Following previous post ("Redneck" disintegrated 7 segments decoder and Hexadecimal 7 segments display (continued)) I decided it was time to try the theory with some practice :-)
The prototype is with DIL parts for practical reasons. However I see no practical way to assemble the diode array with SMB diodes without a proper PCB. My supply of leaded Schottky diodes is too low for this exercise so I used the stock of 1N4148 that was waiting for this moment.
The assembly method is borrowed from other diode matrices I've seen on the 'net (such as A 4-to-16 decoder board) so no breakthrough there ;-)
First the diodes are placed and soldered, if possible line by line because it's impossible to solder them when they are in the middle of a tiny forest of leads.
Then one side of the leads is bent to form the rows. Pretty easy.
All the diodes of one line are soldered together on this side. This is the lines that get pulled low by the HC138 (or '154 in this case, it's more practical).
The diodes are connected in columns on the other side. That's quite tricky but good tweezers help :-)
The assembly has a certain charm. The diodes are quite fragile so the leads are very thin, to avoid stress on the glass beads. I broke one... It's good to not solder the package directly to the pad, but allow one millimeter or two of headroom and bending margin near the PCB. There is no way for the solder tip to access the solder anyway.
Assembly of the 74HC154 is under way, stay tuned ;-)
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