Here is an example by Darrel Taylor for a voltage divider circuit. This makes it possible for a PIC to give an analog measurement for voltage that is more than 5v.
This example divides a 25 volt source into about 4.38 volts for the PIC's A/D converter to handle.
The ratio of the voltage divider is (R1+R2)/R2. To scale up the A/D reading you just multiply the value times the total resistance of the divider (R1+R2) then divide that number by R2.
If the input is 25Vdc then the output voltage will be 25v/((R1+R2)/R2) or 25v / ((4700+1000)/1000)) = about 4.38 Vdc. With 10-bit A/D you should get an ADCin of around 898. If you first scale that A/D value up to match the voltage divider ratio, you can then calculate the voltage the same way you would if you were reading 0-5 V.
898 * 5700 / 1000 = 5118 This is what the A/D would be if it could actually read 0-25 volts directly.
Now then, for 1 decimal place, multiply that by 50 and divide by 1023. For 2 decimal places, multiply by 500 instead.
5118 * 50 / 1023 = 250 or 25.0Vdc
The program below is one way to do it in PBP.
Res1 Var Word Res2 Var Word Rt Var Word Volts Var Word AD Var Word GetReadings: ADCin 0, AD Res1 = 4700 ' Change these to match your Voltage divider Res2 = 1000 ' resistor values for the Solar Cell Gosub CalcVoltage LCDout $FE,2,"Solar= ",DEC Volts/10,".",DEC1 Volts Dig 0," Vdc" ADCin 1, AD Res1 = 2200 ' Change these to match your Voltage divider Res2 = 1000 ' resistor values for the Battery Gosub CalcVoltage LCDout $FE,$C0,"Batt = ",DEC Volts/10,".",DEC1 Volts Dig 0," Vdc" goto GetReadings CalcVoltage: Rt = Res1 + Res2 ' Total resistance of Voltage Divider Volts = AD * Rt ' Scale the AD reading accordingly Volts = DIV32 Res2 Volts = Volts * 50 ' Convert AD to Voltage Volts = DIV32 1023 Return
This article was originally published in forum thread: Using the A/D for Monitoring a Solor Cell and Battery started by chuck.sieveking