Hall sensors can of course be purchased but making them yourself is far more interesting (and satisfying)! According to the theory the crucial thing is to use a touch layer that’s as thin as possible; the length and width are unimportant. An ‘obvious’ starting point for our trials would be copper, which in the form of printed circuit board material is easy to find and handle. Copperclad board may be obvious but not ideal, because it has a very weak Hall constant. Nevertheless we should be able to use it to demonstrate the Hall effect by using very powerful magnets in our sensor.
To achieve detection we need the highest possible level of amplification. In the circuit shown here the voltage amplification is set by the relationship of the two feedback resistors of the first opamp. With the values given (2.2 MΩ and 330 Ω) produce a gain of 6,667. This also creates a convenient bridge connection for taking measurements. The trimmer potentiometer allows fine adjustment. With zero setting that’s accurate to within millivolts we could use this test point to measure Hall voltages of well below a microvolt. Finally in this way we could also measure the flux density of a magnet.
Copper has a Hall constant of AH = –5.3·10- 11 m3/C. The thickness of the copper layer is d = 35 µm. The Hall voltage then amounts to:
VH = AH × I × B / d
When the field B = 1 T and current I = 1 A a Hall voltage of VH = 1.5 µV is produced. The 6,667-fold gain then achieves a figure of 10 mV. The circuit thus has a sensitivity of 10 mV per Tesla. That said, adjusting the zero point with P1 is not particularly easy. The amplifier has a separate power supply in the form of a 9 V battery (BT1). To take measurements we connect a lab power supply with adjustable output current (BT2) to the Hall sensor (the copper surface) and set the current flowing through the sensor to exactly 1 A. Then the zero point must be adjusted afresh.
Next we place a strong Neodymium magnet below the sensor. The output voltage of the circuit should now vary effectively by several millivolts. Note that there are several effects that can influence the measurements we take. Every displacement of the magnet will produce an induction voltage in the power feed wires that is significantly greater than the Hall voltage itself. Every time you move the magnet you must wait a while to give the measurements time to stabilize. With such small voltage measurements, problems can also arise with thermal voltages due to temperature variations. It’s best not to move and inch — and to hold your breath as long as possible!