Compass Detector

Posted on Feb 5, 2014

The LED phototransistor light gate should be mounted about 1 to 2cm apart, in some kind of rigid housing. We mounted it on a piece of brass attached to the circuit board. The housing also needs to be able to fit the compass inside, and also not obscure the view of the needle, as you will need to be able to watch the compass while you are setting i

Compass Detector
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t up. The coil we have suggested is actually two coils and we used them set up in parallel to work with each other (the diagonally opposite legs should be connected together), which is why there are two in the diagram. In the circuit we used (and the diagram above) the collector of the phototransistor was connected to the zero rail, and the emitter was connected to pin 2 of the op-amp. If your circuit does not work, try switching the legs over. When the circuit is built and double checked, then test the output of it (pin 6), it should be around (plus or minus) 10 volts or higher when there is nothing blocking the light, but drop to near zero volts when the path is blocked. Now you are ready for a test. Align the coil and the compass, so that the needle will block the light gate when it pointing East-West. The coil should be about half a centimetre from the end of the needle to start with. Now switch on the circuit, and the compass should swing round towards the coil a little. If this does not happen the chances are that the coil`s magnetic field is parallel to that of the compass, to rectify this you can turn the coil around, or switch the terminals of the coil, or turn the whole detector around. Once the coil is making the needle rotate a little way you need to insert the soft iron into the coil, very slowly. The needle should come further round until it is in the beam, where it should sit. If you do this too fast the...

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