Place the reset button somewhere it can't be easily
pressed by curious hands. I mounted mine on the back
side of the circuit board; I use a paper clip or pen point
to press it through a pinhole in the case. Mount
everything securely in the case with enough room for
sensor cables, phone, and power connections (Figure 6).
Before installing the backup battery and diodes D2
and D3, build a small test circuit with a test load of about
200 ohms to verify that the voltage across the test load is
indeed about 4.75V, since diode voltage drops do vary
somewhat. You may need to adjust the number of diodes
to tweak the voltage, and you'll definitely need three or
more 1N4004s if you choose to use lithium AA batteries
due to their higher cell voltage. Using a low power NAP
command in the code loop reduces the current draw,
while monitoring the alarm inputs to only 10 mA. This
rises to about 15 mA while checking for glitches (using
PAUSE instead of NAP), and will go up to 50 mA or
more when the relay is energized.
Dan Gravatt can be reached at
dgravatt@juno.com.
40
August 2012