> -----Original Message-----
> From: Steve Kasm [SMTP:kcer@COGECO.CA]
> Sent: Saturday, February 09, 2002 6:04 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [EE]:Voltage Comparision Help
>
> Hello,
>
> I have a D.C voltage that sits at 4.35v in a normal state, and drops to
> 2.17v in a second state.
>
> I need a logic output that gives me a logic low or a simple path to
> ground when the above voltage
>
> Drops to 2.17v and sits at a logic high level when this voltage is at
> the 4.35v level.
>
> Is there any simple way to do this with using a comparator chip, can it
> be done using discrete components or a transistor. I need to reset a
> 16F84 processor when this voltage drops to 2.17 volts.
>
You could do this with a comparator and a few resistors, but there are 3 pin
IC's that are designed to do exactly what you required.  They are known as
CPU supervisors, or power supervisors.  As well as monitoring the voltage,
they ensure that the MCLR line is pulled low for the correct amount of time
for a proper reset.

Microchip sell their own range, details at
http://www.microchip.com/1010/pline/analog/anicateg/power/pwrmang/super/inde
x.htm as do Maxim, TI, Analog Devices and a host of other companies.  They
cost barely more than a comparator in most cases, and come in nice TO92
packages for through hole, or SOT23 for SM applications.

Mike

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