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I have an Arduino based circuit that performs a mechanical job and I'd like the circuit to turn on when it detects a magnetic field. I want to use a magnet as a trigger for the circuit. When a magnet is attached to something like say a stick and put near the circuit board I'd like it to detect the magnetic field and turn on.

I have learnt that an operational amplifier can be used for this purpose but need guidance on how to do it as I am a novice in embedded systems & electronics.

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One way or another, you need some kind of transducer that can make a electrical signal that is a function of magnetic field strength. No, a opamp can't do this. A opamp may be useful in tranforming the raw signal from such a transducer into something the microcontroller can read, but by itself a opamp is not a magnetic sensor.

Two possibilities come to mind:

  1. Reed switch. These are the bare switch parts of a relay, usually enclosed in a small glass envelope. The right magnetic field causes the switch to close. This will be the simplest solution if you have a strong enough magnetic field. This usually means a reasonable permanent magnet can be placed with a half inch or so of the reed switch.

  2. Hall effect sensor. The underlying physics of these sensors creates a small electrical signal proportional to the magnetic field in a particular direction. However, you rarely get bare Hall sensors directly. Usually they are integrated into a single package with something that either makes a reasonable size voltage proportional to magnetic field strength, or provides a digital output, usually with some hysteresis, to indicate presence or abscence of some level of magnetic field. Allegro is a supplier that makes a lot of these things with various options.

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  • \$\begingroup\$ The supplier Olin mentioned is Allegro. (allegromicro.com) Another sensor manufacturer that I have used in the past is Melexis. (melexis.com) \$\endgroup\$ – Michael Karas May 28 '13 at 15:11
  • \$\begingroup\$ Note that if you use a reed switch, you can build a device that requires no standby power. On the other hand, a Hall-effect sensor needs to be powered all the time. \$\endgroup\$ – Dave Tweed May 28 '13 at 17:44

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