A. Complicated it is not, but certain
conditions must be met. My design
assumes that your relay emits an external magnetic field when engaged. That
usually means it’s encased in plastic,
not steel. If this is the case, the
magnetic field can be detected with a
Hall-effect device — and nothing more.
Well, you do need an LED and
current limiting resistor, as shown in
Figure 7. When the magnetic field on
the hall sensor exceeds a critical
amount — typically 300 Gauss — the
open-collector transistor turns on. This
causes current to flow through the
LED and light it. That’s it ... almost!
You’re going to have to orient the
Hall device so that it responds to the
magnetic field. This means that you
need to energize the relay somehow
and run the Hall switch over the coil
like a wand (flipping sides) until
the LED lights. Then glue it in place.
Your next assignment will be finding a
suitable Hall-effect device.
Once plentiful, these sensors are
getting harder and harder to find. In
fact, Digi-Key (800-344-4539; www.
digikey.com) has reduced its stock
to just 122 devices, only a handful of
which are in stock at any given time.
I chose the OH360U from Optek
because it meets all the criteria. Other
options are the A3122 or A3213
from Allegro Microsystems (www.
Audio VU On PC
Q. Do you know of any software
package that will take audio input
to the computer and display various
sound level graphics on a monitor, e.g.,
bar graph equalizer, VU meter?
A. In fact, I do — and I like it a lot.
It’s called DXVU Meter from xFX
JumpStart ( www.software.xfx.net/
activex/dxvu). DXVU Meter is an
1 OH360U 1k
Hall Effect Switch
ActiveX control, which can monitor
any audio device configured as a
recording source, such as a microphone or CD-ROM, and display the
monitored audio levels like a standard
VU Meter or as an oscilloscope. DXVU
Meter is also able to save the monitored audio data to a WAV file. Other
functions include a spectrum analyz-
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