worry that he will not be
able to defend himself. I
would like to build a cat
alarm that would sound a
buzzer if he passes
through one of two doors.
I was thinking of
something similar to the
“theft protect” used in
stores, but do not know
enough about these
systems and they are likely
Another thought was
to use a low power
transmitter on the cat’s collar, then
each door would have a receiver and
alarm buzzer. If the cat goes out the
door, the alarm would sound,
warning us he has escaped so we
would have time to grab him before
he got too far. Any ideas on a design
that would work for this application?
Thanks for your help.
— Bill Blackburn
■ FIGURE 3.
transmitter is shown in Figure 3. The
433 MHz transmitter module is
available from several sources.
A Google search of the part
number will find it for you.
The QAM-TX1-433 and TLP434A
are thru-hole, but I chose the QAM-
TX1 because it is available from Digi-Key. The QAM-TX3-433-S
and –ND are surface-mount for reflow
soldering, but you can
hand-solder it if the pads
are made large enough to
extend beyond the
substrate so that a
soldering iron can reach
them. I don’t know how
the ASK is implemented
or whether it is linear or
not, so I tied the input
high and turned the
power on and off to
implement two level ASK. It makes
no difference to the battery. The 555
output is high for 90 mS, turning the
transmitter off; and low for 10 mA,
turning the transmitter on (100 Hz
frequency). The receiver is an
inexpensive AM type available from
Quasar UK (part number QAM-RX4-
AAn RFID system will require research and experimentation, and unfortunately, I don’t
have time for that. A transmitter
on the cat will require a battery
that will last at least 24 hours.
A three volt lithium coin cell
has a rating of 350 mAh at 25 deg
C and 1 mA load. The battery will
last two weeks and costs $6 —
that’s $72 per year; do you feel
the cat is worth it?
Now to find a transmitter that
will run on 1 mA. Unfortunately,
the low cost transmitter vendors
do not supply a datasheet, but
even if the transmitter draws 10
mA it could be turned on and off
with a 10% duty ratio for a 1 mA
A very efficient antenna can
be had that is inexpensive and
about one inch by 1/2 inch.
However, since the cat will be
within three feet of the receiver at
the door, an efficient antenna will
not be required. A short piece of
wire should work. My idea for the
24 August 2013
■ FIGURE 4A.
■ FIGURE 4B.