Pulse Induction Coil and Modification Data

Latest Revisions: 07/12/2002

I have tried to write this in Simple Language so even the unskilled hobby person can understand it, I hope.

See "Schematic and look at IC5, IC6, IC7 & IC8, "The 555 circuits": Frequency: IC5, C11, R3, R4 Determines the frequency. Currently 100 Hz. and a 33% duty cycle. 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 Dead Time: IC7, C17, R7 Determines the Delay Time, before the Sample takes place, allowing the coil to loose back emf. The better the coil, (High "Q") the shorter this time can be. Currently 36 us. Recieve Time: IC8, C21, R9 Determines the turn on time of the fet, "Q3" to recieve the echo from detected metals. Currently set at 50us. 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 will have to change the other values also. (Pulse width, Dead Time and Recieve Time). This is most easily accomplished by putting temporary TrimPots in place of the resistors R5, R7 and R9. Once you have the operation working smoothly again, measure the trimpot values and put equivalent resistors in the circuit. I only recommend this to be done by those persons with reasonable experience! 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 Part": C26 Determines the Integration time. Currently this is a .1 Mfd. Changing it to a .047, Speeds up the response time, but 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." See Views of Various Coils": While not essential for the metal detector to work, best sensitivity on my unit (for my intended purposes) was accomplished with coils pictured below: Click on "Coils 1" and "Coil 2". "Coil 1" is 50 turns, 22 AWG wire "Coil 2" is 42 turns, 22 AWG wire These require a fancy Jig to wind them. It took me a full 1 1/2 days to figure out how to best make the jig and than make it, but just 20 minutes to than actually make the coil. Since I first wrote this I have now made a small coil simular to "Coil 1" but with a inside dia of 1" and an outside dia of 4". It contains 100 turns of #28 Awg. Wire. Very useful for searching a small area for small pieces of metal. All my coils are fitted with an RCA type plug and easily changed in or out of circuit as needed. They can be more easily wound in a rectangle, using Vinal coated solid copper wire. Lay out a board with nails for the window of the coil. Than gradually wind the coil, using pins to temporarly hold the wire in place. Afterwards Epoxy the coil and remove the pins. A simple coil such as seen in the left side of "Coil 3". This will work but has drawbacks when searching for small objects requiring good sensitivity. This coil is 30 turns of 26 AWG wire. Adding more turns to this coil creates too much inter-wire capacitance. and with 40 turns on this coil, it will no longer work. Even a Single turn coil will work better. I put a Ruler in the picture to give a size comparison. The "Coil 4" Picture is a Close-up of the Rectangular coils to show greater detail. The PCB coil on the left is only a few turns, but works quite well also. Both coils have quite high "Q". For the Advanced Experimenter": The coil arrangement in "Coil 5" and "Coil 6" below, can be interesting. The Spiral coil "Coil 1" is connected as LI in the "Schematic 1". Remove the jumper on the circuit board that connects Q1 to R17. The other coil, not shown seperate, is connected between ground and that hole where the jumper was, that goes to R17. The unit now has a seperate recieve and transmit coil. Advantage? The transmit coil can be larger wire for high current draw, while the smaller coil in the picture can made with small wire and more turns. Another interesting advantage to this arrangement can be to place the two coils side by side, at an angle that converges to a point in space, say a foot away. This can be useful for pin-pointing stuff. Unfortunately their are so many Varables to this, I cannot illustrate them. Various sizes of coils and shapes produce varying results! Some really good. 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 a bit unstable, due to the extreme sensitivity involved, but will add greatly to finding smaller items or deeper items.

REMEMBER, Low Capactiance 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.

"HOME"

"Coil 1" Round Spiral Coil

"Coil 2" Rectangular Spiral Coils. (Left one is etched on PCB, Right one is Wound)

"Coil 3" Various Coils (with a ruler as a reference for sizes)

"Coil 4" Close-up of Rectangular Coils

"Coil 5" Close-up of Seperate Recieve and Transmit coils

"Coil 6" Experiment with Seperate Recieve/Transmit Coils.

Close up of mounted REG1117A

Copyright © Mar/2002