83,000 / Seconds Between Each Beep = Instruction Clock
This should give you a reasonable estimate of the
instruction clock's frequency. With fresh batteries, it
should be close to around 8 kHz. As the batteries age, the
It takes four instruction cycles to produce a single
cycle of beeps:
Instruction Clock / 4 = Beep Frequency
The RC clock is always four times greater than the
instruction clock. So, multiply the instruction clock
frequency by four to determine the RC clock frequency.
With fresh batteries, it should be close to 32 kHz.
Grounding pin 3 (J1) shortens the time delay by
roughly 1/120 of the time. This provides a quick way to
hear the random nature of the beeping without waiting
Grounding both pin 3 (J1) and pin 4 (J2) generates a
rapid string of beeps. This proved to be ideal for
measuring and testing the inductive kick in the speaker.
More on that in "Operation & Design Notes."
Pin 5 (Pause) is for possible expansion. While I have
not designed any circuit or tried anything yet, one could
attach a phototransistor or some sort of photodetector
circuit to this pin. Thus, when the lights are on, no beeps
occur; when it's dark, the beeping resumes. This might be
perfect in a bedroom where the victim only hears the
beeping when it's dark. If you chose to explore this type
of operation, you may want to shorten the time between
the random beeps.
The one area I spent the most time testing and
exploring was the speaker. Using a 120Ω resistor for R3 is
a very conservative approach to the design. This allows
you to use any impedance speaker without drawing too
much current from pins 6 and 7.
The All Electronics (CAT# SK- 63) 2-1/4" speaker is
roughly 63Ω resistance and around 1.5 mH of
inductance. Even under worst case conditions — an output
voltage of 3.6Ω and the frequency of the tone is 1.5 kHz
April 2015 35
■ PHOTO 2.