You use a zener diode as a voltage reference in whatever circuit you choose so no matter what the supply voltage is your reference will be stable (within reason). ----- Original Message ----- From: "Pic Dude" To: Sent: Wednesday, May 22, 2002 4:03 PM Subject: Re: [EE]: Voltage Compare > I did this recently with a PIC 16F872. A 5V regulator drops the ~12V > down to the operating voltage of the PIC, as well as providing the reference > voltage. The input is simply a voltage divider on the ~12V supply to limit > the voltage going into the PIC. Actually I set it up so 25.5V at the input > will divide down to 5V. This way, 25.5V reads 255 on the top 8 bits of > the A/D and I saved myself some math that way. But I do lose a bit of > resolution (not a big deal). Also have upper and lower alarms settable > with a single-button interface. > > Cheers, > -Neil. > > > > -----Original Message----- > From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On > Behalf Of Tal Bejerano - AMC > Sent: Wednesday, May 22, 2002 5:35 PM > To: PICLIST@MITVMA.MIT.EDU > Subject: [EE]: Voltage Compare > > > Hi All > > I want to measure and get a warning if car battery is drop under 11.5v. > look at some circuits that use op-amps/comparators and read the explanations > and I wonder, how can it work? if the circuit use the battery voltage to > operate and the voltage drops, then all the adjustments are not correct. as > I understand it I need 2 voltage sources: one is stable and the other is the > battery I monitor. ??????? > > Regards > > Tal Bejerano > AMC - ISRAEL > > -- > http://www.piclist.com hint: PICList Posts must start with ONE topic: > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads > > -- > http://www.piclist.com hint: PICList Posts must start with ONE topic: > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads > > -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads