Not sure this is a good example, but the frequency has similarity and duty cycle appear conjugate. This circuit can be used for hobbyist to trigger a coil at specific frequency. When coil inductance change, the frequency shifted, Arduino uno TMR1 should be able to pick up the frequency change and confirm metal detected. Switching frequency can be change according to resistance and inductance by complying to acceptable current and voltage for 555 Discharge pin input.
