This circuit uses a 555 square wave output to ring the tank circuit containing the unknown inductor, then a common emitter amplifier to boost the signal. After measuring the ring frequency with an oscilloscope the value can be calculated. I built this specifically to measure values under my LC meter’s capabilities, sub 10 uH. The voltage divider on the 555 output reduces the charge pulse voltage enough to prevent it from drowning out the ring and the amplifier isn’t necessarily needed, however it does allow for a higher accuracy frequency measurement. I have built this circuit, with a 10uH (100) inductor and a measured 1.03uF (105) capacitor, I get a frequency of 49.1kHz, 10.2uH.
L=1/(4*pi2*Fres2*C)
L in H
Fres in kHz
C in uF
