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12-input BCD encoder circuit
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Wimberleytech:

--- Quote from: Lomax on January 05, 2020, 10:10:24 pm ---

That's what I'm thinking, though a quick sketch indicates I would need twenty of them. Not a problem; there's plenty of room - just a little fiddly.



--- End quote ---

Just for yucks, I created this LTSpice sim.  Yup...20 diodes.
edavid:
If you'd like to do it with logic ICs, the 74HC148 datasheet shows how.

http://www.ti.com/lit/ds/symlink/sn74hc148.pdf
duak:
There are two optimizations to reduce the number of diodes needed:

1.) the single bit BCD codes don't need diodes as the contacts are already isolated.  This saves 4 diodes for BCD codes 1, 2, 4 & 8

2.) if the control board can safely interpret two diode forward voltage drops (ie. 1.2 V) there is a recursive solution where diodes from another code are re-used. eg., 7 is logically the same as 6 ored with 1 so input 7 will have a diode connected to input 6 and another diode to output 1.  Likewise input 11 will have a diode connected to input 10 and another diode to output 1.
Benta:

--- Quote from: edavid on January 05, 2020, 10:26:45 pm ---If you'd like to do it with logic ICs, the 74HC148 datasheet shows how.

http://www.ti.com/lit/ds/symlink/sn74hc148.pdf

--- End quote ---

^ This, or the MC14532.

Lomax:
This looks doable:

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