Programmable Resistance Box

For prototype electronics projects, most of us have a pile of resistors of various values stored somewhere on our tool bench. There are different methods of organizing them for easy access and identification, but for true efficiency a resistance substitution box can be used on the breadboard to quickly change resistance values at a single point in a circuit. Until now it seemed this would be the pinnacle of quickly selecting differently-sized resistors, but thanks to this programmable resistor bank there’s an even better option available now.

Unlike a traditional substitution box or decade box, which uses switches or dials to select different valued resistors across a set of terminals, this one is programmable and uses a series of sealed relays instead. That’s not where the features stop, though. It also comes equipped with internal calibration circuitry which take into account the resistance of the relay contacts and internal wiring to provide a very precise resistance value across its terminals. It’s also able to be calibrated manually to account for temperature or other factors.

For an often-overlooked piece of test equipment, this one surely fits the bill of something we didn’t know we needed until now. Even though digital resistor substitution boxes are things we have featured in the past, the connectivity and calibration capabilities of this one make it intriguing.

Remoticon Video: Breaking Encrypted Firmware Workshop

If only you could get your hands on the code to fix the broken features on your beloved electronic widget. But wait, hardware hackers have the skills to write their own firmware… as long as we can get the compiled binary into a format the hardware needs.

Luckily, we have Uri Shaked to walk us through that process. This workshop from the 2020 Hackaday Remoticon demonstrates how to decipher the encryption scheme used on the firmware binary of a 3D printer. Along the way, we learn about the tools and techniques that are useful for many encrypted binary deciphering adventures.

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Building A Resistor Substitution Decade Box

[George] built an incredibly tidy resistor substitution decade box. These devices feature a pair of connections and a way to select the resistance between the two of them. In [George’s] case it’s a pair of banana jacks and these eight thumbwheel switches.

What you see above is the side of each thumbwheel switch. These are panel mount devices which show one digit with an up and down button to change the setting. As you can see, the PCB for each provides connections to which a set of resistors can be mounted. This is the difficult part which he goes to great lengths to explain.

At this point he’s got the resistor groups for each digit soldered in place, the next step is to stack the switches next to each other and connect them electrically. From there it’s off to a project box in which they will be mounted.

This project does a great job of explaining the assembly process. If you’re interested in the theory behind a substitution box check out this other project.