[sdiy] Digital Input Circuit

[Tim Ressel]

I use a current limiting resistor (2K) followed by a BAS40-04 dual schottky diode which acts as a clamp, then into a 74HC14 schmitt trigger for cleaning up the edges. This arrangement can handle +/- 15 volts on the inputs without damage. Also good for analog inputs, except the HC14 turns into an opamp buffer.




Tim Ressel
Circuit Abbey
503-750-9331
timr at circuitabbey.com




>________________________________
> From: Stewart Pye <stewpye at optusnet.com.au>
>To: Synth DIY <synth-diy at dropmix.xs4all.nl> 
>Sent: Saturday, June 15, 2013 6:51 PM
>Subject: [sdiy] Digital Input Circuit
> 
>
>Hi,
>
>I'm designing a sequncer based around an AVR XMEGA microcontroller powered by 3.3V. It has some trigger inputs that accept nominal 5V trigger signals, though it should be able to handle any voltage between -15V and +15V as it is part of a modular synth.
>
>There are 8 inputs. The circuit I have drawn is a series input resistor to the base of an NPN transistor. The emitter goes to ground and collector goes to the input pin of the microcontroller (with internal pullups turned on). There is a reverse diode across the B=E junction. Is there a more elegant solution than this? An IC that will handle the input voltage range required with just a series resistor?
>
>Regards,
>Stewart.
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