Nuts and Volts - March 2018

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

Targeted Dunking

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.

Francis Coronado Metairie, LA

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 speak.

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 when hit. 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