3PDT switch

All frequent questions on switching: true or not true bypass, transistor-based or mechanical.
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TimothyT
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Post by TimothyT »

Hi guys, is there like a theory that i can understand how the 3pdt/dpdt switch works? Sometimes, when i see some builds, the true bypass wiring seems a little different from this http://www.tonepad.com/project.asp?id=35. But i guess both work the same way.
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Nocentelli
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Post by Nocentelli »

Switches have a number of poles, i.e. the number of separate circuits which are controlled by the switch, and a number of throws, i.e. the number of separate positions that the switch can adopt. So a SPST = single pole, single throw - This has two contacts which can be connected or disconnected. A SPDT = single pole, double throw = three contacts, with the central common lug (single pole) that can be connected to either of the outer contacts (double throw). True bypass requires DPDT (no LED) or 3PDT (LED): This is ignoring the existance of the "Millenium bypass" pioneered by RG Keen, which uses a DPDT and a bit of clever electronics to switch the LED while still being true bypass.

All 3PDT wiring arrangements should do the same thing: In one position, pedal input>pedal output ("true bypass") and the +9v-to-LED-to-ground circuit is broken (LED off); The other position connects pedal input>circuit input, circuit output>pedal output and +9v-LED-ground is connected (note that you also need a LED current limiting resistor, usually something between 4k7 and 15k, connected between +9v and ground either before or after the LED, doesn't matter which). Additionally, several 3PDT wiring set-ups also connect the circuit input to ground when in bypass to alleviate noise/oscillation that potentially might occur if the circuit input was left "floating". This is the wiring diagram i use (from the Gaussmarkov pedal website):
Gausmarkov.bmp
Gausmarkov.bmp (576.05 KiB) Viewed 7799 times
Note - In this diagram, the pedal input is the grey wire on the left, the output is the grey on the right. The blue wire goes to the circuit input, the yellow comes from the circuit output. Green is ground.
3pdt.gif
3pdt.gif (1.57 KiB) Viewed 7799 times
Note the lugs of the switch are oriented horizontally: In this way, if we number them left-to-right 1, 2, 3 top row, 4, 5, 6 middle row and 7, 8, 9 for the bottom. In bypass switch position, 4 connects to 7, 5>8 and 6>9: This sends the input from lug 4 to lug 7 which is hard-wired to lug 9 which is connected by the switch to lug 6 output. Additionally, ground on lug 5 is connected to lug 8 which is hard-wired to lug 1 (circuit input).

In the other switch position, lug 4>1, 5>2 and 6>3, sending input to circuit input and grounding on the negative end of the LED so it lights up, and lug 3 receives the output of the circuit and send it via lug 6 to the pedal's output.

If you study the various other 3PDT wiring methods, you should be able to trace out what happens in each case.
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

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TimothyT
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Post by TimothyT »

Hey, woah! That's a lot of information! Thanks for your time :D

Now a few questions:

How do i know where is 1/2/3/ on the 3pdt switch? i don't see any marking below the switch.

Also, what do you mean by "5>8 and 6>9"
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Post by Nocentelli »

Hold the 3PDT switch upside down so the lugs are facing you WITH THE LUGS ORIENTED HORIZONTALLY: The middle row are the "common" lugs - on the little diagram in my first post I've labelled them 4, 5, 6 as you go left-to-right. The top row is 1, 2, 3: In one switch position, lug 4 is connected internally to lug 1 (4>1), lug 5 is connected to lug 2 (5>2) and lug 6 to lug 3 (6>3). Now click the switch, and the internal connections change, so 4 is disconnected from lug 1 and connected internally to 7 (4>7), 5 is connected 8, and 6 to 9.
3pdt_2.gif
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modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

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TimothyT
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Post by TimothyT »

ah! Now i get it, thank you so much for the help! Appreciate it :)
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