hall-effect
I am using the a hall effect sensor – model 480-5198-ND. I have tested this simple circuit using a LED and it is working. However, it is "ON" by default. Only when the magnet is near the hall effect sensor , it will be "OFF".
How do I make it such that it is OFF by default and ON only when the magnet is near the hall effect sensor?
Minimal circuit could be one chip like an analogue asic but that would cost many, many thousands of dollars. Minimal could of course mean cheap or small or both. Stretching it a bit, minimal could mean low power.
Putting all this to one side, amplify the hall sensor's output, feed that into a suitably referenced analogue comparator. That will produce a high signal when the magnet is close. Feed that into the clock input of a D type flip flop configured as a divide by two and drive the LED, via a resistor from the output.
Bias the switching transistor on with a resistor to Vcc, then connect the Hall sensor switch from the Base to the Emitter. The sensor will 'rob' current from the base and turn the transistor off.
simulate this circuit – Schematic created using CircuitLab
Best Answer
The simple answer is to add an inverter circuit to the output of the Hall sensor.
Some Hall sensors may be sensitive to the N versus S poles of the magnet and will change their output when you flip the magnet. This would be more the case with a linear type of Hall sensor.