The heart of the meter is PIC16F876A microcontroller with built-in analog to digital converters (ADC) and 2x16 green backlighted LCD display.
Circuit design uses very few external components making it possible to fit this handy meter on a small PCB. The multimeter provides exceptionally accurate readings due to built-in software based calibration and the use of 1% metal film resistors. It needs only one supply voltage that can be acquired directly from the main power supply. Entire voltmeter consumes only 10mA with LCD backlight turned on and 3mA with the backlight tuned off. LCD backlight can be turned off by disconnecting 10 Ohm resistor from the LCD display. The voltage is sampled by using two connected in series 100K and 6.8K resistors.
Additionally the multimeter can be easily calibrated by temporarily connecting three (SETUP, UP & DOWN) tactile buttons or even a piece of wire to C1, C2 and C3 PIC16F876 microcontroller ports.
To enter the calibration setup mode make sure the meter is powered off. Press and hold the SETUP button for two seconds while powering the meter until "Setup Mode" message is displayed on LCD display.
After "Setup Mode" message disappears we will be calibrating the voltage readings and real time voltage reading will be displayed on the display. Connect the highest voltage to Input that you will be normally measuring then connect the commercial multimeter to input as well. We will be matching the voltage of PIC voltmeter with the commercial multimeter. Use UP & DOWN buttons to match the voltage on both devices.
Once the voltage is matched press the SETUP button to start calibrating current readings. You can lower the voltage now and connect a load from 500mA to approximately 2A in series with commercial multimeter to Output of PIC multimeter. Again, we will be matching the current readings on both meters.
Finally press the SETUP button again and calibrating settings will be saved in PIC16F876 microcontroller's non-volatile EEPROM memory. Calibration process is now completed. EEPROM memory is retained even if the power supply is disconnected. Calibration only needs to be performed once. If you ever need to change the calibration settings again you can do so be following the calibration steps. The PIC multimeter is now ready to be used in the power supply or any other project of your choice.
Current sense 0.47 Ohm shunt resistor is connected in series with load at the negative voltage rail and is passed to microcontroller chip through 100K resistor.
You can purchase a complete premium quality PIC Voltmeter Amperemeter Kit at Electronics-DIY store
. Please see the link for more details.