Marsy Electronic

Link Articolo Originale: http://www.best-microcontroller-projects.com/lcd-volt-meter.html

So the input range is from 0V to 5V - the maximum source impedance is 2k5 (for testing use a 1k pot). To improve the circuit add an opamp in front to present greater impedance to the circuit under test. The output impedance of the opamp will be low which is a requirement of the PIC analogue input.

Specification

The LCD volt meter uses the microcontroller power supplies as reference voltages and so it is not likely to be very accurate (e.g. a 7805 5V regulator has an accuracy of 5%). You can make it more accurate by setting the scaling value in the software to match the power supply(measuring the supply using a voltmeter) (see Calibration later).

The most accurate way of all will be to use a voltage reference and feed this into RA3 as the positive reference value but this project simply provides a quick and easy way of measuring volts without circuitcomplexity.

The project mainly shows how to control and setup the software to read an analogue value from the first analogue port RA0.

Note when designing using the analogue inputs its a good idea to start from AN0 as there is only a limited set of ways that more analogue ports can be selected for use (see the chip data sheet - analogue inputs).

For a tutorial on compiling these lcd volt meter files click here.

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.

PIC lcd volt meter using HD44780 LCD.
(Click diagram to open a pdf).

PIC lcd volt meter Hardware

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 program the PIC in circuit through the ICSP connector.

LCD volt meter Software

Project files for the PIC frequency counter circuit

Compiler project files
ADC_4MHz_LCD.ppc

C Source files.
ADC_4MHz_LCD.c
bcd.c

Header files.
bcd.h

Output files
ADC_4MHz_LCD.hex

Description

ADC_4MHz_LCD.c

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. This statement

lng = (ADC_ip * 5000L) >> 10;

Scales the ADC input value to 5000 and then divides by 1024. The output is a number between 0 and 5000. e.g. for a 2.5V input voltage the ADC will read 512. 512 * 5000 is 2560000 Dividing by 1024 gives 2500. So using the fixed decimal point between the 2 and the 5 gives the final result displaying "2.500"

The code converts the 10bit ADC number into a displayable voltage reading.

Calibration

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 acuracy 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 5000L change it to match the multimeter reading e.g. for 4.91V use the statement

lng = (ADC_ip * 4910L) >> 10

This scales the maximum ADC output to 4.91 (The maximum reference value) and displayed readings will now match the multimeter readings.

Obviously this is not the best way to make an accurate volt meter - it would be better to use a more accurate power supply or feed in an accurate voltage reference to Vref+. But it does give a quick and easy method with minimal components.

bcd.c

You can find notes on operation of this code here.