The NE555 with a 50% duty cycle and a frequency of 1kHz in this configuration can also be used (with a few changes) to do a metal detector.
If you want to keep the 50% duty cycle but change the frequency you can use this equation R*C=1/(2*f*ln(2)) keep in mind that the resistence value that you find with this equation is the same for the two resistors (if the resistors are not the same the duty cycle will change) and that the capacitor in the formula is the 470nF capacitor in my schematic, don't replace the 10nF capacitor.
Metal detector circuit coming soon.
