This is a small, hand-made, 3 digits voltmeter with PIC18F1320. The voltage is displayed on three common anode 7 segments LED displays of your choise. They are multiplexed and driven by small transistors. The maximum input voltage is 30 VDC and can be trimmed with a small trimmer resistor that changes the ratio between the two resistors of the input voltage divider. The analog input of the μC is protected by a small zenner diode rated at 5.1 volts. As result, you can see volts with one decimal digit.
The displays need current limiting resistors (R8-R15) in series with the μC outputs. Their values depend on the specific type of display you gonna use. I used resistor networks of 220 Ohms. They can be clearly viewed in the next five pictures.
The circuit is supplied by an LM7805 regulator and is protected from opposite polarity with a diode. The PIC18F1320 is a little bit overkill for such a circuit, but I had it “sitting” in my drawer. Everything was assembled on a small piece of breadboard and wired with Kynar wire. For long lasting opperation, a small heatsink is good to be placed on the regulator. Some pictures of this one-afternoon project:
The code was developed with MPLAB IDE and HI-TECH C Compiler and is free-to-use for anyone who desires to make this mini voltmeter. Just use the link below:
In this link, you can find seperate folders for firmware and hardware. The schematic was designed with Altium DXP, but there is no PCB.
There is also a github repository at https://github.com/manolena/PIC18F1320-mini-voltmeter.