i got it working by picking up a resistor in Q1 emitter chain (including POT resistor value) to around ~12k. this is the point when it stops beeping and flashing by itself, and starts detecting metal. when distance to coin changing, sound and flashes rate also changes, just like your description. but i got VERY LOW sensitivity (despite your description says it is very high) - it detects large 3cm coin from max. 15-20mm where should i dig to?
Last Edit: Mar 24, 2015 8:12:08 GMT -8 by lamazoid
i have rewound the coil using 8x50mm ferrite from AM radio antenna. 180 turns L1 and 75 turns L2. sensitivity is EVEN MORE DECREASED which of your coils on this photo: chemelec.com/Projects/Metal-Pin-3/PinPoint-3.jpg had best detection distance?