[sdiy] Digital Input Circuit

John Cronan john at cronan.cc
Sun Jun 16 04:34:12 CEST 2013


I think I would use an N-channel mosfet. 2n7000 with 100k or so base
to ground. You don't need a series resistance--still just the one
resistor--and this has Vgs +- 20V so no need to clamp it against
reverse breakdown. Otherwise connected just as you said using the
internal pull-up.

Hope that's helpful!

John Cronan



On Sat, Jun 15, 2013 at 9:17 PM, Stewart Pye <stewpye at optusnet.com.au> wrote:
> Hi Pete,
>
> I had been wondering if the 74HC573 would be suitable. It has the same input
> clamping current specs as the 74HC125....
>
> input clamping current Vi <  0.5 V or Vi > Vcc +0.5 V  +/-20 mA *
>
> Though the 74HC573 datasheet doesn't say the following...
> *The input and output voltage ratings may be exceeded if the input and
> output current ratings are observed
>
> So I'm not sure if I'd like to rely on the internal clamp diodes.
>
> Regards,
> Stewart.
>
>
>
> On 6/16/2013 11:58 AM, Pete Hartman wrote:
>>
>> I've seen CD4050 used in this way, and there are 74 series chips that are
>> similar.
>>
>> http://www.electronics-lab.com/blog/?tag=cd4050 is one citation I was able
>> to pull up quickly, although it's not extremely informative, it does list
>> the 74 series chip as well.
>>
>> Thanks
>>
>> Pete
>>
>>
>>
>> On Sat, Jun 15, 2013 at 8:51 PM, Stewart Pye <stewpye at optusnet.com.au
>> <mailto:stewpye at optusnet.com.au>> wrote:
>>
>>     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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>>
>
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