November 2013 21
QUESTIONS and ANSWERS
Video Monitor Alarm
QI have been reading Nuts & Volts for over 13 years now and have greatly enjoyed the Q&A section. I now
must pose a question that I’m hoping
you can answer.
I’m using a 1.2 GHz 30 IR LED
wireless camera and receiver to
monitor my backyard both day and
night. The receiver has two RCA
output jacks: audio and video. I am
looking for a circuit that can generate
a trigger from the video output when
something new comes into the field
of view. The trigger would either be
used to activate a recording device
or visual signal such as an LED. I have
an MC9S12 Dragon12 development
board and have been trying to use
the analog-to-digital converter input
to monitor the analog video output
from the receiver.
— John Foster
AI am sure that it is possible to do what you are attempting using digital signal processing, but I’m
an analog engineer and have no idea
how to do it. What I would do is
rectify and filter the analog video
signal and use a window comparator
to set the alarm if the level went
higher or lower.
The time constant of the filter
would need to be short enough that
the change from daylight to dark
would not cause an alarm, and some
way to deal with passing clouds
would be needed. The DSP (digital
signal processor) would need to deal
with those issues also.
A better solution in my mind is
to use a motion detector — preferably
one based on heat so that moving
branches due to wind will not be
AI have reproduced your circuit here (which was in the September issue) for clarity in Figure 1.
The smaller coil apparently stores
less energy but the field collapses
faster, so it produces a higher voltage
but less current.
If you use a 555 one shot for
your switch circuit (you will find the
circuit and instructions on its
datasheet) and power the U1 circuit
from the 555 output, the power
increase will be minimal.
I doubt that your circuit is
stressing the 12V 5A supply, but
more voltage input will give more
voltage output. However, you run the
risk of internal breakdown of the
insulation of the coil if you exceed
14 volts at the coil. I assume that you
are using an oscilloscope to measure
the voltage loss at the coil because
an analog meter will only measure
the average, which depends on the
duty cycle; a digital meter won’t give
any useful reading.
The waveform at the drain of Q1
should be a ramp as the current in
the coil increases with a sudden rise
to two volts as the coil saturates. You
don’t want the coil to remain in
saturation because it just wastes
power and heats Q1. Reduce the
pulse width to minimize the time in
saturation. You can measure the duty
cycle and the average current from
the source and calculate the peak
current; see Figure 2.
For excellent reliability, the
voltage rating of electrolytic
capacitors should be double the
applied voltage, but for hobby use
you can relax that because the caps
normally exceed their rating and a
failure is merely a nuisance.
■ FIGURE 2.
Can’t figure out
that pesky circuit
Let Russ help!
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