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is it possible to make a logic OR gate with 2 diodes in series like this:

schematic

simulate this circuit – Schematic created using CircuitLab

For my project this would be easier to wire than the usual parallel configuration.

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  • \$\begingroup\$ Should we treat the supply voltages as input? And say voltage across R1 and LED as output? \$\endgroup\$
    – User323693
    Mar 1 '17 at 12:11
  • \$\begingroup\$ diagram updated to show the inputs and the output \$\endgroup\$
    – eadmaster
    Mar 1 '17 at 12:20
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    \$\begingroup\$ Regarding your terminology: V1 and D2 are in series. D3, R1 and led D1 are in series. Diodes D2 and D3 are NOT in series due to the connection to V2. \$\endgroup\$
    – Tut
    Mar 1 '17 at 12:23
  • \$\begingroup\$ D2 - release the magic blue smoke with 1v7 across it. \$\endgroup\$ Mar 1 '17 at 15:57
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No, not as you show.

Even without getting into the logic, clearly this can't be a OR gate. OR gates have two inputs and one output. No inputs are shown at all, and we can at best guess that the LED is intended to be the output.

Getting into the circuit, note that D2 always has a fixed 1.7 V across it. It won't live long that way. Also, whatever the branch of the circuit with the 5 V supply is doing is irrelevant. The top of D3 will always be at 3.3 V by definition of what V2 does.

All this circuit does is unconditionally light a LED and blow up a diode.

Here is the basic idea for a diode-based OR gate:

It can get more complicated from here, and using transistors for some gain is useful, but nonetheless, this is the basic concept.

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  • \$\begingroup\$ To be more specific... it is an OR function... as long as it is OR and NOT And. If he can guarantee only one supply will be used at any given time, it would work as an OR function. But, as you say, both on... and you let the smoke out. LED would be significantly brighter with just the 5V source though. Bad design in general. \$\endgroup\$
    – Trevor_G
    Mar 1 '17 at 12:19
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s it possible to make a logic OR gate with 2 diodes in series like this:

V1 is shorted to v2 through d2.

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