[sdiy] Digital Input Circuit

Stewart Pye stewpye at optusnet.com.au
Sun Jun 16 04:17:09 CEST 2013

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.


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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