2-Legged Passive Component Switching - Micro-Decade Boxes

[Cybercow]

DPDT On-On-On Toggle Switching Techniques for 3-Way Switching:

One of the more common modifications to most any pedal circuit is to inject a means of toggling a capacitor value into the signal path, varying the depth of a chorus, and or some other audio manipulating section of a circuit. Basically, it’s adding a ‘micro-miniature capacitance decade box’ to a circuit.

One of things always that bothered me about many of the 3-option mods, is that the order of the cap values were not linear. In other words, instead of getting “Low, Mid, High” toggling action from a 3-way toggle, one would get a non-linear “High, Low, Mid” or “Mid, Low, High” switching order as the 3-way toggle was activated across its switch pattern. So I set about to see if that could be changed to provide a Low to High transition.

Most such three-way toggles I’ve seen used in such mods are simple SPDT On-Off-On toggle switches. And while those do work as ‘micro-miniature capacitance decade boxes’, they do not allow a Low to High switching action. So I took a DPDT On-On-On switch and started futzing with jumpers and layouts to see if a Low to High switching action could be achieved.

The first one I came up with was for a chorus depth mod which called for a 330pf, 100pF & 47pF set of caps. By applying criss-crossing jumpers across the 4 outer lugs of a DPDT On-On-On toggle switch, adding one leg of each of the 3 different caps in order of value directly onto three of the toggle’s lugs and tying all three of the dangling cap’s legs together, a nice ordered switching appliqué is provided. Just attach the 3 tied cap legs as one leg of the original cap in the circuit and the unused center lug of the DPDT On-On-On toggle to the other leg of the original cap and you get an ordered 3-way swathing toggle that will operate from Low to High. Just mid the orientation of the switch when it’s mounted so it matches the labeling on the enclosure. (see images below)



Another configuration using a DPDT On-On-On toggle for an ordered 3-way toggle is to jumper across one pair of corner-opposite lugs together and the two center lugs together. In this configuration, only two caps are needed and if one uses box-film caps, they can be super-glued directly to the body of the toggle switch. The caps are soldered directly onto the toggle in a manner shown in the images below. (To describe it with words just seems too wordy. And a picture is usually worth a 1,000 words. Image below next paragraph.) And also provides an ordered Low to High switching action.

With only two cap values used, advantage of the jumpered DPDT On-OnOn toggle allows the caps to be used in parallel in the “Low” position. The example images below are being used as the input cap value option for a boost circuit, (LPB, RM, etc), and the Values may vary depending on your tastes/requirements. Here, I’ve used a 6.8nF and a 3.3nF to yield a low of 10.1nF, Mid of 6.8nF and a High of 3.3nF - which work nicely in this circuit.




4-Way Rotary Switching Techniques for 4-Way Switching:

This is just a simple dead-bug faction method of obtaining an ordered switching method of different valued components.




6-Way Rotary Switching Techniques for 6-Way Switching:

For instances where up 6 options are desired/required, SmallBear provides a nice solution with a 6-way (SP6T) rotary switch and a link to a nice little PCB from OSHPark for mounting the SmallBear switch and your components for when & where such switching is called for. The PCB, switch and components will fit neatly into the depth of a 1590B enclosure



That's this months' modification article from me. Happy building!

 

[Robert]

Here's the DPDT method drawn in schematic form:

DPDT ON/ON/ON as 3-way Switch - PedalPCB Wiki

wiki.pedalpcb.com

 

[KR Sound]

Is there an easy way to know when you’re ordering Type 1 vs 2 in order to plan ahead?

 

[Robert]

If you're getting it from Tayda it's almost certainly Type 1, even if it says it's Type 2.

Most vendors will specify when it's Type 2, if they don't specify then it's probably Type 1. The ones who do have Type 1 usually don't specify at all.

Taiway brand is usually Type 2, I've never seen a Taiway that was Type 1.


This is why I always use Type 2... You generally know what you're getting when you order it.


I get mine from StompBoxParts or LoveMySwitches. I've never gotten the wrong switch from either of them.

 

[Brett]

I'm working on a board right now that calls for an ON-ON-ON switch. I thought I had a pretty good understanding of how these worked, but after seeing this page and looking at @Robert's wiki page, I'm not sure I've got it right.

I'll preface by saying that it'd be cool if schematic annotations in CAD programs were aligned with the datasheet in regard to switch pins. Looking at a datasheet for a DPDT, you have pins one through six, not A1, A2, A3, B1, B2, B3. It makes deciphering things like switching positions a little more difficult than it needs to be.

Here's a screenshot from Taiway's datasheet for the DPDT ON-ON-ON switch from LMS:


In the PedalPCB wiki diagram, it shows that on a Type 2 switch, pin one (A1) is active when the switch is in the DOWN position. If I'm not mistaken, Pin one is at the opposite of the keyway:


and if the keyway orientation is...

Up. Always up. No exceptions.

(fwiw, I agree)

Shouldn't the diagram at the wiki show that pin one (A1) and pin six (B3) are UP and DOWN respectively, not the other way around?

If you're getting it from Tayda it's almost certainly Type 1, even if it says it's Type 2.

Damn, this is annoying. Even the datasheet for their DPDT ON-ON-ON toggle shows to be the same as the Taiway switch.

 

[Robert]

We're looking at the switch from the back, the component side of the PCB.
The switch would be installed on the opposite side, so you're looking at the lugs pointing towards you.

When the switch is in the Up position A2 and A3 are connected together, and B2 and B3 are connected together.
When the switch is in the Down position A2 and A1 are connected together, and B2 and B1 are connected together.

I always imagine a fulcrum when I'm trying to visualize out how the switch works.




Also, as another aide when I'm working on a schematic, I have drawn these two components for Type 2 ON/ON/ON switches.

The thin line (A2 > A3) is the connection that is made when the switch is in the middle position.



Also, as a shortcut (although I never use it) I have the same configuration represented as a rotary switch:


When/if I use this component the PCB will automatically have a ratline showing the necessary jumper between A3 / B2

 

[Brett]

Thank you for clarifying, @Robert. Apparently my understanding of the switch pins vs CAD pins was completely bassackwards.

Based on what you're showing above, you label your DPDT switches as follows?
Switch pin 1 = B3
Switch pin 2 = B2
Switch pin 3 = B1
Switch pin 4 = A3
Switch pin 5 = A2
Switch pin 6 = A1

If the above is correct, I'll try to commit it to memory for future reference. I've gotten so used to labeling pins according to the part's pin/lug numbers, but at least I now know why this looked so weird to me.

 

[Robert]

Yep, that's correct.