At 10:46 AM 11/12/97 GMT+2, you wrote:
>Hi All
>My PIC based alarm system have been blown up with a High Voltage
>zapper; those handheld units for shocking people/animals...
>Aparently this is a new craze among burgulars. Thy kill home alarms,
>car alarms and cct video with these zappers.
>
>Short of using screen cable for every sensor, is there other devices
>or goodies that can protect alarm systems from this assult????

Actually its not all that difficult, just takes some care.  Earlier this
year I designed a PIC based unit for a customer that had to get CE
certification for him to sell the product in Europe.  Besides RF emissions
and susceptibility tests, there is an ESD test. Essentially they take a
high tech cattle probe and zap it.  The CE requirements required that it be
able to take an 8KV zap without affecting operation.

I solved the problem with the following techniques:

1.  Good solid ground plane on the PCB.  A multi layer board with power and
ground planes would have been better, but cost was an important factor so I
had to go with a 2 sided board.  You want a very low impedance ground plane.

2.  Current limiting resistors (~100 ohms) and 5.5V MOVs on every input and
output. Harris makes some nice 1206 SMT chip MOVs.  I don't have the part
number handy, email me if you need it.  I think it was the MLA series.

3. Keep the electronics insulated from the case. Be sure there are no
openings that will allow the zapper to get at components or the PCB.  You
don't want to provide a path through the electronics to earth ground.

Good luck!

- Gary


------------------------------------------------------------------------
Gary Sutcliffe,  W9XT          Unified Microsystems
ppvvpp@mixcom.com              PO Box 133 Slinger, WI 53086
http://www.qth.com/w9xt        414-644-9036