I recently built an auto-ranging ohmmeter from simple-circuit.com, which I modified to work with an Oled display. It works accurately enough to sort resistors from 10 ohms to 1Meg ohms, but for some reason does not measure 150K ohms?? 100k and 220k read fine. I double checked all the calibration resistor values and wiring and found no faults. I was wondering if there was something in the code which is causing this problem. Can someone check the attached code for me?
// SKETCH: Ohmmeter_Autoranging_SimpleCircuitcom.ino
// Autoranging Ohmmeter with 128x64 Oled display with Arduino Pro-Mini
// SH1106 Oled display driven with I2C via A4=SDA & A5=SCK.
// Adaped from: https://simple-circuit.com/arduino-auto-ranging-ohmmeter-lcd/
#include <Arduino.h>
#include <U8g2lib.h> // Oled library
// Define digital pins to enable pnp transistors through 4k7 ohm base resistor
#define CH0 12 // Q1 - 100 ohm range
#define CH1 11 // Q2 - 1k ohm range
#define CH2 10 // Q3 - 10k ohm range
#define CH3 9 // Q4 - 100k ohm range
#define CH4 8 // Q5 - 2 Meg ohm range
byte ch_number;
uint32_t res;
const uint32_t res_table[5] = {100, 1000, 10000, 100000, 2000000}; // resistor table array
char _buffer[11];
U8G2_SH1106_128X64_NONAME_F_HW_I2C u8g2(U8G2_R0, /* reset=*/ U8X8_PIN_NONE);
void setup(void) {
//Serial.begin(9600); // initialize the Serial display
u8g2.begin(); // initialize the I2C Oled display
u8g2.clearDisplay();
u8g2.setFont(u8g2_font_lubB14_tf); // Set Lucida 14 font
u8g2.drawStr(10, 20, " < OHM >"); // write to the internal memory
u8g2.drawStr(10, 40, "< METER >"); // write to the internal memory
u8g2.setFont(u8g2_font_luRS10_tf); // Set Lucida 10 font
u8g2.drawStr(0, 60, " by: f.w.mclennan"); // write to the internal memory
u8g2.sendBuffer(); // transfer internal memory to Oled
// NOTE: Millis can only be used once per sketch like this:
while (millis() < 4000) { // Hold Oled display 4 seconds
}
u8g2.clearDisplay(); // clear the display
u8g2.clearBuffer(); // clear the internal memory
// Set digital pins as outputs
pinMode(CH0, OUTPUT);
pinMode(CH1, OUTPUT);
pinMode(CH2, OUTPUT);
pinMode(CH3, OUTPUT);
pinMode(CH4, OUTPUT);
// pinMode(A1, INPUT); // Set analog (adc) as input - Not needed?
ch_number = 4;
ch_select(ch_number);
}
void loop() {
// Read ADC voltage across unknown resistor (digital value between 0 and 1023
uint16_t volt_image = analogRead(A1) + 1;
if (volt_image >= 550 && ch_number < 4) {
ch_number++;
ch_select(ch_number);
delay(50);
return;
}
if (volt_image <= 90 && ch_number > 0) {
ch_number--;
ch_select(ch_number);
delay(50);
return;
}
if (volt_image < 900) {
// This equation computes the value of the unknown resistor
float value = (float)volt_image * res / (1023 - volt_image); // value is unknown resistor in ohms
if (value < 1000.0)
sprintf(_buffer, "%03u.%1u Ohm ", (uint16_t)value, (uint16_t)(value * 10) % 10);
else if (value < 10000.0)
sprintf(_buffer, "%1u.%03u Kohm", (uint16_t)(value / 1000), (uint16_t)value % 1000);
else if (value < 100000.0)
sprintf(_buffer, "%02u.%02u Kohm", (uint16_t)(value / 1000), (uint16_t)(value / 10) % 100);
else if (value < 1000000.0)
sprintf(_buffer, "%03u.%1u Kohm", (uint16_t)(value / 1000), (uint16_t)(value / 100) % 10);
else
sprintf(_buffer, "%1u.%03u Mohm", (uint16_t)(value / 1000000), (uint16_t)(value / 1000) % 1000);
}
else
sprintf(_buffer, "<INFINITY>"); // Measuring probes open circuit
u8g2.setFont(u8g2_font_lubB14_tf); // Set Lucida 14 font
u8g2.clearBuffer();
u8g2.setCursor(5, 30);
u8g2.print(_buffer);
u8g2.sendBuffer();
// Serial.println(_buffer);
// Serial.println();
delay(1000); // update readings once per second
}
void ch_select(byte n) {
switch (n) {
case 0:
digitalWrite(CH0, LOW);
digitalWrite(CH1, HIGH);
digitalWrite(CH2, HIGH);
digitalWrite(CH3, HIGH);
digitalWrite(CH4, HIGH);
break;
case 1:
digitalWrite(CH0, HIGH);
digitalWrite(CH1, LOW);
digitalWrite(CH2, HIGH);
digitalWrite(CH3, HIGH);
digitalWrite(CH4, HIGH);
break;
case 2:
digitalWrite(CH0, HIGH);
digitalWrite(CH1, HIGH);
digitalWrite(CH2, LOW);
digitalWrite(CH3, HIGH);
digitalWrite(CH4, HIGH);
break;
case 3:
digitalWrite(CH0, HIGH);
digitalWrite(CH1, HIGH);
digitalWrite(CH2, HIGH);
digitalWrite(CH3, LOW);
digitalWrite(CH4, HIGH);
break;
case 4:
digitalWrite(CH0, HIGH);
digitalWrite(CH1, HIGH);
digitalWrite(CH2, HIGH);
digitalWrite(CH3, HIGH);
digitalWrite(CH4, LOW);
}
res = res_table[n];
} // end of code.
