I am about to build a few decade boxes as well (and got those cheap decimal and BCD thumbwheels on order, 150mOhms, KM2), which is why i am interested in these posts. I second that the result will be some test equipment. Manufacturer: oneself, precision: if you can afford it. :-) But i´ve got no problem to have it split into another thread if a moderator sees fit.
Haven't thought it through but can you use a BCD switch to build an R2R DAC?
Thought of that too, but it won´t be free of potential (without batteries or isolation or ...) and requires supply voltage, that rules it out for me as a general purpose box you just hook up somewhere into the circuit. One of my adjustable power supplies is always usable in that way as well.
Soldering 9 resistors of the same value in a decimal thumbwheel is relatively straightforward, otoh instead of several single values with high precision. Money wise you rather loose, the price is almost identical between decimal/bcd thumbwheel switches, the single resistors make it a bit more expensive and should something break you need to have that value in that precision. As for the precision: if one really wants to get that last 0.1%: you can still pre select parts and choose the proper ones, like in
and
And just for the thrill of automating test equipment:
Recently i was playing around with connecting to the RS232 of one older cheap multimeter, the output is quite decodable, shouldn´t be too hard to write some script to filter the lowest point in a measurement and recording that value. I also got a more precise
Peaktech 3440 with 4,5 digits/0.01Ohm Resolution, but it is Bluetooth-enabled. Still need to check how to issue the discover and connect of the dongle to get it running, hints greatly appreciated.
Found the VISA docs for python (pyVisa), that would be something to get a bench multimeter for, a Siglent SDM3065X struck my attention so far.
Sent from my keyboard, using my fingers.