How to detect metal with a PIC and a few components ?
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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.
/* ******************************************************************************* * 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++ ; } }
Download picoDetector_project.ZIP file for mikroC : File:PicoDetector-project.zip
Includes :
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