How do I build a Metal Detector?

What about a magnet, very good at sensing metal.......Only joking. I am with harald don't know of one with an analog out. But a few years ago I helped design one which worked like this. Take a tuned circuit and ping it with a pulse the after a time measure the voltage. Then measure again and if metal is present the voltage will be lower. This measures the amplitude of ringing which for a under damped coil is an exponential function
Adam

 

Simple BFO metal detector

BFO ( beat frequency oscillator ) metal detectors use two oscillators, each of which produces a radio frequency. One of these oscillators uses a coil of wire that we call the search loop. The second oscillator uses a much smaller coil of wire, and is usually inside the control box and is called the reference oscillator. By adjusting the oscillators so their frequencies are very nearly the same, the difference between them is made audible as a beat note, this beat note changes slightly when the search loop is moved over or near to a piece of metal. It has been found in practice best to make the search oscillator fixed say at 100khz and to arrange for the reference oscillator to be adjustable 100khz plus or minus 250hz. This gives a beat note of 250hz to 0 to 250hz. The beat note disappears or nulls when the two oscillators are about equal. This type of detector is most sensitive when the beat note is close to zero, about 5hz ( motor boating ) any slight change being noticeable.

 

Posting other people's work without attribution is plagiarism and is bad netiquette.

It is best to post the link to the article. In this case, it is http://www.instructables.com/id/bfo-metal-detector/

 

@chuchito

Are you trying to determine the distance of a piece of "ferrous" metal in air or can it be behind walls, in earth, or otherwise obscured from direct line of sight?

Is it correct to assume you mean ferromagnetic, not ferrous? How important is that distinction to what you are doing?

John

 

I agree Kris. You got me thinking, what about the common circuits we all use every day that we didn't design ourselves that appear time and time again. Someone designed it in the first place but we never mention it but would take credit for it. We all do that.
Adam

 

And that got me thinking years ago, why electronics doesn't have a decent indexing system. Chemists have had a system for indexing structures for well over a century.

John

 

That one was a special case. switychandra posted a whole section of text, verbatim, from an easily accessible source, with no attribution, implying that it was his own words. He's also a new user. This usually means that he's trying to reach the 10 post limit so he can spam users with lots of PMs.

As for standard building block circuits, I guess they all have inventors - thinking of the two-transistor astable multivibrator, for example - but mostly their names are lost in the mists of time. Some could be found through patent searches, and specific clever or unique circuits are still associated with their inventors, but not that many.

Many designs originally appear in data sheets, application notes, and other application material, created by engineers working for component manufacturers, and are usually uncredited. Of course there are famous names in the field, such as Bob Widlar, who has patents for techniques used inside ICs, but I couldn't name any application circuits he designed.

A lot of circuits are trivial and would have been "invented" by many people - who invented the op-amp-based summing amplifier? And some aren't really circuits, as such, just ways to use basic building blocks - who "invented" the emitter follower?

I guess I don't have a problem with the status quo, personally.

 

Yes, definitely.

If you are reproducing a complete circuit design, you should always give the source.

Personally, I would normally link to the original article instead of reposting the diagram. The diagram is only part of the picture; you need the accompanying text.

Many schematic images include the source URL; I normally include it in any designs I post here.

 

Private message SPAM. does that go on, blimey.

 

I've heard that it does. I haven't seen any in my time here. The potential spammers have always been picked up well before they reach 10 posts.

 

What they keep asking stupid questions or what?

 

No, the stupid questions are normally from genuine users We had one guy who would copy a paragraph or two from someone else's post further back on a thread and repost it. And lately there have been quite a few users just posting spam as new threads. They usually get caught within 12 hours or so.

 

I would imagine that it wasn't a big leap for someone who was previously familiar with a Cathode Follower but who invented that? Perhaps a more accurate question would be can either really be considered inventions at all? After all, the inventor of the Triode had mathematically predicted its behavior prior to building the prototype.

Chris

 

Hi to all,
I have build a metal detector circuit based on this idea:
http://www.micro-examples.com/public/microex-navig/doc/076-picodetector.html
I'm using a PIC16F877 and a Bluetooth module RN42SM.
I implemented the detector by oscillating a PWM wave via the PIC,
sending it through an inductor, capturing the frequency at the timer1
and transmitting the data via the BT module to a PC.

Up till now the timer's value slightly changes and doesn't give a good indication
for metal detecting. Does somebody have any ideas of how to improve the
detecting?
I tried changing the frequency's value (521-15000hz) and changing the
captures specifications for flag interrupt between every 4-16th rising edge.
I have a few inductors of 380~680 mH value.

Thanks, Amitai

 

Can you post a schematic please.

 

I have attached the schematic, quiet simple.
hope I didn't forget anything.
I'm attaching the code as well:

Code:

LIST	P=PIC16F877
    	
include	<P16f877.inc>
org		0x00

	__CONFIG _LVP_OFF &_WDT_OFF &_PWRTE_OFF &_CP_OFF & _HS_OSC	

	reset:
	nop
	goto	start
	org		0x20

start:




; configuration
;-----------------------------------

; used_ registers:
	del1			EQU H'0030'  		; for delay
	del2			EQU H'0031'  		; for delay
	to_transmit		EQU H'0050'  		; data register
	led1			EQU H'0070'  		; for led delay

	banksel		ADCON1
	movlw		0x07				; ADC PINS configured as a digital I/O									
	movwf		ADCON1

;UART/*/*/
	banksel		TRISC
	bsf	 	 	TRISC, 6 
	bsf		 	TRISC, 7	
	
	banksel  	TXSTA
	movlw    	0xA7        		; master, transmit EN, Async mode, Transmit Shift Register empty, BRGH =1
	movwf    	TXSTA	       		; Baud Rate = Fosc/(16(SPBRG+1)) , BRGH (baud rate) ignored in sync
	
	banksel		SPBRG
	movlw	 	0x03		   		; = d"16" which corresponds to 125Kbps with 8MHz clock
	movwf	 	SPBRG
	
	banksel  	RCSTA
	movlw    	0x80		  		; enable UART	
	movwf    	RCSTA
	
;PWM, timer2/*/*/
	banksel		TRISC
	bcf			TRISC, 2 			; make the CCP1 pin an output.
	
	banksel		T2CON
	movlw		0x07
	movwf		T2CON				; Prescaler , TMR2 ON
	
	banksel		PR2					; output frequency = 			
	movlw		0xF3
	movwf		PR2					; 1 / [(PR2) + 1] • 4 • TOSC •(TMR2 prescale value)
	banksel		CCPR1L				; duty cycle =(CCPR1L:CCP1CON<5:4>) • TOSC • (TMR2 prescale value)
	movlw		0x7A				
	movwf		CCPR1L				
	banksel		CCP1CON
	movlw		0x0C
	movwf		CCP1CON

;capture, timer1 /*/*/
	banksel		TRISC
	bsf			TRISC, 1 			; make the CCP2 pin an input.

	banksel		T1CON
	movlw		0x01				;  en TMR1
	movwf		T1CON				; (fosc/4) 
	banksel		PIE2
	bcf			PIE2, CCP2IE		; avoid false interrupts 

	banksel		CCP2CON
	movlw		0x07
	movwf		CCP2CON				; Capture mode, every Xth rising edge 

;LED 
	bcf			TRISD, 4
	bcf			TRISD, 5
	banksel		PORTD	
	bcf			PORTD, 4
	bcf			PORTD, 5
	
; end configuration --------------------------------------




;************ MAIN PROGRAM ***************
;*****************************************
	
main_loop:

Led_flicker:
	movlw 		0xFF
	movwf		led1
	t2:
	CALL		DELAY
	decfsz		led1
	goto		t2
		
	bcf			PORTD,5
	bsf			PORTD,4		
		
	banksel		led1		
	movlw 		0xFF
	movwf		led1
	t1:
	CALL		DELAY
	decfsz		led1
	goto		t1

	bcf			PORTD,4
	bsf			PORTD,5
;---end Led flicker

;capture frequency/*/*
	banksel		TMR1H	
	clrf		TMR1H
	clrf		TMR1L
	
	banksel		PIR2
	bcf			PIR2, CCP2IF
	check_capture:		
	btfss		PIR2, CCP2IF					; 1  = A TMR1 register capture occurred (must be cleared in software)
	goto		check_capture
	bcf			PIR2, CCP2IF
	
;transmitting data	
	banksel		CCPR2H							; transmit MSB
	movf		CCPR2H, w
	banksel		to_transmit
	movwf		to_transmit
	sublw		0xFF				; if data = FF change to FE
	btfsc		STATUS, Z
	CALL		data_change	
	movf		to_transmit, w
	CALL 		transmitting_data
	
	banksel		CCPR2L							; transmit LSB
	movf		CCPR2L, w
	banksel		to_transmit
	movwf		to_transmit
	sublw		0xFF				; if data = FF change to FE
	btfsc		STATUS, Z
	CALL		data_change
	movf		to_transmit, w
	CALL 		transmitting_data
		
goto main_loop

	
				
;******** END MAIN LOOP ********************		

transmitting_data:	
	banksel		TXREG
	movwf   	TXREG					    	; writing to TXREG clears TXIF
    trans_wait2:
	btfss		PIR1, TXIF						; wait for the frame to be transmitted
	goto		trans_wait2
	CALL		DELAY
	return
	
data_change:
	banksel		to_transmit
	movlw		0xFE
	movwf		to_transmit
	return
	
DELAY:
	movlw		0xFF
	movwf		del1
	movlw		0xFF
	movwf		del2
	
	loopa:
	decfsz		del1, f
	goto		loopa
	loopb:
	decfsz		del2, f
	goto		loopb
	return
		
end

Amitai