Grid Dip Meter
A Ramsey E
BC D F
- 9V +
A. I know they are not easy to find
— but they are plentiful in old
radios of all kinds. The best source is
from antique radio sales, like Surplus
Sales : www.surplussales.com/
html (CAV) MAPC-50ME. Can’t
afford these treasures, you say? Try
the circuit in Figure 5.
In place of the mechanical variable
capacitor is a semiconductor varactor
diode. A varactor is a reverse-biased
diode that changes capacitance with
voltage. The greater the voltage, the
lower the capacitance. FYI — any
diode can be used as a capacitor in
the reverse-bias mode. The varactor
is special in that it has
predictable characteristics. You
may notice that the schematic
isn’t complete — I’ve just highlighted the varactor section. For those
readers who missed the original
design, I’ve posted it on our website
( www.nutsvolts.com) under
Squelch the Squawk
Q. I have a smoke detector in the
garage, but I cannot hear the beeper
with the door shut. I tried using one of
the Ramsey kits, “The Voice Switch
(VOX).” When I use it with a relay and
small siren, it works, but you have to
reset it manually. Can you help me?
The technology builder's source for kits, components, supplies, tools, books and education.
A. Think about it for a minute. The
VOX is activated by the sound of
the smoke alarm, right? So when the
siren goes off, what does it produce?
Louder sound — sound that prevents
the VOX from turning off. The best
way to prevent this is to pulse
the siren on and off to break the
loop. This can be done using a car
alarm that you can buy at an auto
parts store or by using the circuit in
This is a simple 555 astable timer
with an on/off time period of about
two seconds that drives a relay
(RadioShack 275-005) to pulse your
siren. Not only does it break the feedback path, but a pulsating alarm
draws more attention than one that
just screams. To minimize feedback
from the siren, attach the microphone
as close to the smoke detector as
possible, maybe with duct tape. NV
NUTS & VOLTS
Robot Kits For All Skill Levels
ICs, Transistors, Project Kits
Motors, Frame Components
and Scratch Builder Supplies.
BEAM Kits and Components
Order by Internet, phone, fax or mail.
180 El Camino Real
Millbrae, CA 94030
Visit our store near SFO!
Most orders ship the day received! World-wide shipping. Convenient payment options.
Every month, I collect websites that
are cool, safe, and fun and publish
them ina monthly newsletter.
Here, you will find my past monthly
newsletters. — Mark Donaldson
Take NASA’s interactive“Planet
Quest” quiz toseeif youknow what
is fact and what is sciencefiction.
Likegames? Seeif you can beat my
9 second score. I'm sure youcan.