Designed and Presented by: Abu-Hafss
The circuit is pretty straight forward wherein, two NPN transistors are cross-coupled to form a flip-flop oscillator. The values of R1 and R2 could be anything between 47 – 100R. The frequency of the oscillation is inversely proportional to the inductance and it can be calculated with the following formula:
Frequency (kHz) = 50,000 / Inductance (uH)
Initially the circuit has to be calibrated using a known inductor, as described below:
Suppose, we have an inductor of 100uH. Putting the value of inductor (100uH) in the above formula, we get 500kHz.
Connect the inductor cross point A & B and power on the circuit. It will start oscillating.
Connect the frequency meter at point A or B and ground.
Adjust the POT until the meter reads 500kHz. Now the circuit is calibrated.
Connect an unknown inductor across A & B.
Power on the circuit and read the frequency at point A or B.
The above formula can also be written as:
Inductance (uH) = 50, 000 / Frequency (kHz)
Putting the value of frequency in this formula, the value of the inductor can be found.