Incandenza Bypass

This has turned out to be quite a popular circuit amongst hobbyists and builders alike, and since my short write-up on it got lost with my previous website, I thought I’d take a moment today to revisit the topic and publish it again.

Back in 2017 or 2018, I was thinking that there must be a way to implement soft-switching that didn’t require as many parts as the Boss/Ibanez setup. They’ve been using that circuit since the 1970’s, and given the leaps and bounds in computing and electronics since then, surely there had to be a more efficient means of doing this, right? You can, of course, use a microcontroller. And I now do for most things. But at the time, I was working as a software engineer and the last thing I wanted to do was code in my spare time <shudder>.

I remembered my Digital Logic course in university, and recalled that a T-Type Flip-Flop would be exactly what you’d need to turn a signal that gave out a short pulse (like the press of a momentary footswitch) into a latched output level (such as you’d need to hold a relay closed, or leave it open, as the case may be). Since the 555 timer chip is essentially magic, and can do almost anything, I decided I’d see if it could do this (spoiler: it could).

And the circuit is dead simple.

Named for Hal Incandenza, a main character in the book ‘Infinite Jest’, which I was reading at the time.

Circuit Operation

555

The trigger pin is what trips the action. When idle (ie, footswitch not pressed), the trigger pin is held halfway between 0v and Vcc (approx. 9v), courtesy of the voltage divider created by R1 & R2. When the footswitch is depressed, the current output state (either high [approx. 7.5v] or low [approx. 0v]) is reconnected to the trigger pin, pulling either higher or lower than its normal operating point, causing the IC to toggle its output state. ie, if the output is low and the footswitch is pressed, it flips over and outputs high. If it’s high, it flips low. Perfect.

The 100k resistor (R3) and 1uF capacitor (C1) form a simple first-order low-pass filter that debounce the footswitch.

Relay Coils

The 7.5-ish volt output level of 555 when ‘on’ is a bit hot for a 5v relay, so a 100Ω resistor (R4) is used to help drop the level a bit. You may want to use a slightly bigger or smaller resistor depending on the coil resistance of the particular relay you use. Just measure the voltage at the Coil + terminal when the relay is engaged and check to see if it’s far off from the target. 5-5.5v is probably good. Any higher or lower might mean it’s worth trying to adjust the resistance (lower it to get a higher coil voltage, raise it to get a lower one).

Diodes D3 & D4 prevent spikes in voltage from coming back into the 555 and causing harm. D4 is known as a ‘flyback diode’

LED

The LED indicator is lit when the output goes high. The Resistor shown is 330Ω, but you may want to adjust the value based on the type of LED you use, and/or if 330Ω isn’t a typical resistor value that you use. I’m sure 220Ω or 470Ω will work just fine instead.

Relay Connections

The remaining relay pins are made as one would on a 3PDT switch. My personally preferred method of wiring the relay is shown in the above schematic, though how this translates to the pinout of the relay you decide to use is between you and the datasheet. I like EC2-5NUs.

Power

Some folks have had a bit of trouble with this circuit when it comes to the power. If you encounter strange issues, the most likely culprit is failure to sufficiently separate the switching circuitry’s power from that of the audio circuitry.
I’ve shown one way of doing that in the schematic: right at the DC power jack, use two diodes. One will route power into the audio circuitry and the power filtering circuitry you [hopefully] have there, and the other will route current into the switching circuitry.
The 555 can cause a spike in current draw at the moment of switching, so it’s best to ensure that any current it pulls isn’t coming out of your audio circuit.

And, I don’t think I need to say this, but just in case I do: for the love of god, do not use the R1/R2 voltage divider as a half-supply reference for your audio circuitry.  

Conclusion

There are variations you can make to this. Buffered bypass, triggering a multiplexer or a JFET rather than a relay, etc, etc, and perhaps I’ll add some here in the future. Until then, be safe and have fun.