user34873: I want to use a wired OR connection to connect some pins of the 74HC595 shift register to an Arduino Digital Input Pin. How do I choose proper diodes for this situation?

Maximum rated forward current has to be higher than whatever current a single pin of the 74HC595 produces.
The V_F forward voltage drop has to be as low as possible to make sure that a HIGH from the shift register has still a voltage high enough to result in a HIGH at the Arduino pin after passing through the diode.
Switching time has to be low enough to match the shift register's frequency.

So far, so good? What did I miss?
By the way, I haven't decided on the voltage for VCC, yet. Most likely, either 5 V or 3.3 V.


user20245: For this, almost any diode will work.  Searching, it turns out that the 1N4148 is a popular ubiquitous choice.  But don't go out of your way to obtain this particular diode.

Maximum rated forward current has to be higher than whatever current a
  single pin of the 74HC595 produces.

You should turn your thinking around.  Consider the nominal high voltage of the 74HC595, subtract the forward bias diode voltage drop (usually  0.7V for a silicon diode) and divide that by the resistor you pick.  That will be the current passing out of the 74HC595, through the diode and resistor.  Only a small amount of current will pass into the Arduino.  It should be negligible for most applications.  You should pick a resistor high enough to only allow milliamps of current to pass through this circuit.

The V_F forward voltage drop has to be as low as possible to make sure
  that a HIGH from the shift register has still a voltage high enough to
  result in a HIGH at the Arduino pin after passing through the diode.

Check the voltage margin allow for a logical high input to the Arduino (Atmel processor).  I think 3V - 0.7V = 2.3V is still considered a logical high.  However, if you think this is too close, then switch to the 5 volt Arduino where the 0.7 volt drop will make less difference.

Switching time has to be low enough to match the shift register's
  frequency.

I doubt this will be a problem.  That is, I doubt the CMOS logic will surpass the speed at which the diode can turn on / off.

34873: <p>I want to use a <a href="https://en.wikipedia.org/wiki/Wired_logic_connection#The_wired_OR_connection" rel="nofollow noreferrer">wired OR connection</a> to connect some pins of the 74HC595 shift register to an Arduino Digital Input Pin. How do I choose proper diodes for this situation?</p>

<ul>
<li>Maximum rated forward current has to be higher than whatever current a single pin of the 74HC595 produces.</li>
<li>The <code>V_F</code> forward voltage drop has to be as low as possible to make sure that a HIGH from the shift register has still a voltage high enough to result in a HIGH at the Arduino pin after passing through the diode.</li>
<li>Switching time has to be low enough to match the shift register's frequency.</li>
</ul>

<p>So far, so good? What did I miss?</p>

<p>By the way, I haven't decided on the voltage for <code>VCC</code>, yet. Most likely, either <code>5 V</code> or <code>3.3 V</code>.</p>

<p><a href="https://i.stack.imgur.com/Zx2El.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Zx2El.png" alt="schematic"></a></p>
 20245: <p>For this, almost any diode will work.  Searching, it turns out that the <a href="https://en.wikipedia.org/wiki/1N4148_signal_diode" rel="nofollow noreferrer">1N4148</a> is a popular ubiquitous choice.  But don't go out of your way to obtain this particular diode.</p>

<blockquote>
  <p>Maximum rated forward current has to be higher than whatever current a
  single pin of the 74HC595 produces.</p>
</blockquote>

<p>You should turn your thinking around.  Consider the nominal high voltage of the 74HC595, subtract the forward bias diode voltage drop (usually  0.7V for a silicon diode) and divide that by the resistor you pick.  That will be the current passing out of the 74HC595, through the diode and resistor.  Only a small amount of current will pass into the Arduino.  It should be negligible for most applications.  You should pick a resistor high enough to only allow milliamps of current to pass through this circuit.</p>

<blockquote>
  <p>The V_F forward voltage drop has to be as low as possible to make sure
  that a HIGH from the shift register has still a voltage high enough to
  result in a HIGH at the Arduino pin after passing through the diode.</p>
</blockquote>

<p>Check the voltage margin allow for a logical high input to the Arduino (Atmel processor).  I think 3V - 0.7V = 2.3V is still considered a logical high.  However, if you think this is too close, then switch to the 5 volt Arduino where the 0.7 volt drop will make less difference.</p>

<blockquote>
  <p>Switching time has to be low enough to match the shift register's
  frequency.</p>
</blockquote>

<p>I doubt this will be a problem.  That is, I doubt the CMOS logic will surpass the speed at which the diode can turn on / off.</p>
