Terry,

You can take an enhancement power MOSFET and put in the negative return
line from your circuit. The gate is driven through a 1 Meg resistor from the
positive battery terminal. A correct power connection drives the MOSFET
fully
ON, and everything works. Reversing the supply connections turns the MOSFET
OFF,
and no current flows and you're protected. Correct the power connections,
and you're back to normal. There is very
little voltage drop across the MOSFET. Select a MOSFET with a low "R(on)"
resistance, and there will be very little voltage drop.
You may also require a 5 volt logic mosfet, which will turn on around 2
volts.

The diode approach is probably the best rather than the above if it is for
an inexpensive project.

David




<Hello List,
<
<I have realized an alarm for my bycicle using a 12C508. It is powered by
two AA
<size alkaline battery.
<
<I would like to know if there is a simple (and economic!!) solution which
<I can use to protect my circuit against polarity inversion during battery
change.
<(I know it's a remote possibility, but it exists)
<
<I thought to use a serie diode, but the voltage drop bring the PIC power
voltage
<near the power supply limit of the MCU.
<
Thanks.
<
<
<Best regards,
<Michele