December 2013 21
QUESTIONS and ANSWERS
frequency, and Q are all
lower than calculated. I
blame that on the op-amp
limited bandwidth and phase
shift. I did try a perfect gain
stage (Figure 3) and although
the gain is good, the peak
frequency and Q are about
6% low. I don’t know how to
QI think I have an interesting project. I bought a 3x4 matrix keypad and
I thought I would make a
programmable lock. It didn’t
take long to figure out this is
not a simple thing to do.
LSI/CSI LS7223 is an IC you
can get but it’s expensive. What do you think?
— Ken Brown
AThis is outside my comfort zone. I have never done a keypad entry before. However, Bryan Bergeron in his October issue editorial says we should get out of our comfort zone and learn
something new. I’ll think about it.
12 Volt Control
QI want to upgrade my receiver to one that supports the latest surround formats, but modern receivers use 12 volt triggers instead of switched outlets. My old subwoofer is
satisfactory for now, except for its auto-on feature that falls
asleep and thumps back on.
■ FIGURE 2. ■ FIGURE 3.
Re: ESD Instrument Circuits, pages 16-18, September
Your questions reminded me of an instrument I saw
several years ago, so I don't recall all of the details but the
principle of operation was pretty simple. It used a rotating
grounded electrostatic shield with openings so that a fixed
electrode behind the rotor is alternately exposed and
shielded from the external static field being measured.
The fixed electrode would be connected to the input of a
very high impedance AC amplifier that's tuned to the field
interruption rate. A synchronous demodulator would be
ideal for extracting the final DC measurement value, and
its carrier signal could come from a shield rotation sensor
of some sort. Does that make sense?
Yes, I am sure that would work. Now, how would one
do that solid-state? Kerr cell?
Victor Davis wrote to tell about his reproduction of
Michael Faraday's experiment which shows the principle of
the dynamo. You need a sensitive micro ammeter or
center zero galvanometer, a coil of many turns, and a
strong magnet. If the meter only indicates positive current,
you will need a bridge rectifier. Use germanium or
Schottky diodes for best results. The procedure is simple:
Insert the magnet in the coil; it will cause a current that is
shown on the meter. When the magnet is removed, an
opposite current is generated.
Thanks, Victor; the schematic is in Figure A.
(Victor sent several files on this topic that are posted
at the article link.)
■ FIGURE A.