In this column, Kristen answers questions about all aspects of electronics, including
computer hardware, software, circuits, electronic theory, troubleshooting, and anything
else of interest to the hobbyist. Feel free to participate with your questions, comments, or
suggestions. Send all questions and comments to: Q&A@nutsvolts.com.
; WITH KRISTEN A. McINTYRE
QI’m working on an all-electronic “dunking” booth. I have a solenoid that can drop the seat, but I’m looking for a reliable “shock” sensor to put behind the wooden “target.” Something that
only triggers on a bulls-eye would be best.
AI think it’s a first for me to think about a dunking booth! So, a shock sensor that’s not overly sensitive and one which can only look for something in a localized area is an
interesting project. Ideally, you’d like to be able to set the
sensitivity so you can calibrate it for the ideal “effect,” so to
There are some designs that rely on, for example, a
hanging conducting rod within a conducting ring, but that
is a binary device. Either the hanging rod hits the ring, or
it doesn’t. So, it’s hard to adjust, except by adding mass
to the hanging rod. There are
others based on a conducting
ball in a track. They have a
similar binary characteristic.
Let’s see if we can come up
with something more flexible.
A bit of hunting on
the Internet yielded a class
of devices that look more
interesting for this application.
They are based on the
piezoelectric effect. The
piezoelectric effect is the
generation of electric charge,
and thus an electric field and
voltage (electric potential) as a
result of mechanical stress on a crystal.
The mechanical stress changes the relative orientation
of electric dipoles in the molecular structure of the crystal
such that there is a change in the surface charge density
on some faces of the crystal. That surface charge density
change results in a voltage. The voltage can be quite high
in some cases. This is like a piezoelectric ignitor, similar to
what you’d find on a gas stove or a fireplace lighter.
Piezoelectric shock sensors try to detect acceleration.
Shocks are usually changes in acceleration. Acceleration,
by Newton’s Second Law (F = ma) results in a force on the
crystal, and that causes a voltage to appear. We can detect
that and try to integrate it a bit to smooth it out.
SparkFun Electronics seems to have a few nice ones,
and they’re really cheap. For example, I found one for
under $3 at https://www.sparkfun.com/products/9197
that even has a small mass attached to enhance its motion
under acceleration. You’d want to mount this right behind
the target area such that it would move front and back
The output voltage will probably be high (they state ±
90V max), and it will also be AC, so we’re going to want to
rectify and attenuate this. Shock mounting the target area
and isolating it from the rest a bit would make it so that the
target area moved much more than any surrounding area.
So, you could have a winner’s circle.
Figure 1 shows an analog version of a detector. I have
no idea what the right settings would be for this circuit, but
I’ve included potentiometers so that adjustments can be
made until it works well. The capacitor C1 might have to
• Targeted Dunking
• Sensor Inversion
• Training a Train
Q & A
; FIGURE 1. Piezoelectric shock sensor threshold circuit.
Piezo Input 22K
10 March 2018