Cheap DIY Multimeter Using Atmega328p

In this project, we gonna learn how to make a DIY multimeter using atmega328p (or Arduino UNO). In this Multimeter, I used Interrupt Service Routine ( ISR ) for one push button to switch the modes of the multimeter. There is 4 modes in our DIY multimeter : Voltage measuring (Range : 0-240 VDC), Current measuring, Resistance measuring and Diode testing. The features of DIY multimeter are that building high quality multimeter with high efficiency and accuracy in measuring the electric components.

The components we used in this project are :
Atmega328p MCU
2x 33pf capacitors
16MHz Crystal
push buttons for reset
CP2102 (or any TTL to USB Module)
1uf capacitor
5mm LED
220 ohm resistor

10k ohm resistor
7805 5V voltage regulator
1000uf 25V capacitor

The previous components are the components of Atmega328p microcontroller

You can use Arduino UNO rather than the previous components if you want
Project components:
16x2 LCD
10k Potentiometer

Buzzer
push button

ACS712 30A current measuring module

220 ohm resistor

3x 10k ohm resistor
breadboard and some wires

We used Arduino IDE to make the software of the project
the code in a glance :
we attached an interrupt in the pin 2 of arduino and the MCU will execute the routine if it identified a High-to-Low transition
the routine is that to increase the value of the flag by 1 which determines the mode of the multimeter
in voltage measuring we used a voltage divider circuit in which R1 = 10K ohm and R2 = 220 ohm and the maximum value where the output of the circuit is 5v is about 240v
the ACS712 module sends a value of 2.5 v when the amperage is zero and if the amperage increased by 1, the module will send a 2.5v + 66 millivolts

the resistance and diode testing are excuted by calculating the resistance using Ohm's Law

Voltage measuring of the multimeter measures DC voltage ONLY ! but Current measuring can measure both AC & DC current.
to measure voltage :
connect the ground of the multimeter to the ground of the circuit that it will be measured
connect the 10k ohm resistor of the voltage divider circuit to the positive voltage that you want to measure
to measure resistance and testing diodes:
connect the 10k ohm resistor terminal which is connected to the analog pin to the first terminal of the unknown resistor or the diode and connect +5v to the second terminal of the resistor/diode
to measure current:
get a load (motor or led) and connect it to ground and connect the unknown-current wire to the IP+ of the ACS712 module and connect IP- to the load