Measuring the intensity of changing magnetic field is difficult to do. The changing characteristic adds noise to any metallic sensor that you place in the field (see last post on metallic temperature sensors). To circumvent this a beam of light (Faraday effect) can be used to measure magnetic strength. One of these setups was built and will be referred to as Faraday Effect Apparatus (FEA).
Brief Description of the Problem:
The FEA I made had a bad light source. After troubleshooting the setup the light source got fixed and is now providing much better output data. This means I can measure the intensity of changing magnetic fields better.
Details:
In the FEA it is very important that the source of the light stay at the same intensity so that it doesn't foul the output data. This data fouling happened in my setup and showed itself as a continuously rising voltage in the photosensor amplifier output. The data I needed still showed itself in the output but it had to be separated from the fouled data post measurement. This was not a good use of time and hurt repeat-ability.
After trouble shooting the FEA it was found that the light source, not the photosensor/amp, was the problem. It was constantly changing it's light intensity (the intensity change was small enough to not be perceptible to human eye). In order to make it stay at the same intensity a current mirror was placed inline to prevent the light source (405nm 5mW laser) from drawing additional current as it heated up (diode forward voltage drops more the hotter the diode. This means more power gets provided to the laser if using a constant voltage source) and thus change its intensity.
The FEA output is now stable thanks to the addition of the current mirror to the light source.
Conclusion: Do not use voltage regulators to power LEDs and laser diodes in applications where a very controlled light intensity is needed. Use current regulators.