is the same.






The circuit

Using the capacitance module of the PIC, the circuit is very simple: (click to enlarge)





I will not add a BOM this time. The PIC is the 16F1937. All the job is done by the PIC software. The electrodes are connected to the E1 through E4 inputs. The 4 outputs drive the load. In my case, the load is an LED with a 220 Ohm resistor, but you can interface a transistor to further increase the load.




The software

There are 3 ways to use the capacitance touch module of the PIC. The method used is the frequency change, as described in the theory of capacitance sensors. The 3 different ways mentioned before, have to do with the time base source selection. One way is to use the Timer0 module, the second way is using the Timer2 module, and the third way is using the watchdog timer. All these ways are explained in the Microchip Application Note AN1171, written from Enrique Aleman. The problem with this application note is that it assumes that the reader has plenty of experience using the timer modules and the gate module for timer1. I read it twice and 3 times, and i read the 16F1937 datasheet a couple of times before i can tell that i fully understand the settings. So, i will try to re-explain the method that i use.


The Tmr0

For my circuit, i will use the Timer0 module as a time base. Here is how it works: The Timer0 module receives counts from the internal oscillator, through the prescaller. The Tmr0 overflow interrupt is enabled to watch every time that the Tmr0 overflows. This way, we have a fixed time base, because the internal oscillator and the prescaller remain unchanged. The Timer1 module receives counts only as long as the gate module has not detect a tmr0 overflow, and these counts come from the capacitance sensor module. So, when the tmr0 overflows, 2 things happen: First, the tmr1 stops counting, and second the ISR routine is called for further check. This is the rough explanation of this method.

For me, it is more important that you understand how this works, rather than reproducing the circuit. Human creativity has infinite starting points and no limits. So, let's take a closer look. First, the tmr0 module. Here is the portion of the code that i set it up:

	movlw b'11000101'	; I use the capacitance module with TMR0 as timer.
	OPTION			; So i have to set-it up through the Option Register
				; The prescaler is also connected to TMR0.
				; Bit 5: TMR0 Source is internal instruction clock (Fosc/4)
				; Bit 3: Prescaller assigned to TMR0 module
				; Bits <2:0>: Prescaller rate is 1:64

The option register is used to set up Timer0 module. Bits 7 and 6 are irrelevant to the capacitance module. In bit 5, i set Tmr0 to receive counts from the internal oscillator, which is Fosc/4. In my case, the internal oscillator oscillates at 16Mhz, so the pulses driven to the prescaller have a frequency of 4 MHz. Then, the prescaller is assigned to Tmr0 module to further divide these counts by 64. The pulses that finally are delivered to the Tmr0 module, have a frequency of 62.5 KHz. The following drawing is from the PIC16F1937 datasheet, and shows the diagram of Tmr0. With red, i have marked the route of the pulses to the Tmr0 register:





Timer0 is used as a timebase to measure the frequency. The Tmr1 (explained bellow) is used to measure the frequency (go to page 2).















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  • At 12 May 2014, 12:05:28 user Tomas wrote:   [reply @ Tomas]
    • Hello Giorgos,

      Thanks for your extensive explanations and example videos. Very well done!

      When reading Microchips app.notes, they say that in general you make sure GND is not close to your sensor plate, but you choose to put a ground plane on the bottom side of the board. Which makes a relatively big capacitor of every button. Relative change will be less as compared to, say, only a grounded ring around your button.

      What made you decide to do so? Was sensitivity to high and did you need to make it less sensitive, or why did you do this?

      Thanks,

      Keep up the good work!

      Tomas


  • At 26 February 2013, 21:14:06 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @kibo ???????


  • At 25 February 2013, 1:55:44 user kibo wrote:   [reply @ kibo]
    • hello Giorgos
      ( btfsc zero)Did you think the Zero Status register bit or ...?

      movf CPSCON1,w
      btfsc zero
      goto Cap_Check_Ch_1
      Thank you in advance for your response!


  • At 21 December 2012, 17:36:24 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @sri harsha only assembly listing possible


  • At 21 December 2012, 10:46:05 user sri harsha wrote:   [reply @ sri harsha]
    • [reply @ Giorgos Lazaridis],,,,,
      i know the code is in assembly language but i need the code in C & i am using pic16f1939 i dont know ho to fix the timer in C so please can you send me...
      thankyou
      harshu...:)


  • At 20 December 2012, 5:46:59 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @sri harsha it is in page 2


  • At 19 December 2012, 13:05:36 user sri harsha wrote:   [reply @ sri harsha]
    • guys,
      can you pls give me the coding process......i am in desperate need so pls......mail me to (sriharshu@gmail.com)


  • At 18 December 2012, 12:46:25 user sri harsha wrote:   [reply @ sri harsha]
    • please give me the embedded C code for this program.I am facing lot of problem reading assembly level code and i am using PIC16F1939 so please tell me how to fix the timer and also how to use two timers in the program


      thankyou
      harshu.....:)


  • At 10 December 2012, 19:18:13 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Hamed.mazlum mplab


  • At 10 December 2012, 12:54:55 user Hamed.mazlum wrote:   [reply @ Hamed.mazlum]
    • Dear Sir
      what's name of your compiler that you use?


  • At 6 September 2012, 20:36:19 user Chris wrote:   [reply @ Chris]
    • Hi,
      I have never written in ASM before. Could anyone make this work for 8 inputs and 8 outputs please, hardware im ok with its just the ASM part.


  • At 16 June 2012, 5:57:18 user karim wrote:   [reply @ karim]
    • Thank you very much for the video tutorials, They were a tremendous help for understanding the mtouch system. Keep up the good work!


  • At 11 May 2012, 13:38:25 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Muhammad Awais Tahir The reason for this is probably the threshold setup. Different touch pads have different capacitance, and even if you move them to another position they change their capacitance (due to the cable). First of all, when you change anything to the touchpad (position, wire, etc) you need to restart the program so that it reads the starting capacitance. And second, you may need to change the threshold values of the touch pads (PressThreshold_H,PressThreshold_L and ReleaseThreshold_L)


  • At 10 May 2012, 10:08:12 user Muhammad Awais Tahir wrote:   [reply @ Muhammad Awais Tahir]
    • I'm experiencing a strange behavior. I just kept the CapCheck4 (associated with CPS0) , and removed others just to understand how it works. I've connected an LED to the relevant output port to test the touch. Now, when I connected my touch pad with CPS0 or CPS1 the LED just remains ON whether I touch the pad or not. But, it behaves exactly how it should when I connect the touch pad with CPS2. I'm quite confused on why its working correctly with CPS2 rather than CPS0, and why the port turns ON when I plug the touch pad into CPS0 or CPS1.


  • At 4 May 2012, 15:21:39 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Muhammad Awais Tahir correct, the insulator is not connected


  • At 4 May 2012, 14:58:54 user Muhammad Awais Tahir wrote:   [reply @ Muhammad Awais Tahir]
    • According to the schematics there's no need to connect the insulator part of the touch pad to the ground. Is that right?


  • At 25 April 2012, 20:06:34 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @mina no mate, i'm sorry i haven't


  • At 25 April 2012, 10:01:05 user mina wrote:   [reply @ mina]
    • @Giorgos Lazaridis
      thanks a lot for your answer but i have a question have you ever used the EUSART of 16f1937 because i need to send data to the PC with rs232 .


  • At 18 April 2012, 12:16:42 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Muhammad Awais Tahir first of all, it will be extremely sensitive, so you may need to alter the software variables for sensitivity. Additionally, it is good to cover with some sort of coating the button, since PICs are sensitive to ESD, so a direct touch to an input from a heavily charged body could destroy the port, or reduce its lifespan radically.


  • At 18 April 2012, 12:06:09 user Muhammad Awais Tahir wrote:   [reply @ Muhammad Awais Tahir]
    • Thanks for your reply to my previous query. If I make the touch-sensor using PCB, and I do NOT cover it with plexi glass or any other material what difference would it make?


  • At 16 April 2012, 16:18:35 user ion wrote:   [reply @ ion]
    • @Giorgos Lazaridis
      I know that. Was mine too
      Happy easter and keep looking for easter eggs .... maybe you find a rectangular shape one .... but not BOILED :)
      If you did behave, for sure easter bunny will visit you :) ....


  • At 16 April 2012, 16:05:30 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @ion we had easter this weekend, so it might take some more days to arrive.


  • At 16 April 2012, 14:56:46 user ion wrote:   [reply @ ion]
    • @Giorgos Lazaridis
      Thank you
      The package it is with the post office in Grece acording with my tracking
      You will get it soon
      You can track it also, because you have the number and link.
      Regards
      Ion


  • At 16 April 2012, 14:42:43 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @ion I knew it :D
      Changing the size to bigger will increase the capacitance radically, due the increment if copper area. That is why i state (somewhere) that it is rather difficult to make proper touch-pads. Anyway, it will not be that simple to change the capacitance channels. It will need a lot of changes in the ISR routine. I really do not remember by heart the code since it is rather old.


  • At 16 April 2012, 14:29:41 user ion wrote:   [reply @ ion]
    • @Giorgos Lazaridis
      Thank you Giorgio.
      In fact i changed the the size of the touch pads.
      I put back the old ones and works OK
      As a questions, if i just change the start point from cpscon1 from 00000000 to 00001100 will be suficient to let me use chanel 12 to 15 ?
      Thanks
      Ion


  • At 16 April 2012, 8:35:39 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @ion i think that the problem is that you radically changed the shape or size of the touch pad, which changed the quiescence capacitance and this makes the output oscillate. Before you do anything else, try to change the parameters:
      PressThreshold_H equ b'00000000'
      PressThreshold_L equ b'00010110'
      into something bigger, for example the PressThreshold_L make it b'00110110'


  • At 14 April 2012, 0:12:38 user ion wrote:   [reply @ ion]
    • I had this project working fine for a while
      Then, maybe because due to my messy table, something happen.
      I do not know what
      Except CPS3-RB3 which works fine, all other channels flicker fast between 0 and 1.
      I do not have a scope because i am in China now, but i can see on my LCD.
      What do i have to do to move all the channels to CPS12 to 15
      I am not good at all in assembler , so a good working example will be the best.
      Thank you
      Ion


  • At 11 April 2012, 19:10:46 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Yash yes it is possible but i do not do circuits on demand, so unfortunately i cannot make this one


  • At 11 April 2012, 17:55:56 user Yash wrote:   [reply @ Yash]
    • Hi

      Great project!
      Is it possible to have a total of 16(max) 'buttons'?
      If so is there any change in the code? and which are the 16 connection points?

      Thanks!


  • At 10 April 2012, 15:14:02 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Muhammad Awais I'm almost sure that this program wont work on the pic you mention. I do not have a clue though regarding the changes that have to be done, since i've never worked with the pic you mention.
      As for the SR latches, do you mean the Tmr0 trigger latch? I use it for this program, and if you do not use it then this will not work at all, since the tmr stops from the latch.


  • At 10 April 2012, 13:44:05 user Muhammad Awais wrote:   [reply @ Muhammad Awais]
    • Have you used the SR Latches in the programming ? If I plan to use a PIC that doesn't have SR latches would that make a difference?


  • At 10 April 2012, 12:23:00 user Muhammad Awais wrote:   [reply @ Muhammad Awais]
    • Thanks for posting this. I'm planning to use PIC16F1519 as it comes with 28 cap touch channels, need I make any significant changes to the programming?


  • At 6 April 2012, 19:14:22 user Rogério Francisco wrote:   [reply @ Rogério Francisco]
    • Hi! could you send me the software for these example using four buttons. Thanks.


  • At 31 March 2012, 4:40:21 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @mina This pic has an internal precision oscillator, so you don't need external crystal.


  • At 30 March 2012, 14:32:53 user mina wrote:   [reply @ mina]
    • i just wanted to ask dont u need a crystal for the PIC ?


  • At 19 February 2012, 10:00:38 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Jeff I think there are people that have already make this circuit and burned the hex code and worked properly. Why you ask? Did you face problems uploading it?


  • At 19 February 2012, 6:20:55 user Jeff wrote:   [reply @ Jeff]
    • Hii...is the hex code given is correct ?
      and if i do the cicuit and burn that hex code directly to ic..will it work ?


  • At 28 October 2011, 4:58:04 user Kammenos wrote:   [reply @ Kammenos]
    • @veroo post some screenshots in the forum


  • At 28 October 2011, 3:37:08 user veroo wrote:   [reply @ veroo]
    • Hi.

      Dear Kammenos.
      I did your instructions ( click on the menu project>built options> and then appears "No Active Project". After star a new project and paste the program then follow again your instructions ( project>built options>
      project> and "disable case sensitivity" ) but now not appears "Quickbuild" option.

      You can help me.

      Thank your for you help.


  • At 27 October 2011, 5:48:19 user Kammenos wrote:   [reply @ Kammenos]
    • @veroo Click on the menu Project -> Build options... -> Project.
      The "Buld options" dialog appears. Go to the tab labeled "MPASM Assembler" and make sure the checkbox "Disable case sensitivity" is checked. Then, click "Ok".


  • At 27 October 2011, 3:57:22 user veroo wrote:   [reply @ veroo]
    • Hi:

      Dear Kammenos.

      I already download the MPLAB IDE sofware and found the device pic16f1937, and open the .asm file to edit only to practice, but at the moment of compile the file appears me a list of erros in each one it says Symbol not previously defined.


      You can help me.

      Thank you.


  • At 25 October 2011, 4:40:51 user Kammenos wrote:   [reply @ Kammenos]
    • @veroo download the latest MPLAB which has the 16f1937


  • At 23 October 2011, 22:41:52 user veroo wrote:   [reply @ veroo]
    • Hi:
      Very nice job.

      I Have the compiler, but no theres in the list the pic 16f1937, what I can do. And I need a Programmer for this.


  • At 11 October 2011, 22:13:38 user jay wrote:   [reply @ jay]
    • would anyone be willing to program the PIC16F1937 for me? i will be more than happy to pay for your services.


  • At 10 October 2011, 15:44:06 user Kammenos wrote:   [reply @ Kammenos]
    • @jay http://www.pcbheaven.com/picpages/ i use addembly


  • At 10 October 2011, 15:42:15 user jay wrote:   [reply @ jay]
    • what kind of software do you need to program the PIC?


  • At 17 August 2011, 2:33:09 user Giba wrote:   [reply @ Giba]
    • Hi Kammenos,

      I will do the prototype and I will inform you my finds.

      Regards,
      GIBA


  • At 16 August 2011, 18:49:44 user Kammenos wrote:   [reply @ Kammenos]
    • @Giba go here:http://pcbheaven.com/circuitpages/10_buttons_touch_pad_bcd_output
      and change the code to work with 16 buttons


  • At 16 August 2011, 3:08:20 user Giba wrote:   [reply @ Giba]
    • Hi,

      I am student of Electrical Eng. and my team will have that do a keyboard with touch sensor. Could you send us the programm of touch sensor with 16 ports?


  • At 22 May 2011, 20:47:22 user Keith Elliott wrote:   [reply @ Keith Elliott]
    • Thanks for your article, I think that this might work with my capacitance variable humidity sensor for the weather station I am building.


  • At 18 April 2011, 20:00:10 user Kammenos wrote:   [reply @ Kammenos]
    • @Dylan definitely you need a programmer


  • At 18 April 2011, 18:02:45 user Dylan wrote:   [reply @ Dylan]
    • So, is any programming of the PIC required for this? or does it work "out of the box"?


  • At 28 March 2011, 11:20:05 user Kammenos wrote:   [reply @ Kammenos]
    • @John S 16MHz = 16.000.000/4 = 4.000.000 / 64 = 62.500 Hz = 62.5KHz
      I usually make mistakes with numbers, but this time i was correct :D :D :D


  • At 28 March 2011, 11:08:21 user John S wrote:   [reply @ John S]
    • You mentioned

      "The pulses that finally are delivered to the Tmr0 module, have a frequency of 62.5 KHz."

      is it not 6.25KHz for prescalar of 64?? or I'm wrong?

      ---------------------

      opps... soory! i'm wrong.

      thanks.


  • At 28 March 2011, 10:31:50 user John S wrote:   [reply @ John S]
    • Thanks for the great explanation. You save my time from reading the pic manual and application note again n again. I have seen some of your youtube videos. Great work! Keep up your work. Thank you..


  • At 3 March 2011, 16:13:26 user Kammenos wrote:   [reply @ Kammenos]
    • no, you have to change it


  • At 3 March 2011, 16:08:35 user matt wrote:   [reply @ matt]
    • does this work if use on PIC18f4580


  • At 13 February 2011, 10:49:06 user Kammenos wrote:   [reply @ Kammenos]
    • Graham, read the 3rd page, in which i explain how they are made.


  • At 13 February 2011, 9:56:43 user Graham wrote:   [reply @ Graham]
    • Hi, I'm going to join in and ask what the touch sensors are physically made of?

      They look like pieces of aluminium foil with plastic on top?


  • At 12 February 2011, 21:00:34 user Kammenos wrote:   [reply @ Kammenos]
    • Manish, the change must be done in hardware to make it toggle. Regarding the LED dimmer, this is not that simple. You need to use the PWM output of the PIC and find a way to make the interface. I will make one more circuit, a capacitance touch sensor without microcontroller, and i will not work any more with touch sensors, unless i really need something. As a matter of fact, i did this research to prepare some interface for my coffee machine.


  • At 12 February 2011, 20:54:33 user Manish wrote:   [reply @ Manish]
    • Very nice design,but with this circuit can we make the toggle switch with some external componant & switch on & off any equipment with the help of relay, If possible please upload the schemetic or videos.& also with this can we make the 1 watt LED light dimmer circuit, if possible plz upload the details. Thanking u........


  • At 10 February 2011, 6:26:41 user Kammenos wrote:   [reply @ Kammenos]
    • oh, read this application note from microchip
      http://ww1.microchip.com/downloads/en/AppNotes/01250a.pdf
      Pages 11 and 12


  • At 10 February 2011, 5:14:30 user venkey wrote:   [reply @ venkey]
    • Dear Kammenos, This matrix i know but how to make pcb pads for cap sens module. Thanks


  • At 9 February 2011, 17:54:44 user Kammenos wrote:   [reply @ Kammenos]
    • venkey, read about matrix here:
      http://pcbheaven.com/wikipages/How_Key_Matrices_Works


  • At 9 February 2011, 14:57:59 user venkey wrote:   [reply @ venkey]
    • Hi, nice project. Please explain ho to multiplex 8x8 with pic16f1937 thanks.


  • At 8 February 2011, 22:36:55 user Vasilis wrote:   [reply @ Vasilis]
    • geia sou file mou.
      poli orees oi kataskeues sou kai mpravo.
      ama itheles tha sou elega na ta anaivazeis kai sta ellinika kai ta video kai tis perigrafes...gt opos xereis edo sti ellada den eimaste sainia sta agglika..
      me endiaferei poli auto me ta Capacitance Sensor eidika to allo poy eida xtes pou epiane piso apo to plastiko.
      ama itheles na ta exigisis ligo sta ellinika tha me voithouse poli.
      se euxaristo poli.





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