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flashing emergency lights. The
resistors R3 and R4 control the rate of
charge while the discharging capacitor
facilitates the switching between the
red and blue LED.
Play with the capacitor value and
the R3/R4 resistors to fine-tune the
flashing oscillations. I think diffused
LEDs would make a better effect than
the clear ones I used, but I just had
them laying around.
The NE555 is small (the LEDs are
bigger, so hopefully you can get it to
fit in your design.
See the circuit in action at www.
However, if I reconnect the load — a
90 VDC brake — and have the output
switching device open, the measured
DC output voltage at the rectifier is
approximately 30% greater than the
disconnected output voltage.
off when the main airhandler turns on
The most likely explanation is that
you have a capacitive load on the
rectifier output when you are
measuring the higher output voltage.
You should measure 1.414 (√ 2) times
the input voltage minus diode drops
with a well working peak rectifier.
Since you are measuring close to this
value, this is probably the culprit.
#2 Try using simple 3-5 VDC
flashing LED[s]. Connecting two
small-sized coin batteries together
will provide a fairly long continuous
life cycle. They can be purchased
from vendors such as www.all
They have an excellent supply of
T1-3/4 and T-1 sizes in four colors.
Saint Johnsville, NY
[#11094 - November 2009]
Vehicle ECU Programming
I would like to know what kind of
programming is used in a car ECU. Is it
C, C++, Visual Basic, or something
[#11093 - November 2009]
Bridge Rectifier Voltage Readings
Why, when I read the open circuit
DC voltage with both sides of the
output disconnected from the rectifier,
I read the input voltage ± 10%?
Probably binary code. Computers
speak binary. The translation from
compiler language to binary is
performed in the design lab and then
released to production as a binary file.
[#11096 - November 2009]
Wireless HVAC Fan Controller
I want to install a
duct fan in my
HVAC system. But the
duct where the fan
will go is in the attic
and the main
airhandler is in the
basement. The fan
runs on 120 VAC and
pulls 2A. I need an
system that would turn
the duct fan on and Figure 1
#1 Assuming you simply do not
want to run a control wire between
the auxiliary fan and the air handler,
here's a solution.
X10 powerline control has been
around for a long time. It's an old
technology but in most cases will work
just fine. You can get an X10
powerflash module PSC01 to plug
into an outlet near the air handler. This
device accepts a dry contact closure
of up to 18 volts AC or DC on the
control input. You could wire in your
24 VAC air handler fan control signal
through a voltage divider to drop the
voltage below 18V, or you could
mount a vane switch inside the air
path of the air handler. A vane switch
is a small sensitive microswitch (one
that requires very little pressure to
operate the switch); install a sail on the
switch lever. The sail is nothing more
than a piece of lightweight thin
cardboard or similar material. When
the air "blows on the sail," the switch
will click on. Either of these methods
will cause the PSC01 to send an X10
command into the AC power line. At
the other end, get an X10 appliance
module AM466 and just plug the
auxiliary fan motor into the module.
The advantage of the vane switch is
that your HVAC technician won't
accuse you of causing problems with
the system. But it's mechanical and
more prone to failure than a direct
Set the house code and unit code
the same on both X10 units. Set the
switches on the PSC01 to Mode 3
(only transmits a single house/unit
code, on/off). Set to Mode A for up to
18 VAC or DC control input or Mode
B for dry contact control.
Of course, one side effect with
the older X10 technology is that your