3PDT and true bypass wiring

I found out that it was easier for beginners to make a proper true bypass wiring when they fully understand how does it work. Here is a blog post to explain it in an easy way!

The 3PDT Footswitch

Before going any further, you have to understand how functions a 3PDT footswitch. The 3PDT stands for "3 poles, double throw". Here is a diagram showing the different possibilities :

Here, we can see that a simple pole, simple throw (SPST) is equivalent to a simple on-off switch! When you press it, it connects 2 parts of the circuit. It is used for example in temporary switches to make noise with a feedback loop, like on my Coda Effects Dolmen Fuzz, where the self "Solstice" footswitch is a SPST.

When you use a double pole, simple throw (DPST), it is the same thing, but you connect two things at the same time! It is rarely used but can be useful for specific circuits.

A simple pole dual throw (SPDT) is equivalent to a toggle switch. So basically, double throw just means that you have two positions for the switch. You can send the signal to one side of the circuit or the other side. It is widely used in guitar pedals! For example you can choose between to type of diodes with this kind of switch: on one side it will be a germanium diode and on the other side a silicium diode.

A dual pole DPDT is equivalent to 2 SPDT. It is also widely used!

From that we can easily conclude that a 3PDT is equivalent to 3 double throws at the same time. You have two position for 3 switches with 2 positions each:

 

That means to say that on one position, the middle pins are connected to the ones of the top (2 is connected to 1 for instance), and in another position to the ones on the bottom of the switch.

How to use it for true bypass?

When you want to do a true bypass, you want a simple thing:

• in one position (position 1), input and output jacks are directly connected, the signal does not goes through the circuit (bypass)
• Position 2: input jack goes to the input of the circuit, the output of the circuit is connected to the output jack, and a beautiful shiny LED is on.

For position 2, there are 3 conditions to be filled, that is why we need a 3PDT !

 

With this type of wiring, we have the two following positions: 

That is to say on and off for the guitar pedal!


If you do not want to bother to solder directly on the 3PDT (you have to be precise and it is not very practical if you want to modify it afterwards, and most of the times it does not look so good), you can directly use a small PCB to make your true bypass like a boss. You can either make it yourself (model here and here), or buy it (very cheap here).


There it is!
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