The schematic of the circuit board. The values of capacitors are in uF,
and resistors
in Ohms (unless where noted). Numbers in bold were sent to me
by
Bart McCormick
(sterling_sound.at.bellsouth.net) on 9/05. I have not
verified
them. Thanks to him.
Starting in the top
left, the power
supply section consists of the two AAA batteries and the C11
electrolytic capacitor. The C19 ceramic is the bypass cap for
the
PIC. Below it in the schematic is the Sun Sensor, which
employs a
standard Cadmium Sulfide cell to detect the light level. Its
resistance drops with increasing light, thus the output at the jumper
R1 goes low when it is daytime. That is connected to pin 5 of
the
PIC, which also serves as the control input for the red LED.
Due
to the high value of R14, the Sun Sensor is unable to provide much sink
current to turn
on TR4 to light the LED, but the PIC (with its totem-pole output) is
able to do that without difficulty. Presumably, the code in
the
PIC programs pin 5 as in input to sense the light level, but drives it
low to turn on the red LED.
The Motion Sensor portion in the top right starts with the output from
a pyroelectric sensor that connects to the U2A
TLC27
Op-Amp.
This, along with the
RC following it formed by C12 and R18, is configured as a band-pass
filter with a peak pass frequency of about 1 Hz and a peak voltage
gain of about 233 (47dB). The result of a SPICE simulation is
shown below. The plot is the ratio of the input voltage of
U2B
over the input voltage of U2A.
Characteristics of band-pass filter formed by U2A, C12, R18 and
associated components.
The filter peak is around 1Hz and 47dB (gain of ~233).
The output of U2A is then threshold detected by
U2B, which feeds pin 7 of the PIC via a 1Meg resistor. This
latter
pin also serves double
duty, and provides current to light the Green LED when the internal
code drives it high. Pin 3 of the PIC forms an enable signal
for
the threshold
detector. From observations of the operation of the circuit,
this
line goes high (disabling the threshold detector) for 10 seconds after
every X-10 command
sent via RF. The need for this disable is unclear.
Also,
why this
disable function is not performed in software inside the PIC (and thus
saving one pin) is not clear to me either.
Key components of the motion detector are the three RC pairs around
U2A. To ignore false triggers associated with slow thermal
events, we could shift the peak of the band-pass higher in
frequency. This can be done by decreasing the value of the
capacitors. More on that is mentioned in the modification
section
below.
Sensitivity could in principle be increased by decreasing R5, however
range is controlled geometrically by the arrangement of the fresnel
lens in front of the pyroelectric sensor, and increasing the gain could
degrade the reliability of the sensor in terms of false trigger events.
The transmitter is keyed by pin 2 via R3. It is perhaps
possible
to control the power level with R3, but since I did not bother with
tracing the transmitter itself, I do not know for sure. That
section is encased in wax to stabilize the components, and I did not
want to destroy this particular unit by disturbing it.
Perhaps I
will trace this section in the future.
The other parts of the circuit such as the two switches for the user
interface are minor, and not worth much mention. The
microcontroller is a
12C508
from Microchip, and uses its internal RC
oscillator, which means it is running at around 4MHz, or about
1MIPS. I would think that its code is protected, and that
reading
it would be fruitless. It is worth noting that the 1 minute
expected duration for the OFF command delay was really about 70 seconds
with this particular sensor.
Modified band-pass filter response. The peak frequency is now
shifted up to 3.3Hz.
