Nuts and Volts - August 2012

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.