AND/OR Switching Part II: Active and Passive components

One of the goals with this switching system is to make it with as small of a footprint as possible. With the increased access to SMD services the use of the 14 and 16 pin CMOS chips for switching purposes are no longer a barrier which would take up large amounts of real estate. However, they are not without their drawbacks .

The challenge:

The BYP/CH1 and SEL/CH2 switches would need to pass the signal when not engaged. This is where we run into a road block. Its probably already apparent to most, but I struggled with it and figured I'd share it anyway, but the CMOS chips such as the CD4053 are electronically controlled, even when logic levels are low and you would expect that the SPDT would pass a signal, it will not if there is no power connected to the pedal.

This raises a small issue. Normal pedals using mechanical 3PDT bypass switches, are entirely passive. It doesn't matter if power is supplied to the pedal, when the 3PDT has its connections being met correctly it will pass the signal. This is not the case with the 4053.

I don't know how often people are trying to play through their pedals when they have their pedals unplugged. If power fails to your pedal board, I imagine it would be close to an outlet supplying power to your amp and trying to play anything would be pointless. Nevertheless it does present a challenge.

As I alluded to in the last post, this system would be very difficult to implement with a purely mechanical switching system. Instead I will be developing a microcontroller & CMOS system. If you haven't read through it already, I recommend checking out the GEOFEX article on the electronic bypass system. This project will be based on this and an of course expand upon it.

The first step is figuring out the signal paths and number of switches that will be passed through.

After sketching out some ideas, I have a rough draft where ≥10 SPDT/SPST switches would be required to move between the OR and AND switching. This corresponds to Channel 1 OR Channel 2 as well as Channel 1 INTO Channel 2. With this many switches involved at the moment, I think its safe to stop here for debugging purposes. Drawing anything further will get complicated quickly.

Given the digital nature of all of this we can already start to think of how a sort of "TRUTH" table could be set up to switch between modes and what each of the switches would need to do in order to achieve a given outcome.

Word to the frustrated:

When using photoFETs for muting, make sure you use a current limiting resistor. I had burned some out and not re-tested them and kept wondering, why isn't this muting?