Choosing a op amp for a hall effect keyboard

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ndlu2

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I am trying to make a keyboard using these hall effect keyswitches I have. Unlike other switches that are basically shorted or not shorted, these switches each has its own IC with three pins: two for the power source, and one for output.

I measured the output voltage to be 100 mV when pressed and 300 mV when not pressed. The issue I'm having is that the analog input pins aren't changing when the switches are pressed, and the output from the ADC has noise that can't be removed by averaging.

After testing the switch with a source meter, I found that the output signal is strongest when powered with 5 volts which is what my Teensy 2.0 delivers.

I've never used op amps before, so I need some help choosing which one to purchase.
 
If you're just looking to turn a 0.1 to 0.3 volt signal into 0 to 5 volts, I'd use a LMV339 or LM339. These are technically not "opamps". They're voltage comparators, which are optimized for this task.

You'll need to connect 0.2 volts (or whatever voltage you want for the low-vs-high threshold) to one of the inputs and the signal to the other. The simplest way to get 0.2 volts is a resistor divider from 5 volts. A 100 ohm and 2.2k or 2.4k resistor will get you close enough.

It doesn't matter which inputs connect to the signal and 0.2 volt threshold: one way will invert the signal, the other will keep it the same polarity.

These chips have open drain output, so be sure to configure the Teensy pins as INPUT_PULLUP mode.
 
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