Here are instructions for the search coil of the metal detector above. It requires 70 turns of 30 swg (22 awg) enamelled copper wire on a 120 mm (4¾") diameter former. Neither the winding of the coil nor the wire gauge are critical. Wind the coil around the former, and temporarily hold it together with stubs of insulating tape. Then bind it tightly with insulating tape all round. Scrape the enamel off the ends of the coil's enamelled copper wires. Then add a Faraday electrostatic shield, for stability. Prepare some long, thin strips of tin foil. Twist a 100 mm (4") length of bare wire around the coil, over the insulating tape. This provides electrical contact with the foil. Now wind the foil around the circumference of the coil -- beginning at the base of the bare wire -- yet the foil should not quite complete a full 360°. Leave a small gap as shown. Now tightly bind the whole Faraday shield with insulating tape. Attach the coil to IC1 by means of microphone cable, with the cable's screen(s) going to the Faraday shield at the one end, and to -6V (negative) at the other. The coil may be mounted on a rigid plate, and a suitable handle attached. OBSERVATION: Click on the diagram to blow it up. You could wire the coil directly to the circuit if you're in a hurry.
NOTE: You may re-publish this design, on condition that you acknowledge the designer (Thomas Scarborough) and this blog (http://thomasscarborough.blogspot.com).