Turning Impedance into Frequency: An LC Experiment

In earlier versions of this LC meter (up to V6), calibration was handled automatically by a relay. The relay would insert the reference capacitor into the tank circuit at the start of each measurement session. Simple, automatic — but not without problems.

Problems with the relay approach

Cost: small signal relays suitable for ~1.5 MHz switching are not cheap

Availability: sourcing a consistent relay across builds was annoying

The V7 solution: a 6P DPDT switch

I replaced the relay with a readily available 6-pin double-pole double-throw (DPDT) switch. The switch physically inserts or removes the reference capacitor from the LC tank — the same function, but done by hand.

This makes calibration a conscious 3-step manual operation:

Set switch to CAL position

Press the calibration button — the sketch measures f1 and f2, solves for L and C of the tank

Set switch to MEASURE position — ready to use

"Manual calibration is a feature, not a bug"

Once you understand that calibration only needs to be done once per power-on (or when ambient temperature changes significantly), the manual switch becomes perfectly acceptable. In fact, it makes the measurement process more transparent — you always know exactly what state the circuit is in.

The sketch for V7 is available on GitHub, along with the KiCad PCB files and assembly manual (English and Japanese):

👉 https://github.com/Nobcha/ARD_LCM_MANUAL

A detailed build log is also on my blog:

👉 https://chitose6thplant.fc2.page/lc-meter-v7-dpdt-calibration-3-steps-layers-pcb-model/

One note on the PCB

The first version of the V7 PCB had three errors (OLED header pinout, R6/R7 silkscreen, ICSP header). These are documented in the repository as a "Mistake prevention list." A corrected schematic is also included.

Next up: V8, where the sketch was restructured to reduce flash usage from 95% down to 80%, making room for future improvements.

kpa radio ©nobcha 

The LCD is connected via I2C, so I replaced it with an OLED 128x64 and rewrote the sketch. I uploaded it to GITHUB as V6.4. 

I used KiCAD and uploaded the Gerber files on GITHUB.

I used LTSpice analysis to verify that the Franklin oscillator circuit works with an LC tank circuit and an inverting amplifier using a 74HCU04.

I found the history for this meter. I'm interested in the principle how to measure the values.

At first, the tank parameters (C1 and L1) are derived by adding a known standard capacitor to the tank. Please refer the equation note.

Why a Franklin Oscillator, Not a Precision LC Meter.