Category Archives: Electronics

AM radio, variometer tuned

variometer-tuned AM radio

This was an experiment to see if there was an easy way to avoid having to make a variable capacitor. The previous designs I made using a home-made variable capacitor, although cheap to  make, still needed quite a bit of time and effort. This one was surprisingly easy and works really well. It’s a bit strange having to tune it by swinging the coil around, but one gets used to it.

This design used fixed capacitors in the tuning circuit. Tuning is accomplished by varying the inductance instead. This is accomplished by a variometer – a coil wound in two halves, one inside the other in such a way that the angle between them can be varied.

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DIY fluxgate magnetometer for experimenters


I don’t think I’ve ever made anything with such an impressive-sounding name before. Doesn’t it sound like something out of a sci-fi movie?

I didn’t make this as a project for my students – it’s a bit advanced. Rather I made it as part of my ongoing quest to record electromagnetic earthquake precursors: one of the ways in which they might be observed is by recording small changes in the Earth’s magnetic field. Thus my interest in magnetometers.

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Simple AM and Short Wave Radio Projects for Beginners

AM radio with spiral-wound coil and home-made tuning capacitor


The radios described here are a regenerative design utilizing a single JFET and cheap audio amplifier integrated circuit.

(JFET: “Junction Field Effect Transistor”, a small electronic device which can amplify radio signals and is used as the basis for these radios.

Regenerative: employing “regeneration” (feedback), a technique used in simple radio designs to increase the amplification of radio signals.)

Here in Taiwan, there don’t appear to be many people dabbling in radio as a hobby. My recent experiments with simple radios that students might be able to make, have been regarded somewhat like “black magic”. So it hasn’t been too difficult to get students interested in it and a few of them have made the radios described below with very good results. Continue reading

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Cheap seismometer using sound card and Amaseis – now running under Windows

After about a year and a half of messing around, I finally got this very cheap seismometer system working and displaying real-time data under both Windows and Linux (More on how to set up Linux later.) Educationally, it has great potential.

It consists of the sensor and an interface box. This interface is radically different from traditional digitizing methods. The earthquake signal, after being amplified, then modulates a fixed 5 KHz audio tone. That plugs into the microphone socket of a computer running three pieces of software under Windows (XP in the one shown here). They are, 1) the one I put together – “seismochop”, which retrieves the earthquake signal from the audio tone and converts it to serial data, 2) a virtual serial port package called com0com which acts as a relay, and 3) Amaseis – software that records and displays earthquake data. These three can be downloaded free.

This very cheap and rather unusual way to digitize an earthquake signal was inspired by my experience in ham radio, when AM transmitters were common. When I found the same principle (amplitude modulation of a carrier wave, or more correctly here, pulse amplitude modulation) used in “cheapchop“, even including the basic software, I was able to get it working.

Here is a screen shot of a local quake I recorded today. There’s a DOS window I brought to the foreground for the screen shot. That’s seismochop showing the data samples at about 11 samples per second. Continue reading

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BFO Metal Detector – Student Project

(* BFO: “Beat Frequency Oscillator” a technical term describing the most common operating method used by metal detectors.)

The previous post mentioned the metal detector our students made during the winter vacation.

Following are the technical details:

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Home Made Speaker Experiment

Here is an easy way to demonstrate how sound is reproduced electronically.

This is how this simple speaker works:

(Note that on most speakers the magnet is stationary and the coil is attached to a moving cone.)

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Student Seismology Research Station – Seismometer and Magnetometer Recording Via Computer Sound Card

(Seismology = the study of earthquakes
Seismometer = device for measuring earthquakes
Magnetometer = device for measuring magnetic fields)

One cannot ordinarily record earthquakes via a computer sound card as they don’t respond much to the slow vibrations experienced in earthquakes. (Update: not strictly true. See my latest on this: Simplest Seismometer – Experiments with direct recording through PC sound card) However, with some cheap electronic trickery and clever software, it can be done, and very adequately for the purposes of study. The seismometer above, made by a high school student, in its first night of testing recorded this local 3.8 quake – a magnitude 1 in Taipei:

(click on the pictures for larger versions)

and as a surprise bonus, some filtering of background noise revealed this distant quake as well. It was a 5.9 from 1000 km away in the Philippines.

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Simple Magnetometer Science Project Using Water as Lens

(Click on photos to see full size image.)


A magnetometer such as this can detect small changes in the Earth’s magnetic field. It can be useful to monitor these changes as they may indicate the occurrence of natural phenomena that can influence our lives. Certain kinds of solar activity for example, have the potential to disrupt communications and power systems. Large earthquakes are also known to produce magnetic changes prior to their arrival. (This project was originally intended to work in conjunction with the search coil magnetometer project for predicting earthquakes.) Continue reading

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Can earthquakes be predicted? A simple magnetometer for detecting earthquake precursors

magnetometer: n. [Magneto- + -meter]
An instrument for measuring the intensity of magnetic forces [1913 Webster]

precursor:  One who, or that which, precedes an event, and indicates its
approach; a forerunner [1913 Webster]

Scientists, particularly in Japan, have for many years been gathering evidence of electromagnetic signals that come from under the ground before earthquakes. The bigger the signal, the bigger the quake is likely to be. This can occur hours before, and in some cases, days or even weeks before the quake itself. Why it happens is not well understood but current theory is that it has to do with rocks creating large electric currents as a result of crushing or grinding under high pressure. Several methods have been used to detect these reportedly erratic, electromagnetic pulses. The method described here is one of the simplest. Technically, it’s called a “search coil magnetometer.” This does not respond to very slow magnetic changes, but it can sense short-term ones, making it suitable for detecting earthquake precursors. Continue reading

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Recording Earthquakes for Beginners – The Background Story


In my search for educational projects that can engage a kid’s interest, I recently began looking into seismology (from Ancient Greek, “seismos”, an earthquake and “logia”, study of.)

Being a rather high-tech subject, I was skeptical of my chances of reducing it to something kids could do, but I gave it a shot anyway, since it’s very applicable here in Taiwan, where earthquakes average a couple a day. (You can see them on this website: )

The goal was to come up with some kind of a detector; the simplest and cheapest design possible that a student could plug into a computer and record real earthquakes. Although still in progress I wanted to share what I’ve done so far as it’s a fascinating field and full of good, observable data that will give anyone a deeper understanding of the planet we live on. Continue reading

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Home made battery tester/galvanometer

A galvanometer is a device for measuring the flow of an electric current. (From Galvani, a professor of physiology at Bologna, and meter, measure.)

In this day and age, electricity has found its way into our lives in the most unexpected areas. Nobody could have imagined how useful computers and cellphones would become, but how far can it go? I thought electric toothbrushes were pushing the limit once, but then I saw a computer controlled toilet seat….. Continue reading

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Morse Code Practice Key

This project utilizes a wide variety of materials and technologies: woodwork, electronics, computer software if desired, and the code itself.

Morse code is still used by many amateur radio operators. One can also still find military, maritime and aviation applications for it. Pilots, for example, need to know it so they can recognize the location code sent out by radio beacons. Continue reading

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