with TJ Byers
In this column, I answer
questions about all aspects
of electronics, including
software, circuits, electronic
and anything else of
interest to the hobbyist.
Feel free to participate
with your questions, as
well as comments and
You can reach me at:
software, and a 555
A bunch of timers:
grade, precision on/off,
NUTS & VOLTS
and a weekly timer.
A guitar preamp and
electronic push button
Q. I found your October 2003
article on zero-crossing detectors
(“Another Zero-Crossing Detector”)
very interesting. However, I do not
know much about these detectors.
Please provide me with a few practical
A.Here are a few applications
where zero-crossing detectors are
1. Video blanking signals for blanking
the screen during the retrace of a
2. Breaker-less ignition systems.
3. Microcontroller sensor position
4. Phase and frequency measurements.
5. Speech/music discriminators in
The most prolific use of zero-crossing is for the switching of power
circuits. Let’s take the typical lamp
dimmer as an example. Cheap lamp
dimmers simply chop up the sine
wave to adjust the brightness of the
lamp. They do this by detecting the
zero-crossing of the waveform and
waiting a specified time before
turning on the conducting switch —
typically a triac or SCR. (Figure 1)
However, this method generates vast
amounts of RFI that can cause noise
and interference in electronic devices.
A better solution is to turn on the
switch only at the zero-crossing point.
While I used a light dimmer for
the example of zero-crossing, they
have far-reaching power switching
applications, including industrial
motors and solenoids. In addition to
reducing RFI emissions, zero-crossing
switching provides a “soft-start” that
eliminates surge currents.
Q. I read the microphone preamp
answer in the December 2003
issue and I was wondering if this same
preamp would work for an acoustic
guitar that has a pickup in it?
A. Nope. The amp is designed for
a very small input voltage in the
range of 2 mV to 10 mV. The output
of a high performance guitar pickup
can be as much as 2.5 volts when you
start rockin’. The best guitar preamp
circuit I’ve found to date was designed
by J. Donald Tillman way back in 1993
and uses a single FET transistor. You
can download it from his website
or find it here in Figure 2.
Unlike the design in the
December issue, the gain of this preamp is just 3 dB (double the input
voltage) and acts more like an
impedance converter than an amplifier.