•

Published March 20, 2022 © GPL3+

Precision DIY voltage-current meter

Using ATtiny24's capable ADC, this probe can measure voltages from 1mV to 20V and currents from ~1mA to 2Amps with reasonable accuracy.

IntermediateFull instructions provided10 hours466

Things used in this project

Hardware components

Microchip ATtiny24A

Slide Switch

Software apps and online services

PlatformIO IDE

Hand tools and fabrication machines

Soldering iron (generic)

Story

The Idea:

I usually had problems when I wanted to measure supply voltage and current consumption of a circuit using multi-meters. They can measure either voltage, or current and can't measure them simultaneously; You have to change probe terminal and the selector to measure each one at a time. That's the idea behind this device.

ATtiny microcontrollers are usually used in little projects where the demands on memory or pin count is low. So their other capabilities are usually overlooked.

ATtinys' capable ADC:They [not all of them] have a very capable ADC with 20x gain, differential inputs with uni-polar or bi-polar measurements and a 1.1V reference. You can use them to measure very tiny signals like thermocouples or RTD sensors.

I used an ATtiny24A for this project. as the internal 1.1V reference has a big error, I used a 2.5V voltage reference with a 1/3 divider to provide 0.8V reference for it. you have to measure the VREF on the PCB with a multimeter and write down the value in the defined part of code in milli-volts, e.g. 806.

The features:

It's Self powered; it doesn't use the measured circuit's supply. so it can measure tiny voltages like 10mV
The main board has a header input. you can design any probe with that same 3 pins layout to best suit your case
It automatically changes the range to show the value with best precision
The code uses 54 bytes out of 128 bytes of RAM on ATtiny24A, and 1234 bytes out of 2048 bytes of flash. there is a lot of room if you are going to make it better.

1 / 3 • The main board assembled on 2xAA battery holder + external probe

Using the onboard push-button you can switch between these values:

The calculated internal voltage reference (you should read something very near to 1100. my device shows 1084)
The voltage of the probe. It automatically switches the "dot" point to show 2 digit voltages. (0.000 to 00.00)
The current flowing through the probe. It automatically switches the "dot" point to show 2 digit currents. (0.000 to 00.00)

Zoom in the schematic to read the text carefully

The things you should tweak in the main.cpp file:

The VREF:measure the VREF on the PCB with a multimeter and write down the value in the defined part of code in milli-volts, e.g. 806.
The Divider value:Measure the resistor divider ratio and write it down. as the resistors have errors, measure R6 and R7 with a multimeter, calculate R6/(R6+R7), multiply it by 10 and write it down.
The Shunt resistor value:Measure the shunt resistor with a multimeter and write it in milli-ohms

Write the measured values like that in the main.cpp file

Schematics

Code

The main.cpp

#include <avr/io.h>
#include <util/delay.h>
#include <stdbool.h>
#include <t7segment_sr.h>
#include <avr/interrupt.h>

#define EX_VREF   804UL // write the measured VREF in milli Volts
#define RES_DIV   232UL // write 10x the exact value of resistor divider. 232 means 23.2
#define SHUNT     125L  // write or tweak the shunt resistor value in milli ohms



uint16_t sample(uint8_t ad_mux_chn);
void adc_init(void);
uint16_t  sampling_interval, key_interval, vref, vin, iin;
extern volatile uint16_t milli_tick16;

//0 = VREF, 1 = VIN, 2 = IIN
uint8_t sample_mode = 0;
uint8_t display_mode = 0;


int main()
{
  CLKPR = 0b10000000;
  CLKPR = 0b00000010;
  DDRB  = 0xFF;
  DDRA    |= (1<<DDA4) | (1<<DDA5) | (1<<DDA7);
  PORTA   |= (1<<DDA6);
  TCCR0B  |= (1<<CS01);//divide by 8.
  TIMSK0  |= (1<<TOIE0);
  adc_init();
  sei();
  t7seg_write_int16(sample(0b000010));
  while(1)
  {
    if(milli_tick16 - sampling_interval > 200)
    {
      if(sample_mode == 0)  vref = (EX_VREF*4095L)/sample(2);
      else if(sample_mode == 1)  vin = ((uint32_t)vref*sample(1)*RES_DIV)/40950UL;
      else if(sample_mode == 2)  
      {
        uint16_t temp_var = sample(0b001011);
        if(temp_var < 4000)
        {
          iin = (((uint32_t)vref*sample(0b001011)*1000L)/(4095L*SHUNT*20))  - 1;
        }
        else
        {
          iin = (((uint32_t)vref*sample(0)*1000L)/(4095L*SHUNT)) - 1;
        }
      }
      sample_mode++;
      sample_mode %= 3;
      sampling_interval = milli_tick16;
    }
    
    if(milli_tick16 - key_interval > 250)
    {
      if(!(PINA & (1<<PINA6)))
      {
        display_mode++;
        display_mode %= 3;
        key_interval = milli_tick16;
      }
    }

    if(display_mode == 0)
    {
      if(vref < 10000) t7seg_write_int16(vref);
    }
    else if(display_mode == 1)
    {
      if(vin < 10000) 
      {
        t7seg_write_int16(vin);
        t7seg_set_dot_single(0,1);
      }
      else
      {
        t7seg_write_int16(vin/10);
        t7seg_set_dot_single(1,1);
      }
    }
    else if (display_mode == 2)
    {
      if(iin < 10000) t7seg_write_int16(iin);
      t7seg_set_dot_single(0,1);
    }
    else if (display_mode == 3)
    {
      
    }
    t7seg_looper();
  }
}

Credits

Tirdad Sadri Nejad

6 projects • 21 followers

AI masters degree student. a guitar player and an electronic hobbyist.

Contact

Amir Kazemi

4 projects • 3 followers

Material science engineering MSc nano material researcher

Contact

Comments

Please log in or sign up to comment.

Precision DIY voltage-current meter