[sdiy] Digital Input Circuit

[Neil Johnson]

Tim Ressel wrote:
>>> 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.

cheater00 wrote:
>>In a parallel thread on TekScopes, TVS diodes were recommended.

Tim Ressel wrote:
> TVS diodes have a higher
> voltage. Schottky diodes have a 0.3 volt drop when conducting. This
> means the inputs to the processor do not exceed specifications.

TVS diodes are good for absorbing ESD spikes, and you generally want
them as close to the signal input as possible to minimise the
radiating loop, sometimes with a low-value series damping resistor.
But they'll clamp the spike at a highish voltage.  A higher-value
series resistor and Schottky diodes as Tim suggests then protects the
internal gate inputs.  And use Schottkys rather than ordinary silicon
diodes or you won't know whether the specific diode you soldered down
(good) or the IC's internal diode (bad) will conduct first.

That said, some digital devices specifically have built-in protection
diodes, but they still need a series current limiting resistor in any
case.  I remember reading a Microchip app note showing how with a
single high-value resistor from the mains Live you could make a very
simple phase-triggered lighting controller (the Live went to an
interrupt input that kicked the simple firmware at the start of every
AC cycle - crude but very cheap).

And while today's small geometry ICs have ESD diodes built-in, they
don't have them on all pins (just "most") and they're there to protect
the IC during assembly and packaging, not for normal everyday
protection.  As always, refer to the datasheets of the devices you're
using.

Neil
--
http://www.njohnson.co.uk