ULTRA LOW POWER CMOS
My family recently moved into a
two-story house with a water heater in
the attic just above our bedroom. This
situation made me somewhat nervous,
especially after hearing stories about one
of our neighbors who had returned from
a vacation to find more than $10,000
worth of water damage from a leaking
They did have a drip-pan under the
heater but as often happens, the drain line
did not work. I don't get into my attic very
often, especially in the summer. As it was,
my first indication of a problem might be
when water started dripping from the light
fixtures. I needed a water alarm.
By Michael Mullins
To my surprise, I was unable to locate a suitable water
sensor with an alarm. I wanted a battery powered
device that would operate unattended for at least one
year. It needed to be inexpensive because I also wanted
to place devices in other places in the attic, especially
near vents where leaks might occur.
Battery operation requires careful attention to power
demand. For example, a 9V battery has a typical capacity
of about 600 mAh. For a 9V battery to last a year, this
means the average power draw must be less than 68 µA
(600 mAh, 8,760 hours/year). A power draw of 50 µA is
a more reasonable target because at the end of the year
there needs to be remaining power to sound the alarm.
➤ Design #1 — I started out with a microcontroller-based design using a float switch as a sensor. This
functioned, but it did not meet my power budget. It was
not difficult to lower the average power draw for the
microcontroller using sleep functions and a slow clock.
However, the linear voltage regulator necessary for the
5V microcontroller drew far too much quiescent current.
I considered using a pair of AA batteries and using a low
voltage microcontroller, but this introduced another problem. Piezoelectric buzzers generally need higher voltages
for adequate sound output. It would be possible to add a
step-up switcher to resolve this issue, but now the circuit
■ PHOTO 1. This useful device can be
assembled in two hours.
was becoming complicated. For a nice example of how
this can be done for a glass-breaking sensor, there is an
application note from Texas Instruments using one of their
low power microcontrollers (R. Kammel, K. Venkat, Texas
Instruments Application Report SLAA351, Apr 2007).
Rethinking the Problem
➤Design #2 — How about the old 4000 series CMOS
devices I had in my junk box? They have extremely low
current requirements at idle, and with their wide range of
power supply voltage (3-15V), they don't need a voltage
■ FIGURE 1. Gated
oscillator from one
CD4001 NOR gate.
June 2009 41