If you’ve ever worked with guitar pedals or analog audio gear, you’ve probably realized the value of a resistor decade box. They substitute for a resistor in a circuit and let you quickly flick through a few different values at the twist of a knob. You can still buy them if you know where to look, but [M Caldeira] decided to build his own.
At its core, the decade box relies on a number of 11-position rotary switches. Seven are used in this case—covering each “decade” of resistances, from 1 ohm to 10 ohm and all the way up to 1 megaohm. The 11 positions on each switch allows the selection of a given resistance. For example, position 7 on the 100 ohm switch selects 700 ohms, and adds it to the total resistance of the box.
[M Caldeira] did a good job of building the basic circuit, as well as assembling it in an attractive, easy-to-use way. It should serve him well on his future audio projects and many others besides. It’s a simple thing, but sometimes there’s nothing more satisfying than building your own tools.
We’ve seen other neat designs like this in the past, including an SMD version and this neat digital decade box. Video after the break.
Nice to see the resistors are easily-replaceable robust through-hole components. Because, inevitably, one resistor gets burnt one day, rendering the whole box markedly less useful.
agreed, and I like how the pcb is broken up into smaller repeatable patterns. I’ve done that for keyboards, and it’s both economical and facilitates repair. No need to desolder everything to replace a trace, just the pcb in question will do.
Well that’s nicer than mine. I built one that’s (externally) based on the Elenco RS400. Two rotary switches, each one selects a single resistor value, and a switch to choose between the two rotary switches. I opted for banana jacks instead of the alligator clips the Elenco uses. I didn’t see any need for a PCB with such a straight forward design.
Now I’m wishing I had built a decade box instead, especially one as polished as that one.
Meh, to big for me.
Recently I bought a few resistor decade PCB’s from aliexpress. Cost around EUR5 and 8 decades. Size of the PCB is around 30 by80mm. These have SMT resistors and you set the value with a bunch of jumpers. I had bought similar versions before, but the new batch has a 0.1 Ohm resolution. That’s quite handy if you want it for example to emulate temperatures of a PT100
How do you get a decently accurate .1ohm res? Ive never found decade boxes very usefull under 10ohms, every time ive tried to make a low ohm box the traces and switches can have up to .5ohms of resistance.