62 December 2016
READER - TO - READER TECHFORUM
My young son frequently
leaves the fridge door open. I’m an
electronics beginner and I’d like to
build a simple circuit to alert me
when that happens. It should be
simple enough that I could build it
with the help of my son, so he learns
two lessons in one sitting. Schematic
and design appreciated!
#12161 Michael Calhoun
Does anyone have a simple
circuit that will allow me to mix my
iPod audio out with my computer’s
audio to play through the same
speakers? I want to avoid un-plugging/
re-plugging just to hear some music.
#12162 Jonnie Vanalstyne
Hit By A Speeding Card
My Raspberry Pi Model III uses a
microSD card. The cards are available
in different “speeds.” Is there any
significance to how “fast” a card I get?
#12163 Rick Holtz
Grand Rapids, MI
Getting Started With SDR
I want to start experimenting with
Software Defined Radio (SDR) There
seems to be a fair amount of info,
but I’m still not sure where to start.
What are the minimum requirements
to get started on a small budget that
the wife will live with? Is there a best
#12164 Amaranto Melgar
[#8166 - August 2016]
I would like to add a “rain
detector” to my sprinkler system
that would automatically disable the
sprinklers. The system uses an old-style
mechanical “dumb” timer. How about
a simple circuit to do the job?
You can buy a normally closed
SPST switch containing a water
absorbing material that expands when
wet to open a microswitch. This is
placed in between the common side
of the valves and the return line. The
switch can be adjusted to open after
an appropriate amount of rain has
fallen. They are sold by most sprinkler
manufacturers in home improvement
and hardware stores.
[#8167 - August 2016]
It has fallen to me to teach a short
course on electronics for a summer
camp program near our home. I
would welcome suggestions for
curriculum! The kids are ages 8-12.
#1 Suggestion depending on
length of camp:
1. Build an LED flashlight.
2. LED flashlight with dimmer.
3. Crystal radio.
4. Crystal radio with one-transistor amplifier.
#2 Summer is about over so this
might be useful for next year, but this
is what I would teach kids if I had
about a week to do it.
• Batteries and LEDs
• Types of electricity
• DC, AC line power
• Power line safety
• Tesla coil RF
Series and Parallel Game:
• Lines on floor
AC Line Safety:
• Wet areas
• Reducing power use with CFL and
• DVD players
• Computers and hard drives
• Problems with TVs — High voltage
• Dangers of microwave ovens and
how to avoid them with proper
The series and parallel game is
played on a gym floor or grass area
with lines laid out with painter's tape
or yarn and 4” nails. Use small rugs
for batteries and lamps. Have kids
to man the switches which are yarn
sections with weights on the free
The teacher yells out “switch
one close," etc., then the switches
move and the kids stationed along
the “wires” move like electrons if the
circuit is complete. Do a short circuit
too where the kids have to move
quickly and the teacher breaks the
circuit and yells stop/sit down. This
comes last, followed by a comment
and question time. This is modeled
after the on-stage demos that are part
of Harvard’s famous CS 50 class.
Have a local fire official do the
line voltage safety lesson because
they do this all the time.
Be sure to have tiny torx drivers
for taking the super magnets out of
I would like to see some
guidelines for the Takeapart Day like:
• Unplug all wires rather than cut
• Watch out for anything that has
ink in it or on it, and wear gloves
while removing that part to the