• # EEVblog #212 – DIY Decade Resistance Substitution Boxes

Dave feels a tad nostalgic, so takes apart his 25+ year old DIY decade resistance substitution box. And some info on how you can build your own.
DIY thumbwheel box:
http://www.eevblog.com/forum/index.php?topic=4244.0

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## EEVblog #1055 – How to Design a Custom LCD

How to design a custom multiplexed LCD display. Dave takes you through what is required ...

• Richard C

Another DIY hobbyist approach to a resistance substitution box is to wire several potentiometers in series, using various values. The obvious big disadvantage is that, in order to set a particular value, you’ve got to take the box out of a circuit and use a meter to adjust the value. Also, it’s easy to knock a knob and change the value. Still, for many purposes, that may be only a minor nuisance. You may not even need every decade — a 1Meg, 10K, and 100 Ohm pot in series may be able to do much of what you need, especially if you use high quality (10 turn?) pots. It all depends on what you’re going to use it for.

It was fun to hear the nostalgia in your voice as you cracked open the old project with the old screwdriver. I’ve felt the same feeling with a few of my old projects, almost like looking at an old school yearbook or childhood photo album.

• Chris

Can’t you get BCD switches to short out 1k, 2k, 4k and 8k resistors? That way you cut down resistors and don’t have to do math in your head

• Gerald

You need special, so called complementary BCD switches for this. They are available, but you’d have to look for them. And they cost more. More than the few additional resistors if you go for 9 resistors/decade at 1%.

And don’t forget, the usual E series ranges of resistors don’t contain 4 and 8 values. Not even the E192 series. To get the 4 and 8 you need two x 2 and four x 2 resistors, for a total of eight resistors/decade. Not a huge saving.

Now, there are also some special resistor decade switches with four (1-2-3-6) or five contacts (1-2-2-2-2). You even get Kelvin-Varley thumbwheel switches. But not for cheap.

If you get some cheap/used 10-contact decimal tumbwheel switches use them for a resistor substitution box. If you get some 0-9 BCD switches use them for a capacitor substitution box. If you get complementary BCD switches sell them on ebay :-). If you get resistor decade switches consider yourself lucky and have fun building your resistor substitution box. If you get Kelvin-Varley thumbwheel switches throw a party and brag in front of your friends 🙂

• codeboy2k

I built a Heathkit version when I was about 12.

I still have it, but it’s gotten awful dirty contacts, just like yours.

I’d like to see a teardown of the digital variety decade boxes. These look like calculators and have an LED or LCD display.

I always wondered if they used relays, or very low Rdson switches, or discrete mosfets to select resistors (1 of 10) or short out resistors (1-2-3-4) style. I suppose I would use low Rdson mosfet switches in a 1 of 10 configuration for the lowest 2 decades or so and perhaps change to 4-3-2-1 style for the higher decades where the additional resistance of the switches shorting the resistors is not significant anymore.

• codeboy2k

Replying to myself here 🙂 I see that a 2-2-2-2-1 configuration would give a lower overall Rdson through the switches. Maximum 2 x Rdson for each decade using the 1-2-2-2-2 configuration. The 4-3-2-1 configuration has a max 4 x Rdson per decade. And of course 10(or 9) resisters per decade gives maximum 1 x Rdson per decade. so it’s the best, but using fewer parts is always cheaper, but not always better.

Cheers!

• codeboy2k

again replying to myself.. I think it’s not possible on the lowest decades to use any mosfet switches. the 10’s decade (0-9 ohms) @ 1% would need switches with Rdson of < 5 mOhm to stay in tolerance. I don't think these exist. If we wanted 0.1% tolerance would need Rdson of 500 uOhm .. not possible yet. So I expect that low tolerance digital decade boxes would probably use relays at the lowest decades and maybe switch to mosfets at higher decades.

This just gives more reasons why I'd like to see one of these units in a teardown 🙂

Cheers!

• Raff

Gees Dave, your soldering at age 12 was WAY better than mine! But then again, the soldering iron I was given to use had to be heated on the stove! 🙂

• %100 i aggreee
buddy

gain replying to myself.. I think it’s not possible on the lowest decades to use any mosfet switches. the 10′s decade (0-9 ohms) @ 1% would need switches with Rdson of < 5 mOhm to stay in tolerance. I don't think these exist. If we wanted 0.1% tolerance would need Rdson of 500 uOhm .. not possible yet. So I expect that low tolerance digital decade boxes would probably use relays at the lowest decades and maybe switch to mosfets at higher decades.

This just gives more reasons why I'd like to see one of these units in a teardown 🙂

Cheers!

• Warren

One shortcoming of decade resistor boxes is the need to dial in each digit. I’ve been pondering an idea that would use binary substitution and would give you, for example, the ability to scroll through a large portion of the E12 or E24 range, or even finer, with a single knob. Of course, it would have to use a microcontroller. Anyone think this is a good idea?

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