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I bought the LS7220 IC which is designed to check passcodes.

For some reason, the pin 8 Lock Status Output does not illuminate the LED when I connect the power, and when I bridge the VCC in sequence pin 3, 4, 5, 6. I have not yet attached anything to the other pins, I just wanted to get this basic functionality to work first.

Hopefully I am not shorting the IC by connecting the battery straight to the IC, or forgetting to include a pulldown or pullup resistor somewhere? It is likely something basic, perhaps it does not output enough power to illuminate an LED?

Here is a diagram with how I have wired it, excuse the simplicity: First design attempt

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    \$\begingroup\$ Tip: Note that when you use the CircuitLab button on the editor toolbar and "Save and Insert" on the editor an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. \$\endgroup\$
    – Transistor
    Commented Jul 3, 2021 at 16:48

1 Answer 1

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The LED will never light up connected like that. The LOCK output is open drain, so it can only light up a LED when sinking current in, it cannot source current out.

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    \$\begingroup\$ The datasheet is poorly done. The internal block diagram shows open-drain outputs but the typical use schematic (figure 5) shows the outputs connected as if they are push-pull outputs (I.e. output connected to LED to ground). \$\endgroup\$
    – DoxyLover
    Commented Jul 4, 2021 at 8:17
  • \$\begingroup\$ @DoxyLover So you are saying one side of the LED should be connected to the pin, and the other side to the GND? \$\endgroup\$
    – questioner
    Commented Jul 4, 2021 at 14:09
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    \$\begingroup\$ @questioner No, because that is how you have connected it now. The exact opposite way, LED anode to supply, LED cathode to the active-low output pin. \$\endgroup\$
    – Justme
    Commented Jul 4, 2021 at 14:42
  • \$\begingroup\$ @Justme Sorry a second question. I think it says I need to connect to pin 7 Lock In to turn the LED back on, but this did not work, any ideas? \$\endgroup\$
    – questioner
    Commented Jul 5, 2021 at 2:10
  • \$\begingroup\$ @Justme Edit of my first reply: I connected the LED's anode to the GND on my breadboard and the cathode to the pin. It lights up the LED, and then when I input the passcode correctly it goes dark. This is not what you said, but it seems to work? \$\endgroup\$
    – questioner
    Commented Jul 5, 2021 at 2:16

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