Calibration
When Completely Assembled, The Only Calibration is to set the Voltage at Pin 6 of IC9's TP to between 0.2 and 0.5 volts, Using VR1.
Revision: Sept 20 2007
However For Initial Operation, I also Recommend These Settings: R5A which is VR5, Set to a 10K Resistance. R7A which is VR6 Set to a 4K7 Resistance. R9A which is VR& Set to a 3K3 Resistance. The reason for this is it puts these setting about equivalant to the Pulse-1. And it will operate with almost any coil. Otherwise you may have the wrong settings for the coil your using. This May result in Not get any Response to metals.
Gain: IC9 controls most of the Gain in this circuit. Increasing the value of R23 will increase gain. But at the same time, Increase noise. Some increase might be acceptable But don't go overboard. More about IC9 and also IC10 Below.
"The 555 circuits and Functions": Frequency: IC5, C11, R3, R4 Determines the frequency. Currently 100 Hz. and a 33% duty cycle. If you decide to change the frequency, just change the capacitor to a larger value to lower the frequency or a smaller value to raise the frequency. However in doing so, you may have to change the other values also. (Pulse width, Dead Time and Recieve Time). Pulse Width: IC6, C14, R5 Determines the Pulse width to the coil. Currently 165 us. Making it longer or shorter, requires changes to dead time R5A is VR5 on the Parts List Dead (Delay) Time: IC7, C17, R7 Determines the Delay Time, before the Sample takes place, allowing the coil to lose back emf. The better the coil, (High "Q") the shorter this time can be set for. Currently 36 us. R7A is VR6 on the Parts List Recieve Time: IC8, C21, R9 Determines the Recieve Time. (Sample Time) Currently Set a 50 us. R9A is VR7 on the Parts List
SOME PRACTICAL VALUE EXAMPLES "Use Linear Taper Pots and they will give a Fairly linear range of adjustment".
Frequency
C11A = .22 = 47 hz C11B = .15 = 68 hz C11C = .1 = 100 hz C11D = .047 = 217 hz C11E = .022 = 470 hz C11F = .01 = 1000 hz Pulse Width
Rotation Resistance Ohms Time in uS Min. 5100 56
1/4 7600 84
1/2 10100 111
3/4 12600 139
Full 15100 166
Dead TimeC19 & C22 = 100pf & Caps + R7 = 3k3 C19 & C22 = 50pf Caps + R7 = 1K8
Rotation Resistance Ohms Time in uS Resistance Ohms Time in uS Min. 3300 17 1800 9
1/4 4550 24 3050 16
1/2 5800 30 4300 22
3/4 7050 36 5550 29
Full 8300 43 6800 35
Recieve Time
Rotation Resistance Ohms Time in uS Min. 1500 16
1/4 2750 30
1/2 4000 44
3/4 5250 58
Full 6500 72
If you Use the Values I've given for the Pots and Caps above, results are pretty good. I have also given the Calculated Times for Minimum Rotation, 1/4 turn increments and Maximum Rotation of each control. ** I Recommend these controls should be calibrated for ease of reference.I did not find any advantage in Changing Frequency, But that is Your Choice
There is No Guarantee that your detector will function
correctly at all these settings. ** Lead Lengths, Layout, Coil Size as well as Numerous other Factors can cause problems. Play with it. **Reducing the turns on the coil, as well as a Smaller Diameter coil should help.To keep the board small and Avoid Wiring Problems: The additional caps for C11, (a, b, c, d, e, etc) mount off-board on S1. As well for the Expanded Meter, the 2 meter diodes and one resistor mount off-board, on the back of the meter.
ADDITIONAL EXPERMENTATION:
I have also experimented with Small 1/2 inch OD coils wound on a Ferrite Core and achieved some very good results for Pin-Pointing. "Pot Cores" seem to work Best. "These types of Coils gave me the Best Results in Detecting my 1/4 and 1/2 Gram Gold Nuggets".
I only recommend this experiment be done by those persons
Having "GOOD Electronic Knowledge"!
Interconnecting Cable: When connecting the coil to the circuit board, it is best to use a shielded cable to prevent these wires also picking up metal objects. However too much Capacitance in this wire will reduce the detectors sensivity. Below are a few cables and their capacitance per foot. RG174/U 30.8 pf per ft. RG58U 28.5 pf per ft. RG59U 20.5 pf per ft. RG62A/U 13.5 pf per ft. RG6U 16.2 pf per ft. Beldon 1223A ---- 12.8 pf per ft. Beldon 89269 In my unit, I'm only using 3 feet of cable and I'm using the RG174U. However if you plan to run long lengths, consider this capacitance.
On the Schematic, IC9 & IC10, "The TL071 or TL081 IC's": "C26" Determines the Integration time. Currently this is a .1 Mfd. Changing it to a .047, Speeds up the response time, but is Harder to control. Changing it to a .47, Slows down the response time, easier to control, but less responsive. A NOTE about VR2: This is the main control over the unit, and once the unit is finalized, this control and the volume control are the only two controls that will typically need further adjustment. The "Meter Zero" is quite stable and seldom needs adjustment. To Facilicate Easier adjustment, here is another modification. Once the circuit is completely built and Optomized the way you want it, A new value of a Resistor can be put in place of R26 and R27, at either end of VR2 and the value of VR2 can be reduced to a 10 K or even a 5 K pot. Try to keep the total resistance close to 150K or slightly higher. Method to determine resistors: set up the unit for normal operation, with about 1 beat per second. Now turn off the unit. Without turning the shaft of it, Remove VR2 from the circuit and with an ohm-meter measure the resistance from the center to each side. Now, with the lower value pot that you have chosen to be used, put an appropriate resistor value for R26 and R27 to equil the resistance readings above. "In my detector", I finally used a 10 K pot, making R26 = 82 K and R27 = 68K. Resistance total = 160 K ohm." While not essential for the metal detector to work, best sensitivity on my unit (for my intended purposes) was accomplished with my flat coils. But these coils require a fancy Jig to wind them nicely. Meter: If Higher sensitivity is wanted, replace the meter with a 100 ua or even a 50 ua. meter. ** Ultimate sensitivity: Replace the conventional meter with a 200 MV. Digital Panal Meter. Set Adjustment for a slight positive reading. The reading is is somewhat unstable, due to the extreme sensitivity involved, but will add greatly to finding smaller items or deeper items.REMEMBER, Low Capacitance and High "Q" is always IMPORTANT in the coil designs.
Please, If any persons building this come up with other "Useful Modifications". Please contact me. Lets share the info. It will bennifet others as well.
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