16 March 2016
connection; without the bypass capacitor, the 100K
voltage divider resistor will drop voltage, and this voltage
will become an extraneous input to the non-inverting
terminal.) For the DC, however, the 100K resistor
provides a 6 VDC for the output level, so you can use
both phases of the input audio signal.
The 100K resistor connected between the +12V
power supply line and the voltage divider resistor is the
feedback resistor for the R channel. With the same input
and feedback resistances for R and L channels, the signals
should be balanced and the input impedances equal.
There must be an audio common somewhere between
the input and the output in order to make a complete
circuit for the audio signals in the input channels and the
output, but these do not have to be common.
I have added trimmer potentiometers to adjust for
resistor mismatch in the feedback circuits and DC level
circuit. The “correction” to the circuit proposed in the
November Reader Feedback actually rendered the R
channel asymmetric with the L channel, so the voice
would not be suppressed.
Thank you for your fresh insight into this circuit.
Re: Q&A Tips — Batteries (May 2015)
According to the Huffington Post, freezing alkaline
batteries extends their life by only five percent.
To freeze or not to freeze, that is the question.
The March 9, 2010 Huffington Post guest post “Freezer
Freaks: 5 Weird Things I Freeze To Save Money,” by
Jeff Yeager from The Daily Green’s says “... for alkaline
batteries, freezing extends their shelf life by only about
5%” ( www.huffingtonpost.com/2010/03/09/freezer-
I did not see any documentation with the HuffPo
post, but from personal experience I have been able to
keep batteries for many years in the freezer and they
still performed as well as a new battery. This is a matter
of chemistry (the Arrhenius Equation) where the higher
the temperature, the more active the electrolytes in the
battery are at corroding the metallic elements (electrodes
and casing) in the battery. When the electrodes are
dissolved, they cannot “make” electricity. When the
casing is breached, the electrolyte leaks out, thus
reducing the battery’s capacity and ruining the device in
which the battery is installed.
Remember to warm the batteries to room
temperature using the ambient air before using them
to insure proper electrical capacity (low temps = low
Re: Breaker Tripped/Sump Pump Alarm #1
Just perusing the December 2015 column, my eye
was caught by Figure 11. I hope you don’t have any
circuits in your home wired this way! That circuit breaker
will always be tripped — tied from hot to neutral — and
the alarm will sound constantly. I’d go with either an LED
(or optoisolator, if hooking up an audible alarm) in series
with a diode and appropriate resistor across the breaker
or a neon bulb with appropriate resistor. These will be in
series with the load.
Another solution is to have the monitored breaker
also power a relay, and tie the alarm via an NC contact
to another circuit not prone to tripping.
You are correct the circuit shown is a dead short,
but it was not intended to represent an actual circuit (I
left off the circuit’s load to simplify the drawing). In the
December Q&A, Figure 11 (modified circuit shown here
in Figure 9 with a load inserted) was meant to be an
illustration of connecting an alarm device across a circuit
breaker, but there should also be a load (sump pump) in
the circuit between the circuit breaker and the neutral
The device wired to the circuit has sufficient
impedance/resistance to prevent the short-circuit. The
alarm circuit wired around the circuit breaker would draw
current only when the breaker trips because there would
be full line voltage across the alarm circuit, whereas when
the breaker is not tripped, there is zero voltage across the
breaker. A relay, LED/diode, or neon lamp could be used
to implement the alarm circuit. I would insert a current-limiting resistor in series with the LED/diode or neon
lamp to protect the device, but high enough so as not to
shunt too much current from the load.
Re: Breaker Tripped/Sump Pump Alarm #2
I suppose you have already had lots of reaction
n FIGURE 8.