piclist 1999\04\06\221015a >
Thread: Single-button combination lock project
www.piclist.com/techref/microchip/devices.htm?key=pic
picon face BY : Harold Hallikainen email (remove spam text)



       There is a similar single button combination lock in the Ideas For
Design of a recent EDN.  That was done entirely in hardware, however.
       This sounds like it'd ideally be a Morse Code receiver.  Just key in
your password to open the door.  This could also be a little like bar
code decoding in that we need to judge the width of a pulse in relation
to the width of other pulses in the stream.  Years and years ago I wrote
some 6800 code to save and load files from a tape drive called a "stringy
floppy."  On recording I sent a transition some time period after the
previous transition.  I think a 0 was one time period and a 1 was 2 time
periods.  I established a threshold that was sqrt(T0*T1) where T0 is the
time of a zero "pulse" and T1 was the time of a 1 "pulse" (pulses in
quotes since I timed between a positive edge and the succeeding negative
edge for one bit, and the negative and the next positive for the next
bit).  The duration of each zero and one bit were stored and used to
recalculate the threshold.
       So, in Morse code, a dah is three times the duration of a dit.  I'd
probably force the password to start with a dit so you can get an idea
the speed of the doorbell operator.  Then set a threshold based on this
duration and sort the dits and dahs to figure out the characters being
sent.  As they are being received, the threshold can be adjusted for
operators who speed up or slow down.


Harold



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