double coil

[gold1958]

Hi Guys
I am building HH rev d. could someone please explain L2 (the second coil) option.

Thanks

[bbsailor]

The latest revision of the Hammerhead circuit board has provisions for using both a mono coil as well as a DD coil.

A mono coil functions as the Transmitter (TX ) coil as well as the Receiver (RX) coil. Look on the schematic and see a jumper that links the mono coil to the input of the NE5534 amplifier. In the DD configuration, there are two coils in the same housing shaped like two letter "D" s back-to-back overlaped on the straight side by about 1 inch. One coil is the TX coil and the other coil is the RX coil. Each coil is connected to the respective circuit board connections by independent shielded cables.

Read the article written by Carl as it describes the Hammerhead circuit and initially the mono coil. Some forum posts by Carl describe the changes he made to accomodate a DD coil but are not in his article. Then, look at the schematic for the Rev D and see where he placed jumpers to allow either a mono coil or double D (DD) coil to be used. If a DD coil is used you need a damping resistor for each coil. For the 11" Hays Elecronics coil housing, each DD coil is made about 8" inside diameter and shaped into a "D" shape to fit inside the coil housing. A single mono coil of about 10.5" inside diameter will fit inside the same housing.

Mono coils are simplier to make and should be tried first to get the circuit working. Then, if you want more sensitivity on small gold items, a DD coil can work at a few microseconds lower delay.

Use the forum search feature and look for posts by "Reg" and "Carl-NC" about "DD coils" or "Double D coil".

Once you get started building the Hammerhead there are many who have experience with it who will jump in and help you if you have specific questions.

bbsailor