module, and then tap the button, the circuit may not be
on long enough to power-up and send the signal. I need
to be sure the transceiver has enough time to fully power-up, send a signal, then continue to stay on long enough to
receive a confirmation signal. This, in turn, lights the
doorbell button to let the user know the bell has actually
The processor provides the necessary logic and
timing. In this particular scenario, the pushbutton is only
used to turn the system on. Once the processor has
performed its tasks, it goes to sleep on its own, waiting for
the next visitor.
The entire package fits into a plastic single-gang
junction box behind the doorbell button. When idle, the
circuit draws less than 50 nA. On average, the system is
powered to 20 mA once a day for only a few seconds.
The batteries will easily last more than a year, which was
the design target.
Figure 3 includes a PIR motion sensor trigger circuit
for the same wireless doorbell project. I have several
transmitters, and the receiving base station plays a
different tone depending on which transmitter is triggered.
Panasonic makes a low power PIR motion sensor that
draws less than 1 uA while waiting to detect motion.
When motion is detected, the signal is amplified by an
NPN switching transistor that, in turn, triggers the interrupt
on the processor, waking it up.
In addition to driving the radio with the necessary
timing, the processor has some delays built into it so the
receiver isn’t constantly ringing while something is passing
the motion sensor. The radio is turned off during these
delays. Figures 4 and 5 show the front and back of the
wireless motion sensor module.
The front has the radio, antenna, and battery pack.
The back has the PIR sensor, power control, and
processor. This is designed to be mounted discretely in a
junction box with only the sensor dome exposed. I used
OSHPark (see References) to manufacture the circuit
board. Assembly is done via my toaster oven reflow
I hope this has inspired you to “toss the toggle” and
include a pushbutton power control into your next project.
While I have focused on a particular microcontroller and
compiler, these principles can be applied to other
hardware and software development systems. Email me
with questions and/or comments at firstname.lastname@example.org.
November 2014 45