Easy precision hoby meter
STM32F103C8 This is simply precision LC meter 0.0 nH to tens mH, 0.0 pF to hundrets nF and ohmmeter from 0.0 to 33MOhm with automatic scale. Display is LCD1602 A, coneected with 4 data D4,D5,D6,D7 are PB15,14,13,12; EN, RS are PA8,9; RW is permanently on ground, V0 for LCD connect to GND via 2.2(1.8) kOhm res to GND. Suply voltage LCD is 5V. Library is very simply. Very important is chain timers TIM1 and TIM4 started with TIM3 (prescaler to 1s) for measure resonans frequency LC oscillator. Oscillator is easy with OP amplifier LF353 with J-fet imputs, gain bandwitch - 4Mhz, power suply is +5V,GND,-5V. In my case is used L on ferit-core with inductans around 32 microHenry, after calibrations is saved to memory and cannot by bigger how 65 microHenry, size memory is 65535. Working capacitor is 1nF on 250V (styroflex). Feedback capacitors have 270 pF. Inductor and capacitor must have good quality for frequeny stability and precision measure. Calibrations capacitor must by very precision 1% or better, after calibrations we can compare measure value with this capacitors.
Calibration is in more steps. On first is need set working capacitor with feedback cap. + parasitic capacitans (1nF + 270pF + 10pf..). Next calculate 10* Inductors, for bigest precision :
resonant working frequency - Fw=1/(2xΠ√LC) working inductors L=(2π〖Fw)〗^2Cw
next connecting calibrating capacitor, frequency decrease on to Fcal; Fcal=1/(2xΠ√(Lx(C+Ccal))) for precision measure 10*
Thats we have Fw and Fcal and working inductor.
In realation Fw/Fcal is the same as Cw/(Cw +Ccal) and we can calculate Ccal for compare.
If calculate Ccal is unequal with Ccal, we must change working capacitor on start.
If Ccal is OK, we can save Cw and L to Flash memory, by careful, to flash can be save max 10000 times.
LINEAR OHMMETER
Ohmmeter is composed fom 2 circuits, high scale M33, 3M3, 33Megaohm, and low scale 330, 3k3 33kilohm. Low scale measure voltage on measure resistor via going current 100mikA from LM334 (current source R26). 1/2 LF353 gain 1* for 33k - 3.3V; for 3k3 - 10*; for 330 ohm 100* (trim R15, R14, R13). High scale measure resistors via negative feedback oposite scale resistors with hexfets. When is probe open ( no measure resistor) need setting R19 output voltage around 4-4.2V. Next with quality resistors M28, 2M8, 28M - 1% setiing scale (trim R16, R17, R18).