PicoDetector : a PIC-based simple and cheap metal detector

A short video clip is sometimes better than a long explanation :

The idea of this circuit is to hack PIC oscillator circuit, by replacing the crystal by a coil : the frequency of the oscillator then depends on presence of metal near the coil, just like in a classic metal detector.

To detect changes of main oscillator frequency, we use the PIC watchdog as internal time reference.

By comparing both oscillators frequencies, we can know if a piece of metal is near the coil, and then light a LED.

It shares the same idea as PicoBat, the ultrasonic bat detector.

Circuit Schematic

• Coil L1 replaces crystal in PIC oscillator circuit : try with the coils you have to find the best one ! if calibrate LED blinks on power up, it works.
• D1 is a dual color LED, replace with two classic LEDs if you don’t have one.

C Source code

/*
 ************************************************************
 * picoDetector: an ultra-simple and cheap metal detector
 ************************************************************
 *
 * Author : Bruno Gavand, april 2009
 * see more details on http://www.micro-examples.com/
 *
 * source code for mikroC PRO compiler V1.65
 * feel free to use this code at your own risks
 *
 * target : PIC12, oscillator in HS mode, watchdog enabled
 *
 * PIC PIN Assignemnt :
 *
 * GP0 : detect LED indicator
 * GP1 : calibrate LED indicator
 * GP2 : NC
 * GP3 : NC
 * GP4, GP5 : inductor
 *
 ************************************************************
 */

#define MAXTRY 15       
// number of watchdog restart to calibrate loop counter

unsigned char   ctr ;           
// number of loops between two watchdog resets
unsigned char   previous ;      
// previous value of ctr
unsigned char   calibr ;        
// calibration value when oscillator runs free
unsigned char   restarts ;      
// number of watchdog restarts
unsigned char   en ;            
// enable flag, allows detection

/*
 * main loop
 */
void    main()
        {
        unsigned char   i ;
        

        /*
         * configure GPIO as digital port
         */
        CMCON0 = 7 ;
        ANSEL = 0 ;
        TRISIO = 0 ;
        GPIO = 0 ;

        /*
         * power up ?
         */
        if(STATUS.NOT_TO)
                {
                /*
                 * yes, init variables
                 */
                restarts = 0 ;
                calibr = 1 ;
                }

        /*
         * watchdog reset counter
         */
        if(restarts < 255) restarts++ ;

        /*
   * if counter differs too much from calibration value
         */
        if((previous ^ ctr) > calibr)
                {
                /*
                 * turn detect LED on
                 */
                GPIO.F0 = en ;
                
                /*
                 * if not on power up
                 */
                if(STATUS.NOT_TO == 0)
                        {
                        /*
                         * while in calibration mode
                         */
                        if(restarts < MAXTRY)
                                {
                                /*
                                 * shift calibration value
                                 * and wait a little bit
                                 */
                                calibr <<= 1 ;
                                Delay_ms(5) ;
                                }
                        }
                else
                        {
                        /*
                         * turn detect LED off
                         */
                        GPIO.F0 = 0 ;
                        }
                }

        /*
         * save last counter
         */
        previous = ctr ;

        /*
         * is calibration over ?
         */
        if(restarts > MAXTRY)
                {
                /*
                 * yes, turn calibrate LED off
                 * and set enable flag
                 */
                GPIO.F1 = 0 ;
                en = 1 ;
                }
        else
                {
                /*
                 * no, turn calibrate LED on
                 * and clear enable flag
                 */
                GPIO.F1 = 1 ;
                en = 0 ;
                }


        /*
         * set watchdog prescaler
         */
        OPTION_REG = 0b11111001 ;
        
        /*
         * start counter, to be interrupted by watchdog
         */
        ctr = 0 ;
        for(;;)
               {
               ctr++ ;
               }
        }

For more detail:  PicoDetector : a PIC-based simple and cheap metal detector

About The Author

Ibrar Ayyub

I am an experienced technical writer holding a Master's degree in computer science from BZU Multan, Pakistan University. With a background spanning various industries, particularly in home automation and engineering, I have honed my skills in crafting clear and concise content. Proficient in leveraging infographics and diagrams, I strive to simplify complex concepts for readers. My strength lies in thorough research and presenting information in a structured and logical format.

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