Since our first few lessons covered resistors, and in at least one case demonstrated the utility of having a Resistor Substitution Box, that’s what we’re going to build for our first project.
Introduction
One of the most useful pieces of equipment in a hobby electronics lab is a good quality resistor substitution box. As seen in the lab video for our 2nd Beginner’s Corner Lesson, “Resistor Circuits”, a resistor substitution box offers many benefits. With a resistor substitution box you:
– Don’t have to keep every resistor value on hand
– Can test a non-standard resistor value before you order it
– Have the ability to fine tune a resistor value in circuit
– Can quickly change a resistor value to see the results
But a good quality resistor substitution box is expensive ranging from a few hundred dollars to many hundreds of dollars. And while cheaper ones are available, they offer limited utility as they only allow you to select from a few standard values. Even some of the boxes in the $200 range have very limited power handling capability (1/2 Watt).
Requirements
I’ll bet we can build a more capable resistor substitution box for less than the expensive ones. Let’s lay out some requirements/specifications so we have something to design to:
– At least 6 digits (we’ll design to 6 – it’s easy to add more)
– Range of 1Ω to 999,999Ω in 1Ω steps
– +/- 1% accuracy
– 3W power rating
– Binding post connections
– Sturdy project case
– Professional look and feel
– Total cost well under $100.00
Those requirements will give us a top notch resistor substitution box at less than half the cost of purchasing a good one. And, if you are willing to spend a little more, you can add a digit at the start and end for a range of 0.1Ω to 9,999,999.9Ω.
How Rotary Switches Work
Now that we’ve got a set of design requirements, let’s start laying out a schematic. We need to make a design decision right up front. While thumbwheel switches are an option they tend to be more expensive – in some cases significantly more expensive – than rotary switches. Based on that, we’ll choose to use rotary switches in our design.
Before we begin, let’s talk about how a rotary switch works. A rotary switch allows you to select from one of a number of positions by turning the shaft. Important factors to consider when choosing a rotary switch are how many unique items (positions) from which you need to select, three, five, ten, twenty, all are possible with the right rotary switch. In our case, we need to select from 10 (0-9) different resistance values.
The number of decks, indicates how many planes of pins the switch has. multi-pole switches combine multiple independent switches on the same rotary switch. Multiple poles can reside on a single deck or on multiple decks.
We only need single pole, single deck rotary switches for our project. The good news is that’s also the least expensive rotary switch option.
Ideally, we’d like to have 10 position rotary switches so that we can select between 0 to 9 for each significant digit, – x1, x10, x100, x1,000, x10,000, and x100,000. However, if we find a really good deal on some 11 or even 12 position rotary switches we can take advantage of them and just have one or two unused positions on the far right (clockwise positions) of each switch.
Above is a schematic diagram of a single pole, 10 position rotary switch. Note that like a potentiometer, the rotary switch has a wiper, which is the common connection for the switch, and then a corresponding connection (terminal) for each switch position. Turning the switch to a given position connects that terminal directly to the common. For any given position the switch routes current through the common to the selected terminal (position).
One other important characteristic we need to keep in mind when selecting our rotary switches is the switch type. Types include “shorting”, sometimes called “make before break”, or “non-shorting” sometimes called “break before make”. When you switch a shorting type rotary switch, between position 2 and 3 for example, it makes the contact with position 3 before it breaks the contact with position 2. Positions 2 and 3 are momentarily shorted together. When you perform the same operation with a non-shorting rotary switch, it breaks the contact between position 2 before it makes contact with position 3. The two positions are never shorted. However, there is an open circuit between the initial break with position 2 and the make with position 3. When we select our rotary switches we will choose the non-shorting type.
Circuit Design
Also recall from our “Resistor Circuits” lesson that resistors in series add. We are going to use that rule, combined with the way the rotary switch works, to design each significant digit (1s place, 10s place, etc.) in our substitution box. Each switch will be wired exactly the same. The only difference will be the value of the resistors.
Let’s trace our way through a few switch positions to see how it works. To start, look at the schematic above. Note that when the rotary switch is fully counter-clockwise, the common is connected directly to the “0” terminal with no resistors in the circuit. The resistance is 0 x R, or 0Ω which is exactly what we want.
Now let’s rotate the switch one click clockwise. Now when we trace the circuit we can see that the current passes through only one resistor. The resistance is 1 x R. Again, this is exactly what we want.
Let’s rotate the switch a few more clicks to the right. We’ll set it to position “5”. Following the current path through the switch we can see that it passes through five resistors for a total resistance of 5 x R, and so on and so on.
We can see from this configuration that whatever position we dial the switch to, 1, 2, 3, etc., that’s the resistance value we get. If we wire a switch with all 1Ω resistors then position 1 gives us 1Ω resistance, position 2 gives 2Ω resistance, and so on.
Likewise, if we wire a switch with all 10Ω resistors then position 1 gives us 10Ω, position 2 gives us 20Ω, and so on.
The third, fourth, fifth, and sixth rotary switches will each be wired with 100Ω, 1kΩ, 10kΩ, and 100kΩ resistors, respectively. Then we just need to wire all of the switches in series so that they all add up to the total selected resistance.
Complete Schematic
Here’s the full schematic diagram. Each switch is wired exactly the same. Only the resistor values vary from switch to switch.
That’s the complete design: six single pole, 10-position rotary switches, ten each of 1Ω, 10Ω, 100Ω, 1kΩ, 10kΩ, and 100kΩ resistors.
Well, that’s a good stopping point for this installment. In the next installment we’ll learn how to search for and select components including the switches, knobs, resistors, binding posts, and a suitable enclosure. In addition, we’ll complete a Bill of Materials (BOM) and order our parts. Stay tuned for the next installment, coming in a few days.
If you enjoyed this lesson and found it valuable then please give it a like and a share. And thoughtful, on topic comments or questions are always appreciated.
Cheers
Dominick
