Kits are available for this project from
Talking Electronics for $15.00 plus postage.

See more projects using PIC micros:
Elektor,EPE,Silicon Chip

You can add this circuit to all sorts of projects that
require on-off control.

Our design allows up to 4 touch sensors using a PIC12F629. The output of each touch sensor is active LOW and this can be connected to an additional circuit to control a LED, motor or relay etc.

The photo of the project shows one output with the two LEDs connected in series to produce an infinitely high input impedance so that one of the input/output lines of the micro can be used as both input and output. Two LEDs drive the base of a transistor and a third LED is connected to the collector of the transistor to indicate it is being activated.
The input lines to a PIC microcontroller contain FETs and a FET has an almost infinite input impedance (resistance). This mean it is very sensitive and will detect
voltages that are called STATIC ELECTRICITY.

The Prototype PC board with one input

These voltages (Static Electricity and radiation from power cables and wiring) are all around the home and are produced by a number of things including the electromagnetic radiation from the 110v or 240v mains wiring, the radiation from a TV that uses a picture-tube (Cathode Ray Tube) by the electrons being fired by the gun towards the screen, the static produced when walking on carpet or produced by clothing hen it is moved on the body and the movement of paper and plastic items.
You will be amazed at where static electricity can be found and the input to the microcontroller we are using in this project will detect these charges, simply by connecting a wire to one of the input pins.
In fact the micro is so sensitive it will react uncontrollably when moved around the home.
The whole essence of this project has been to remove the uncontrollability-factor and create a touch wire that will only respond when it is touched.
This is a very difficult thing to do as we are using the very sensitive input of a pin to detect the charge (or lack of charge) produced by a finger and at the same time masking the charge from the surroundings.
The answer is to charge a small capacitor and see if the finger discharges it. This means the input will not be responsive to any static charges in the room.
For this to work, we are assuming the body is uncharged and some-times clothing etc will create a charged condition and the touch sensor will not work.
That's why this project will not work in all situations and with all users.
However when it does work, it is amazing.
The slightest touch of the wire with a finger will turn on the LEDs.
You can build 1, 2, 3 or 4 sensors and use them with touch pads to control all types of devices.

One of the clever features of this circuit is the use of a single input/output line as both an input and output.
This is called multiplexing or "sharing."
The line is firstly set up as an output and the 100p capacitor is charged. It is then turned into an input line and a 20mS delay is called to give a short period of time for a finger to discharge the capacitor.
The charge on the capacitor is then detected after this time and if it is low, the line is turned into an output to activate the base of the transistor via two LEDs.
These LEDs have been included to produce an infinitely high impedance on the line so that the charge on the 100p capacitor will not be affected.
The two LEDs and base-emitter junction of the transistor will produce an infinite impedance to voltages below the turn-on voltage of the combination.
Another clever circuit-design is placing a diode between the "ground" or Vss line of the micro and the 0v rail.
This reduces the 6v supply to 5.4v (as this is the maximum voltage the chip can be delivered). But more importantly it increases the sensitivity of the input line and makes the project much more sensitive by actually raising the zero-detection-point by about 0.5v. 

ouch the Touch Plate or wire and allow the capacitor to discharge. The LED corresponding to the sensor will illuminate and the transistor will sink up to 100mA.

The circuit can be built on any type of Proto board. We have used one of our surface-mount PC boards as all the components and wiring can be seen at the same time and this makes construction and diagnosis easy.
The photo shows only one input however the board is big enough to add the three other sections. 

This project is part of a course in PIC Programming. See left index on Talking Electronics website: List of PIC Projects: for the projects in this course.
The course consists of building these projects and modifying the programs to learn the art of programming PIC microcont
Here are the files you will need for "burning" your chip and/or modifying the program:


The following program is for viewing. It may contain spaces or hidden characters that will not compile correctly to produce a .hex file. Use the .hex file above to burn your chip or the .asm file to modify the program.
;Touch Switch.asm
	list	p=12F629
	radix	dec
	include	"p12f629.inc"
	errorlevel -302	; Don't complain about BANK 1 Registers 

		& _WDT_OFF & _INTRC_OSC_NOCLKOUT  ;Internal osc.

;_MCLRE_OFF  - master clear must be off for gp3 to be input pin 

; variables - names and files

temp1	equ 20h	;
temp2	equ 21h	;
temp3	equ 22h	;
temp4	equ	23h	; 
_flash     equ	24h	; 

status	equ	0x03
rp1	equ	0x06
rp0	equ	0x05
GPIO 	equ 	0x05

status		equ	03h
option_reg	equ 	81h

	; bits on GPIO
pin7	equ	0	;GP0  
pin6	equ	1	;GP1  
pin5	equ	2	;GP2  
pin4	equ	3	;GP3  
pin3	equ	4	;GP4 
pin2	equ	5	;GP5  

rp0	equ	5	;bit 5 of the status register

;Beginning of program
	org	0x00
SetUp	bsf	status, rp0 	;Bank 1			
       	movlw	b'11101000'	;Set TRIS  GP1,2,out  GP5 in
	movwf	TRISIO	   	   ;		
	bcf	status, rp0	;bank 0
	movlw   07h         	;turn off Comparator ports
        movwf   CMCON       	;must be placed in bank 0 
        call	flash 
	clrf 	GPIO       	;Clear GPIO of junk	
	goto 	Main	
;* Delays 			*

_10us	goto	$+1
	goto	$+1
	goto	$+1
	goto	$+1		
	retlw 	00

_250uS	movlw	.80
	movwf	temp1
	decfsz 	temp1,f
	goto 	$-1		
	retlw 	00

_1mS	nop
	decfsz 	temp1,f
	goto 	$-2		
	retlw 	00	
_10mS	movlw	.10
	movwf	temp2
_10	nop
	decfsz 	temp1,f
	goto 	_10
	decfsz 	temp2,f
	goto 	_10	
	retlw 	00	
_20mS	movlw	.20
	movwf	temp2
_20	nop
	decfsz 	temp1,f
	goto 	_20
	decfsz 	temp2,f
	goto 	_20	
	retlw 	00	
_250mS	movlw	.250
	movwf	temp2
_250	nop
	decfsz 	temp1,f
	goto 	_250
	decfsz 	temp2,f
	goto 	_250	
	retlw 	00			

;* Sub Routines 			*
	;flash LED at start-up 
flash	movlw	0ffh
	movwf	_flash
	bsf	gpio,1
	decfsz	_flash,1
	goto	$-2
	bcf	gpio,1
	retlw	00		
;* Main 							*
Main	bcf	gpio,4		;make line LOW to discharge 100p
	call	_10uS	
	bsf	gpio,4		;make line HIGH to charge 100p
	bsf	status, rp0 	;Bank 1			
       	movlw	b'11111000'	;Set TRIS  GP4 in
	movwf	TRISIO	   			
	bcf	status, rp0	;bank 0		
	call	_20mS
	btfss	gpio,4
	goto	$+3
	bcf	gpio,2
	goto	Main
	bsf	status, rp0 	;Bank 1			
       	movlw	b'11001000'	;Set TRIS  GP4 out
	movwf	TRISIO	   			
	bcf	status, rp0	;bank 0
	bsf	gpio,4		;turn on LEDs and transistor
	call	_250mS
	goto	Main

You can add additional features to this project by writing your own program or adding to the program above. Send any additions to Talking Electronics for inclusion in this article. 


Touch Switch
Parts List

Cost: au
$15.00 plus postage
Kits are available

4  -  220R   SM resistor
4  -  470R   SM resistor

4  -  100p SM capacitors
1  -  100n SM capacitors
4  -  10u SM electrolytic

12  -  SM yellow LEDs
1  -  SM diode
4  -  BC847 SM transistors

1  -  SPDT mini slide switch
1  -  8 pin IC socket 

20cm fine enamelled wire
10cm tinned copper wire for touch wires
20cm  - very fine solder 

1  -  PIC12F629 chip (with Touch Sw routine)
1  -  battery holder
4  -  button cells

1  -  Prototype PC board