LCD voltmeter circuit

Posted on Feb 6, 2014

You can use any PIC microcontroller that has an ADC and enough memory to hold the program. The LED is pulsed after every ADC acquisition to indicate that the processor is alive - so you can tell if the software is active. You can recompile the lcd volt meter files if you want examine code operation (using the built in simulator) or change the source code. Note the hex file is contained in the download. The code is simple showing how to set up the ADC using direct control (it does not use built in routines) so that the code can easily be ported to other compilers. It enters a continuous loop collecting ADC samples and displaying them on the LCD. The only complexity is that the code uses fixed point maths so that it avoids using floating point variables (saving memory space). Long variables are used to scale the output result.

LCD voltmeter circuit
Click here to download the full size of the above Circuit.

Note: The circuit does not feed in an accurate voltage to Vref+ so the accuracy of the reading is only as good as the power supply voltage regulator (not shown on circuit diagram) - this will usually be a 7805 standard 5V regulator. This has an accuracy of 5% so the displayed output will be wrong by 5%. If you want to calibrate the output to get a more realistic a reading you can set the scale factor in the software to match the 7805 output voltage value. All you do is use a multimeter to measure the power supply (mine read 4.91V) then instead of the 'ideal' scale factor of 50000L change it to match the multimeter reading e.g. for 4.91V use the statement lng = (ADC_ip * 49100L) >> 10 This scales the maximum ADC output to 4.91 (The maximum reference value) and displayed readings will now match the multimeter readings. For production use this is not a good idea and you would be better off adding a voltage reference.

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