0
\$\begingroup\$

Edit

I did a poor job of soldering and forgot to solder the pull-down resistor on the input to the buffer. My mistake for posting a question too soon.


I built a circuit to read a button with a single board computer. Wanting to protect the input of the computer, I used a TI 74HC4050 buffer with a diode on the output to avoid any consequences if the SBC's input pin was mis-configured as an output.

I'm confused about the behavior of the buffer at the anode of the diode. It goes from 0V to 5V, but does not return back to 0V. Removing the diode seems to solve the issue (and anyways the resistors limit current enough to protect misconfigured pins), but I still want to understand why this was happening.

The datasheet description for the buffer says that it is a CMOS non-inverting buffer, so I imagine that there is a pull-up network and a pull-down network. Why doesn't the pull-down network successfully clear the voltage at the input of the diode?

schematic

simulate this circuit – Schematic created using CircuitLab

This is my understanding of how the buffer looks on the inside. I understand that there may also be some ESD protection circuits attached, but I don't see how those could be causing issues.

schematic

simulate this circuit

\$\endgroup\$
  • \$\begingroup\$ What is connected to the output? \$\endgroup\$ – The Photon Jun 7 '17 at 3:37
  • \$\begingroup\$ Once the buffer output switches from 0 to 5V, the voltage across R3 is 5V - diode drop. If the buffer output becomes 0, the diode is not conducting (reverse biased), so the output can't become 0. Probably you have a capacitor at the Output (After R3) \$\endgroup\$ – Ash Jun 7 '17 at 3:42
  • \$\begingroup\$ An input pin on a single-board computer is connected to the "output", so it's essentially a capacitor as @Ash suggests. The voltage on that capacitor will simply drain to ground through R3 once the output of the buffer goes to 0V; note that the other end of R3 is connected to ground. Anyways, I'm not asking about the output node, I'm asking about the anode of the diode. \$\endgroup\$ – johnny_boy Jun 7 '17 at 3:47
  • \$\begingroup\$ If the buffer doesn't drive the anode of the diode to 0V either the buffer is broken, the input isn't logic zero or there's something mis-wired. \$\endgroup\$ – John D Jun 7 '17 at 3:49
  • \$\begingroup\$ @JohnD Embarrassed to say that I forgot to solder the pull-down resistor on the input to the buffer. \$\endgroup\$ – johnny_boy Jun 7 '17 at 4:06

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.