R1, R2, and R3 are 1/4W resistors; higher wattage
resistors can be used if needed. R1 sets the timing for the
RC clock. While not super critical, I suggest ±5%
tolerance. R2 and R3 values are not that critical, so even if
they were off by ±20%, the circuit will still work.
The best speaker I found was a small 63Ω unit from
All Electronics. The small size and low cost offered the
loudest sound for the money. Larger 48Ω speakers also
work, but were overkill for my wishes. Using a lower
impedance than 48Ω is possible, but the volume is
In earlier versions of the AOM, I did use 48Ω mini
transducers (All Electronics CAT# PE- 52), but even with
two in series, the sound was not loud enough for my taste.
Standard AAA alkaline batteries should last for at least
three months. If you wish to use standard AA alkalines,
the minimum running time would be increased to at least
six months. I chose two AAA batteries in a holder because
they were small and easy to conceal. Still, even AA
batteries aren't so large that the unit couldn't be hidden
The IC is the PIC12F629, but the PIC12F675 will also
work in its place. I strongly recommend using an IC
socket. Should you wish to reprogram the chip, it's easy to
Pins 3, 4, and 5 of the IC allow for special test
features and an additional function for the AOM. Pins 3
and 5 make use of the internal weak pull-up resistors
inside the PIC. Pin 4 uses the external pull-up resistor, R2.
While you don't have to wire any connecting wire or post
to these pins, I recommend wiring at least one wire to pin
4. Once built, this pin allows for quick and easy testing.
The test/feature pins are defined as follows:
Pin 3 GP3 Fast: When connected to ground, it
shortens the timing cycle by around 1/120 of the time.
This causes the unit to beep every few seconds
(random time period); (J1).
Pin 4 GP3 10 Sec: When connected to
ground before power is applied, the AOM beeps
around every 10 seconds; (J2).
Pin 5 GP2 Pause: When connected to
ground, this pauses the time count; (NC).
Once built, you could install the batteries and
wait roughly three to eight minutes before you
might hear a beep. Both pins 3 and 4 allow you to
shorten the time you need to wait.
There are two choices for speeding up the
beep cycle: pin 3 and pin 4. Pin 4 (J2) — the 10
second cycle — provides the most information
about the circuit. Remember, this pin must be
grounded before the power is applied. Once the
power is applied, if you measure the time between
each beep, you'll have a reasonable estimate for
the instruction clock frequency, the initial beep
frequency, and the RC clock frequency.
34 April 2015
■ FIGURE 1.
■ PHOTO 1.