Nuts & Volts

transformer, then isolate the outputs
for each stepper (using a diode) and
filter each using a hefty capacitor (C =
I out / 120 * V RIPPLE ).
DOOR OPEN BUZZER
QI have a Chamberlain garage
door monitor that notifies
me when the garage door is
open or closed. When the
door is closed, a coded radio signal is
sent to a receiver, which displays a
green LED. When the door is open, a
red LED flashes once every second for
as long as the door remains open. The
system works great, but I would like to
add a buzzer in addition to the flashing
red light when the door opens.
However, I don’t want the buzzer on all
the time when I’m working in the
garage with the door open. I want the
buzzer to just initially sound for the
door opening and then stop. I was
thinking of using the “Adjustable
Buzzer” circuit in the December ‘05
column, but I’m not sure how to adapt
it for my purposes. Do you have a good
solution?
— Christopher Rust
Maple Grove, MN
the garage door illuminates the green
LED and effectively disables the timer
— until the next open-door incident.
WHICH WAY
THE WIND BLOWS
QI would like to build a weather
vane to show the direction
of the wind on a display
inside my shop. Ideally, it
would have LEDs to indicate at least
eight wind headings. Instead of a mechanical
contact at the vane, is there a
simple circuit to indicate the position
■ FIGURE 3
24VCT
17Vp-p
XL = 45 ohms
60mH
2Vp-p
Choke Input Design
QUESTIONS & ANSWERS
~10.5 VDC
44 mVp-p
Rs
+
+V
10,000uF
RL = 6 ohms
of the vane? Perhaps a series of Halleffect
components have to be used? I’m
not sure if I understand the Hall-effect
components correctly, but I’m open to
anything and willing to try (read:
limited electronic, self-taught, hobby
education).
— Larry
AWelcome to the world of selftaught
hobby electronics.
The road is bumpy, but filled
with lots of serendipity detours
that lead to exciting discoveries.
Let’s start with the Hall-effect.
In operation, a constant bias
+12V
■ FIGURE 2
AIt’s fortunate that you
have a steady green
LED in addition to the
blinking red LED, because
it’s nearly impossible to trigger
a timer from a pulsing source.
I suggest you take the trigger
signal from the green LED when
it extinguishes — the opposite of
the “Adjustable Buzzer” design.
Instead of a 555 timer, I chose to
go with a NOR gate monostable
timer (Figure 3). When the green
LED goes dark, the 4N25 optoisolator
turns off and applies a
high to the 4001 NOR gate. This
starts the timer and turns on the
buzzer. The time is determined by
t = 1.1RC — about five seconds
for the values shown.
Want the buzzer to beep in
step with the blinking red LED?
Use the optional beeper circuit
under the buzzer diagram. When
the red LED is lit, Q1 turns on
and sounds the buzzer. Closing
Receiver
Green LED
Receiver
Red LED
Break
Circuit
Break
Circuit
4N25
LED Off Buzzer
4N25
10K
+12V
10K
4001
R1
1K
4001
4.7uF
t = 1.1RC
5 Sec
4001
LED Off Beeper (Optional Add-On)
+
1M
4001
1K
R1
1K
+
-
Q1
2N2222
Q1
2N2222
Buzzer
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