In designing the AOM, my key criteria was to keep it very simple. The AOM uses a minimum of parts; take a look at the schematic and Parts List.
Like many other PIC projects I've designed, the real
power to the AOM is found in software. The AOM uses
an external RC clock, R1 and C2, for an external clock
frequency of around 32 kHz. The low frequency clock
keeps current consumption to a minimum — 168 µA to 90
µA depending on battery voltage — and allows a fresh set
of AAA alkaline batteries to last at least three months.
Microchip does not provide a lot of information as to
suggested RC values for a 32 kHz external clock ( 8 kHz
instruction clock), and after testing the AOM with different
voltages, I now understand why. It seems that the external
RC clock is very sensitive to changes in the supply voltage.
Refer to Table 1 for the measurements I made.
For other projects, this radical change in the clock
frequency would be unacceptable. For the AOM,
however, this change adds to the intensity of the
For example, when the batteries are fresh, the initial
tone is around 2 kHz every three to eight minutes. When
the batteries are near the end of their life, the tone is
around 3 kHz every two to five minutes.
I found the Datak Experimenter's protoboard with
standard IC and component spacing (#12-607) to be ideal
for building this project. If you can't find this exact board,
any small 1.8" x 1.8" standard IC protoboard will do.
Before soldering, start by drawing the parts layout on
the component side of the board; see Figure 1. As
tempting as it is to skip this step, by doing it you'll avoid
the aggravation of making a mistake and having to
unsolder and re-solder components back into the correct
place. Just be sure to double-check your work against the
schematic and Photo 1.
C1 is a tantalum capacitor; the shorter lead is
connected to the negative of the power supply. Connect
this capacitor as close as possible to the IC power supply
pins 1 and 8. This capacitor removes any noise spikes that
might reach the PIC. Also, do NOT substitute an
electrolytic capacitor. A tantalum capacitor is the best
choice because it can deal with the sudden demands for
current. The electrolytic is the worst choice because it has
the slowest response for abrupt current demands.
For all prototypes I constructed, I used a ceramic
capacitor for C2. However, I see no reason why any type
of 500 pf capacitor or other values couldn't be used. (See
"Operation & Design Notes.")
April 2015 33
Supply Voltage vs.
Voltage Instruction Clock
5.5V 5,395 Hz
5.0V 5,715 Hz
4.5V 6,132 Hz
4.0V 6,680 Hz
3.5V 7,471 Hz
3.0V 8,671 Hz
2.5V 10,655 Hz
2.2V 12,543 Hz
C1 1 μF Tantalum Capacitor
C2 500 pF Capacitor, any type (see "Operation & Design Notes")
R1 75KΩ, 1/4W ±5% recommended
R2 10KΩ, 1/4W
R3 120Ω, 1/4W
Speaker 48Ω or higher. I suggest using All Electronics CAT# SK- 63, 2-1/4" 63Ω
speaker ($1.25 each)
PIC12F629 DIP, plastic (PIC12F675 may be used as a replacement). You will need to
program the PIC.
Eight-pin DIP IC Socket
IC Protoboard, Datak #12-607 recommended
AAA, Two-cell Battery Holder or AA Two-cell Battery Holder
■ TABLE 1.