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PIC inputs and diode clamps and switching regulators, oh my.

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dfnr2:
I agree, you need a schmitt trigger on each channel to ensure a reliable response.  However, before designing in the 'HC14, you will need to check out the waveform and make sure that the dead zone of the 74hc14 is well within the detector output range under all conditions.  Otherwise you will get valid TTL signals that don't accurately reflect the quadrature behavior.  In that case, you would be better off using a dual op-amp configured as two schmitt triggers (one per detector).  You can then set the trigger points to whatever voltage you want.  Tie the open-collector outputs up to 3.3V with a resistor much smaller than the feedback divider network.  This will stabilize and square up the quadrature signals.  Any workaday comparator such as an LM2903 will work fine.  You can do the detection in software if you hook the conditioned detector signals to some "change on interrupt" pins, or if your software has other stuff to do, you can use one of the great encoder chips from usdigital (http://usdigital.com/products/interfaces/ics/) with up/down outputs hooked up to counter increment inputs on the uc.

These encoders actually incorporate schmitt triggers, but unless the dead zone is perfect for your application, you're better off making your own schmitt trigger anyway.

By the way, if your quadrature detector is a couple of phototransistors or whatever, what's to keep you from just powering them (and the LED's) with the 3.3V?  Then you could avoid having two supplies.  You'd still want a schmitt trigger, though.

Dave

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