Experimenting with magnetic sensors.

This month, we put our flux-gate sensors to work and build a practical magnetometer and gradiometer.

In the June, 1998 issue of Electronics now, we introduced the FGM-x series of fluxgate magnetometer sensors from Speake & Co. Ltd. These relatively low-cost devices are made in the United Kingdom, but are distributed in the United States by Fat Quarters Software [24774 Shoshonee Drive, Murrieta, CA 92562; Tel: 909-6987950; Fax: 909-698-7913: Web: www. dconn.com/fatquarterssoftware]. For those not with us then, let's quickly review some of the important features of the FGM-3, which is used in the projects that are described in this article.

The FGM-3 is a three-terminal device [ILLUSTRATION FOR FIGURE 1A OMITTED] that will measure magnetic fields over the range [+ or -] 0.5 oersted ([+ or -] 50 mteslas). It can be used in a wide variety of magnetometer and gradiometer circuits. The three terminals on the FGM-3 are: RED: POWER (+5 VDC); BLK: GROUND (OV); and WHT: SIGNAL

The output signal is a train of TTL-compatible pulses with a period that ranges from 8 to 25 mS, or a frequency range of 40 to 125 kHz. The magnetic field strength is indicated by the frequency produced, and can be read on a variety of display devices including digital frequency counters, digital period counters, analog meters, and even computers. It is relatively easy to interface the FGM-3 device to microcontrollers - such as the Parallax BASIC-Stamp or Micromint PicStic devices - using a line of interface chips offered by Speake and Fat Quarters.

The sensitivity pattern of the FGM-3 device is shown in Fig. 1B. It is a figure-8 pattern …