However For Initial Operation, I also Recommend These Settings:
If your using the Dual Frequency Make Sure you are set to the 100Hz.
ALSO SET THESE CONTROLS:
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 and Duty Cycle.
Currently 100 Hz. and a 33% for Ver-3,
or 100 Hz and a 7% duty cycle for Ver-3B.
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).
(Updated Aug 2010)
{100 Hz is good for detecting Medium to Large objects,
using a 8 to 12 inch coil.}
{1000 Hz is good for detecting Small Objects, Using coils
8 inches in diameter or Smaller.}
But 1000 Hz Draws Considerably MORE CURRENT.
NOTE: An External SPDT Switch could be added to
switch between these two frequencies.
But be careful on the 1000 Hz, as when using higher Pulse
Width at 1000 Hz the Current Draw can become EXCESSIVE and
Cause Damage.
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 can be considered as VR5.
Dead (Delay) Time: IC7, C17, R7 Determines the Delay Time, (Dead 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 can be considered as VR6.
Recieve Time: IC8, C21, R9 Determines the Recieve Time. (Sample Time)
Currently Set a 50 us.
R9A can be considered as VR7.
"Use Linear Taper Pots and they will give a Fairly linear range of adjustment".
Frequency (Updated June 2009) (I Only Recommend using these Two Frequencies.)
Pulse Width
1/4 7600 84
1/2 10100 111
3/4 12600 139
Full 15100 166
Dead Time
C17 = .01 & R7 + R7a C17 = .0047 & R7 + R7a
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
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.
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 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. For the "ADVANCED EXPERMENTER", To Facilicate Easier adjustment, here is another possible 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.
REMEMBER, Low Capacitance and High "Q" is always IMPORTANT in the coil designs.
Meter: If Higher sensitivity is wanted, replace the meter with a 100 uA, 50uA or even a 25 uA. meter.
I Do NOT Recommend This:
** But for Ultimate sensitivity: Replace the conventional meter with a 200 MV. Digital Panal Meter.
Set Adjustment for a slight positive reading. These readings are somewhat unstable,
due to the extreme sensitivity involved, but can help somewhat to finding smaller or deeper items.
PLEASE "CLICK HERE" for More info on using a "LCD Meter".
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.
"My HOME Page"
Copyright © June 2013